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  • richardmitnick 1:47 pm on July 19, 2017 Permalink | Reply
    Tags: , , , , Cosmic Inflation, , Scientists Are Using the Universe as a "Cosmological Collider", Standard Model of Particle Physics   

    From CfA: “Scientists Are Using the Universe as a “Cosmological Collider” 

    Harvard Smithsonian Center for Astrophysics


    Center For Astrophysics

    July 19, 2017
    Megan Watzke
    Harvard-Smithsonian Center for Astrophysics
    +1 617-496-7998
    mwatzke@cfa.harvard.edu

    Peter Edmonds
    Harvard-Smithsonian Center for Astrophysics
    +1 617-571-7279
    pedmonds@cfa.harvard.edu

    1

    Physicists are capitalizing on a direct connection between the largest cosmic structures and the smallest known objects to use the universe as a “cosmological collider” and investigate new physics.

    The three-dimensional map of galaxies throughout the cosmos and the leftover radiation from the Big Bang – called the cosmic microwave background (CMB) – are the largest structures in the universe that astrophysicists observe using telescopes.

    CMB per ESA/Planck

    ESA/Planck

    Subatomic elementary particles, on the other hand, are the smallest known objects in the universe that particle physicists study using particle colliders.

    LHC

    CERN/LHC Map

    CERN LHC Tunnel

    CERN LHC particles

    A team including Xingang Chen of the Harvard-Smithsonian Center for Astrophysics (CfA), Yi Wang from the Hong Kong University of Science and Technology (HKUST) and Zhong-Zhi Xianyu from the Center for Mathematical Sciences and Applications at Harvard University has used these extremes of size to probe fundamental physics in an innovative way. They have shown how the properties of the elementary particles in the Standard Model of particle physics may be inferred by studying the largest cosmic structures. This connection is made through a process called cosmic inflation.

    The Standard Model of elementary particles (more schematic depiction), with the three generations of matter, gauge bosons in the fourth column, and the Higgs boson in the fifth.

    Inflationary Universe. NASA/WMAP

    Cosmic inflation is the most widely accepted theoretical scenario to explain what preceded the Big Bang. This theory predicts that the size of the universe expanded at an extraordinary and accelerating rate in the first fleeting fraction of a second after the universe was created.

    Lambda-Cold Dark Matter, Accelerated Expansion of the Universe, Big Bang-Inflation (timeline of the universe) Date 2010 Credit: Alex MittelmannColdcreation

    It was a highly energetic event, during which all particles in the universe were created and interacted with each other. This is similar to the environment physicists try to create in ground-based colliders, with the exception that its energy can be 10 billion times larger than any colliders that humans can build.

    Inflation was followed by the Big Bang, where the cosmos continued to expand for more than 13 billion years, but the expansion rate slowed down with time. Microscopic structures created in these energetic events got stretched across the universe, resulting in regions that were slightly denser or less dense than surrounding areas in the otherwise very homogeneous early universe. As the universe evolved, the denser regions attracted more and more matter due to gravity. Eventually, the initial microscopic structures seeded the large-scale structure of our universe, and determined the locations of galaxies throughout the cosmos.

    In ground-based colliders, physicists and engineers build instruments to read the results of the colliding events. The question is then how we should read the results of the cosmological collider.

    “Several years ago, Yi Wang and I, Nima Arkani-Hamed and Juan Maldacena from the Institute of Advanced Study, and several other groups, discovered that the results of this cosmological collider are encoded in the statistics of the initial microscopic structures. As time passes, they become imprinted in the statistics of the spatial distribution of the universe’s contents, such as galaxies and the cosmic microwave background, that we observe today,” said Xingang Chen. “By studying the properties of these statistics we can learn more about the properties of elementary particles.”

    As in ground-based colliders, before scientists explore new physics, it is crucial to understand the behavior of known fundamental particles in this cosmological collider, as described by the Standard Model of particle physics.

    “The relative number of fundamental particles that have different masses – what we call the mass spectrum – in the Standard Model has a special pattern, which can be viewed as the fingerprint of the Standard Model,” explained Zhong-Zhi Xiangyu. “However, this fingerprint changes as the environment changes, and would have looked very different at the time of inflation from how it looks now.”

    The team showed what the mass spectrum of the Standard Model would look like for different inflation models. They also showed how this mass spectrum is imprinted in the appearance of the large-scale structure of our universe. This study paves the way for the future discovery of new physics.

    “The ongoing observations of the CMB and large-scale structure have achieved impressive precision from which valuable information about the initial microscopic structures can be extracted,” said Yi Wang. “In this cosmological collider, any observational signal that deviates from that expected for particles in the Standard Model would then be a sign of new physics.”

    The current research is only a small step towards an exciting era when precision cosmology will show its full power.

    “If we are lucky enough to observe these imprints, we would not only be able to study particle physics and fundamental principles in the early universe, but also better understand cosmic inflation itself. In this regard, there are still a whole universe of mysteries to be explored,” said Xianyu.

    This research is detailed in a paper published in the journal Physical Review Letters on June 29, 2017, and the preprint is available online.

    See the full article here .

    Please help promote STEM in your local schools.

    STEM Icon

    Stem Education Coalition

    The Center for Astrophysics combines the resources and research facilities of the Harvard College Observatory and the Smithsonian Astrophysical Observatory under a single director to pursue studies of those basic physical processes that determine the nature and evolution of the universe. The Smithsonian Astrophysical Observatory (SAO) is a bureau of the Smithsonian Institution, founded in 1890. The Harvard College Observatory (HCO), founded in 1839, is a research institution of the Faculty of Arts and Sciences, Harvard University, and provides facilities and substantial other support for teaching activities of the Department of Astronomy.

     
  • richardmitnick 3:05 pm on June 3, 2017 Permalink | Reply
    Tags: , , , Cosmic Inflation, , , What does the edge of the Universe look like?   

    From Ethan Siegel: “What does the edge of the Universe look like?” 

    From Ethan Siegel
    June 3, 2017

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    The simulated large-scale structure of the Universe shows intricate patterns of clustering that never repeat. But from our perspective, we can only see a finite volume of the Universe. What lies beyond this edge? Image credit: V. Springel et al., MPA Garching, and the Millenium Simulation.

    Universe map Sloan Digital Sky Survey (SDSS) 2dF Galaxy Redshift Survey

    “The Edge… there is no honest way to explain it because the only people who really know where it is are the ones who have gone over.”
    -Hunter S. Thompson

    13.8 billion years ago, the Universe as we know it began with the hot Big Bang. Over that time, space itself has expanded, the matter has undergone gravitational attraction, and the result is the Universe we see today. But as vast as it all is, there’s a limit to what we can see. Beyond a certain distance, the galaxies disappear, the stars twinkle out, and no signals from the distant Universe can be seen. What lies beyond that? That’s this week’s question from Dan Newman, who asks:

    If the universe is finite in volume, then is there a boundary? Is it approachable? And what might the view in that direction be?

    Let’s start by starting at our present location, and looking out as far into the distance as we can.

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    Nearby, the stars and galaxies we see look very much like our own. But as we look farther away, we see the Universe as it was in the distant past: less structured, hotter, younger, and less evolved. Image credit: NASA, ESA, and A. Feild (STScI).

    NASA/ESA Hubble Telescope

    In our own backyard, the Universe is full of stars. But go more than about 100,000 light years away, and you’ve left the Milky Way behind. Beyond that, there’s a sea of galaxies: perhaps two trillion in total contained in our observable Universe. They come in a great diversity of types, shapes, sizes and masses. But as you look back to the more distant ones, you start to find something unusual: the farther away a galaxy is, the more likely it is to be smaller, lower in mass, and to have its stars be intrinsically bluer in color than the nearby ones.

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    How galaxies appear different at different points in the Universe’s history: smaller, bluer, younger, and less evolved at earlier times. Image credit: NASA, ESA, P. van Dokkum (Yale University), S. Patel (Leiden University), and the 3D-HST Team.

    This makes sense in the context of a Universe that had a beginning: a birthday. That’s what the Big Bang was, the day that the Universe as we know it was born. For a galaxy that’s relatively close by, it’s just about the same age that we are. But when we look at a galaxy that’s billions of light years away, that light has needed to travel for billions of years to reach our eyes. A galaxy whose light takes 13 billion years to reach us must be less than one billion years old, and so the farther away we look, we’re basically looking back in time.

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    The full UV-visible-IR composite of the Hubble eXtreme Deep Field; the greatest image ever released of the distant Universe. Image credit: NASA, ESA, H. Teplitz and M. Rafelski (IPAC/Caltech), A. Koekemoer (STScI), R. Windhorst (Arizona State University), and Z. Levay (STScI).

    The above image is the Hubble eXtreme Deep Field (XDF), the deepest image of the distant Universe ever taken. There are thousands of galaxies in this image, at a huge variety of distances from us and from one another. What you can’t see in simple color, though, is that each galaxy has a spectrum associated with it, where clouds of gas absorb light at very particular wavelengths, based on the simple physics of the atom. As the Universe expands, that wavelength stretches, so the more distant galaxies appear redder than they otherwise would. That physics allows us to infer their distance, and lo and behold, when we assign distances to them, the farthest galaxies are the youngest and smallest ones of all.

    Beyond the galaxies, we expect there to be the first stars, and then nothing but neutral gas, when the Universe hadn’t had enough time to pull matter into dense enough states to form a star yet. Going back additional millions of years, the radiation in the Universe was so hot that neutral atoms couldn’t form, meaning that photons bounced off of charged particles continuously. When neutral atoms did form, that light should simply stream in a straight line forever, unaffected by anything other than the expansion of the Universe. The discovery of this leftover glow — the Cosmic Microwave Background — more than 50 years ago was the ultimate confirmation of the Big Bang.

    Cosmic Microwave Background WMAP

    NASA WMAP

    CMB per ESA/Planck

    ESA/Planck

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    Schematic diagram of the Universe’s history, highlighting reionization. Before stars or galaxies formed, the Universe was full of light-blocking, neutral atoms. While most of the Universe doesn’t become reionized until 550 million years afterwards, a few fortunate regions are mostly reionized at earlier times. Image credit: S. G. Djorgovski et al., Caltech Digital Media Center.

    So from where we are today, we can look out in any direction we like and see the same cosmic story unfolding. Today, 13.8 billion years after the Big Bang, we have the stars and galaxies we know today. Earlier, galaxies were smaller, bluer, younger and less evolved. Before that, there were the first stars, and prior to that, just neutral atoms. Before neutral atoms, there was an ionized plasma, then even earlier there were free protons and neutrons, spontaneous creation of matter-and-antimatter, free quarks and gluons, all the unstable particles in the Standard Model, and finally the moment of the Big Bang itself. Looking to greater and greater distances is equivalent to looking all the way back in time.

    The Standard Model of elementary particles (more schematic depiction), with the three generations of matter, gauge bosons in the fourth column, and the Higgs boson in the fifth.

    5
    Artist’s logarithmic scale conception of the observable universe. Galaxies give way to large-scale structure and the hot, dense plasma of the Big Bang at the outskirts. This ‘edge’ is a boundary only in time. Image credit: Wikipedia user Pablo Carlos Budassi.

    Although this defines our observable Universe — with the theoretical boundary of the Big Bang located 46.1 billion light years from our current position — this is not a real boundary in space. Instead, it’s simply a boundary in time; there’s a limit to what we can see because the speed of light allows information to only travel so far over the 13.8 billion years since the hot Big Bang. That distance is farther than 13.8 billion light years because the fabric of the Universe has expanded (and continues to expand), but it’s still limited. But what about prior to the Big Bang? What would you see if you somehow went to the time just a tiny fraction of a second earlier than when the Universe was at its highest energies, hot and dense, and full of matter, antimatter and radiation?

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    Inflation set up the hot Big Bang and gave rise to the observable Universe we have access to. The fluctuations from inflation planted the seeds that grew into the structure we have today. Image credit: Bock et al. (2006, astro-ph/0604101); modifications by E. Siegel.

    You’d find that there was a state called cosmic inflation: where the Universe was expanding ultra fast, and dominated by energy inherent to space itself. Space expanded exponentially during this time, where it was stretched flat, where it was given the same properties everywhere, where pre-existing particles were all pushed away, and where fluctuations in the quantum fields inherent to space were stretched across the Universe. When inflation ended where we are, the hot Big Bang filled the Universe with matter and radiation, giving rise to the part of the Universe — the observable Universe — that we see today. 13.8 billion years later, here we are.

    Inflation theorist Alan Guth:

    4
    Alan Guth, Highland Park High School and M.I.T., who first proposed cosmic inflation

    HPHS Owls

    Lambda-Cold Dark Matter, Accelerated Expansion of the Universe, Big Bang-Inflation (timeline of the universe) Date 2010 Credit: Alex MittelmannColdcreation

    5
    Alan Guth’s notes. http://www.bestchinanews.com/Explore/4730.html

    6
    The observable Universe might be 46 billion light years in all directions from our point of view, but there’s certainly more, unobservable Universe, perhaps even an infinite amount, just like ours beyond that. Image credit: Frédéric MICHEL and Andrew Z. Colvin, annotated by E. Siegel.

    The thing is, there’s nothing special about our location, neither in space nor in time. The fact that we can see 46 billion light years away doesn’t make that boundary or that location anything special; it simply marks the limit of what we can see. If we could somehow take a “snapshot” of the entire Universe, going way beyond the observable part, as it exists 13.8 billion years after the Big Bang everywhere, it would all look like our nearby Universe does today. There would be a great cosmic web of galaxies, clusters, filaments, and cosmic voids, extending far beyond the comparatively small region we can see. Any observer, at any location, would see a Universe that was very much like the one we see from our own perspective.

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    One of the most distant views of the Universe showcases nearby stars and galaxies seen along the way, but the galaxies closer to the outer regions are simply seen at a younger, earlier stage of evolution. From their perspective, they are 13.8 billion years old (and more evolved), and we appear as we did billions of years ago. Image credit: NASA, ESA, the GOODS Team and M. Giavalisco (STScI/University of Massachusetts).

    The individual details would be different, just as the details of our own solar system, galaxy, local group, and so on, are different from any other observer’s viewpoint. But the Universe itself isn’t finite in volume; it’s only the observable part that’s finite. The reason for that is that there’s a boundary in time — the Big Bang — that separates us from the rest. We can approach that boundary only through telescopes (which look to earlier times in the Universe) and through theory. Until we figure out how to circumvent the forward flow of time, that will be our only approach to better understand the “edge” of the Universe. But in space? There’s no edge at all. To the best that we can tell, someone at the edge of what we see would simply see us as the edge instead!

    See the full article here .

    Please help promote STEM in your local schools.

    STEM Icon

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    “Starts With A Bang! is a blog/video blog about cosmology, physics, astronomy, and anything else I find interesting enough to write about. I am a firm believer that the highest good in life is learning, and the greatest evil is willful ignorance. The goal of everything on this site is to help inform you about our world, how we came to be here, and to understand how it all works. As I write these pages for you, I hope to not only explain to you what we know, think, and believe, but how we know it, and why we draw the conclusions we do. It is my hope that you find this interesting, informative, and accessible,” says Ethan

     
  • richardmitnick 2:09 pm on June 1, 2017 Permalink | Reply
    Tags: , , , Cosmic Inflation, , The Inflated Debate Over Cosmic Inflation   

    From Nautilus: “The Inflated Debate Over Cosmic Inflation” 

    Nautilus

    Nautilus

    June 1, 2017
    Amanda Gefter

    Why the majority of physicists are on one side of a recent exchange of letters.

    On the morning of Dec. 7, 1979, a 32-year-old Alan Guth woke up with an idea. It had come into his head the previous night, but now, in the light of a California day, he could see the shape of the thing, and was itching to work through the math. He hopped on his bike and rode to his office at the Stanford Linear Accelerator Center. His excitement got him there in record time: 9 minutes, 32 seconds. At his desk, Guth neatly carried out the calculations in his notebook, forming the numbers and symbols in tight, careful lines. Then, at the top of a fresh page, he wrote in all caps: SPECTACULAR REALIZATION.

    4
    Alan Guth, Highland Park High School and M.I.T., who first proposed cosmic inflation

    HPHS Owls

    Lambda-Cold Dark Matter, Accelerated Expansion of the Universe, Big Bang-Inflation (timeline of the universe) Date 2010 Credit: Alex MittelmannColdcreation

    5
    Alan Guth’s notes. http://www.bestchinanews.com/Explore/4730.html

    A year later and some 6,000 miles away, in Moscow, in the middle of the night, Andrei Linde, having read Guth’s paper, had his own spectacular realization. He had been working on his own idea and now he saw how to bring it to life by fixing the difficulties that plagued Guth’s theory. He woke his sleeping wife. “I think I know how the universe was created.”

    Guth and Linde had worked out the beginnings of the theory of cosmic inflation. The theory would go through several incarnations over the next few decades, as kinks were worked out and details honed. But the core idea was spectacularly simple: In the earliest fraction of a second of time, a small patch of universe expanded faster than the speed of light, doubling its size again and again, growing a million trillion trillion times bigger in the blink of an eye. A little patch of world, about the size of a dime, grew into our entire observable universe.

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    The objectors: From left to right, Anna Ijjas, Paul Steinhardt, and Avi Loeb.

    What began as a radical notion has now become standard wisdom among physicists—except, notably, Paul Steinhardt, Anna Ijjas, and Avi Loeb. The three physicists recently wrote a scathing article in Scientific American arguing that it’s time to abandon inflation and look for a competing idea. (What idea, you ask? Steinhardt, conveniently, has one that he’s been pushing for decades.) Inflation is too unlikely to occur, too flexible to be confirmed or rejected experimentally, and too messy in its implications, the threesome argued. It “cannot be evaluated using the scientific method.”

    It’s not surprising, then, that Guth and Linde—along with physicists David Kaiser and Yasunori Nomura—published a terse response in Scientific American earlier this month defending their theory. What is more surprising, perhaps, is that 29 more of the world’s leading physicists signed it—including four Nobel laureates and a Field’s medalist.

    In the media flurry that followed, the disagreement between these groups of physicists was presented as a straight debate, of the kind that often occurs in science when there are multiple interpretations of data. But describing an equivalence between the opinions of Steinhardt, Ijjas, and Loeb on the one hand, and nearly the entirely cosmology community on the other, is a mistake.

    The long list of signatories to the recent rebuttal letter in Scientific American puts the lie to the claim that the community is divided. When Ed Witten, Steven Weinberg, Leonard Susskind, Frank Wilczek, Juan Maldacena, Eva Silverstein, Sir Martin Rees, and Stephen Hawking (to name a few) write a letter saying you’ve gotten something wrong … well it’s probably worth considering.

    The rebuttal letter also challenges us to understand more clearly why so many scientists are passionate about inflation. What is it about this theory that has the greatest minds in the known universe leaping to its defense?

    From its inception, inflation has offered a remarkable synthesis of seemingly unrelated aspects of physics. It draws on a concept from particle physics called scalar fields—fields that look the same no matter how you view them, but can contain energy or pressure. Their high level of symmetry suggests that one would be most likely to find them in the earliest moments of the universe’s history, which makes them relevant to cosmology. And scalar fields have the special property that they can have negative pressure—a curiosity that takes on deeper meaning in the context of general relativity, where pressure contributes to the curvature of spacetime. Normal pressure produces gravity. Negative pressure produces inflation.

    Inflation shows us that we can take a scalar field from particle physics, apply Einstein’s theory of general relativity, sprinkle in some quantum fluctuations, and get the entire universe replete with all the features we observe when we look around us—features that otherwise would remain inexplicable—from the homogenous distribution of stars and galaxies in all directions and the large-scale geometry of spacetime to the precise bumps and wiggles in the remnant heat from the Big Bang to the very existence of matter itself. When the incredible burst of inflation begins, the uncertainty that rules nature’s tiniest scales is stretched to astronomical proportions, linking the quantum world with the cosmic. When it ends, the expansion energy is transformed into matter and radiation, which sinks into the ripples and divots of spacetime forged by those quantum fluctuations, laying a blueprint for the formation of stars and galaxies.

    As Kaiser, the letter’s co-author and a physicist at the Massachusetts Institute of Technology, explains, “Inflation is a conservative, minimalist outcome of two of the most important conceptual principles in physics: the equivalence principle of general relativity and the uncertainty principle of quantum mechanics.” If a theory’s explanatory power, as co-author Nomura of the University of California, Berkeley says, lies in the ratio of the number of things it can explain to the number of assumptions it takes to explain them, inflation packs a serious punch.

    To be fair, when Guth and Linde first put forth the idea, there were aspects that seemed a little, well, odd—as if inflation arose by exploiting loopholes in physics. Yes, it’s true nothing can travel through space faster than light [in a vacuum], but space itself can expand faster than light; no one had ever seen a scalar field, but physics didn’t expressly forbid it; Einstein himself had conceived of an antigravitational force that would stretch space, but no one had ever observed the stuff. But in the decades since, all of those weird ingredients have shown up. In 1998, astrophysicists discovered an antigravitational force—“dark energy”—that’s driving the accelerated expansion of space right now, with the galaxies farthest from us crossing out of our cosmic horizon faster than the speed of light. In 2012, physicists at the Large Hadron Collider in CERN found clear evidence of a scalar field—the Higgs field. As Murray Gell-Mann once quipped, in physics, “Everything not forbidden is compulsory.” Including, it seems, the universe itself.

    LHC

    CERN/LHC Map

    CERN LHC Tunnel

    CERN LHC particles

    CERN CMS Higgs Event

    CERN ATLAS Higgs Event

    In its early days, too, testing inflation seemed nearly impossible. It predicted that the background heat from the early universe that pervades the sky today—stretched into microwave wavelengths now by 13 billion years of cosmic expansion—would be uniform in temperature to 1 part in 100,000, but beyond that, tiny temperature fluctuations would take on a specific statistical distribution across the sky. No one in 1980 could have imagined the minute level of detail with which spacecraft like NASA’s WMAP, launched in 2001, and the ESA’s Planck Satellite, launched in 2009, would map those tiny differences, bearing out the expectations of inflationary models.

    Cosmic Microwave Background WMAP

    NASA WMAP satellite

    CMB per ESA/Planck

    ESA/Planck

    “The most astonishing thing for me is that it has proved so susceptible to experimental-observational confirmation,” says Susskind, the Stanford physicist and founder of string theory who co-signed the letter. “The outlook for further confirmation is excellent and to say otherwise indicates a lack of appreciation for the accomplishments to date.” Ongoing experiments—including telescopes that are now searching the skies for primordial gravitational waves—could further confirm the predictions of inflation.

    (Susskind, incidentally, was one of the first people with whom Guth shared his spectacular realization on that fateful December day back in 1979. Susskind listened to his colleague’s jubilant news, then said, “You know, the most amazing thing is that they pay us for this.”)

    One of inflation’s early predictions seemed, at the time, flat out wrong. A universe that underwent inflation ought to appear extremely flat (its spacetime metric Euclidean to any measurable degree), because any curvature it had will have been stretched to such huge proportions that you’d never notice it—just like you don’t notice the Earth’s curvature when you’re standing on a flat sidewalk. But the metric of spacetime, according to general relativity, is determined by how much stuff is in it, and a flat metric requires a critical amount of mass and energy. Adding up the weight of all the galaxies in the observable universe, physicists came up about 96 percent short.

    That’s not a little wrong—that’s absurdly wrong. And yet, in the days since, astrophysicists have discovered that the universe is full of invisible dark matter, detectable by its gravitational effects, which brought the total up to 27 percent of the critical mass. Then came the discovery of dark energy, which could be weighed by its effect on the expansion of space, bringing the total to a perfect 100 percent—the critical number for a flat universe, far too conspiratorial to be a fluke.

    Given all of inflation’s successes, why are Steinhardt, Ijjas and Loeb claiming that it isn’t science?

    First, they argue that inflation is too unlikely to occur. The initial conditions required for that scalar field to start inflating, they say, requires an improbable amount of fine-tuning. Within the chaotic conditions at the beginning of time, they argue, you’d need a large, smooth, flat patch of spacetime within which inflation could occur. But large, smooth, flat patches of spacetime are hard to come by—in fact, they are the very features of our universe that inflation was invented to explain. Getting the right initial conditions to trigger inflation, then, would render inflation totally unnecessary.

    That might have been true in the past, but it was never clear that it really mattered. If you start with chaos, there’s bound to be a smooth patch in it somewhere, just randomly, and it doesn’t matter how rare it is because once it inflates it grows so big it completely dominates the universe. That’s the thing about cosmology—you don’t have to show that the universe began easily or quickly. You just have to show that it could begin at all.

    In any case, the story has since changed dramatically in the last few years. Major developments [1] in what’s known as “full numerical general relativity”—powerful computer simulations of the actual dynamics of spacetime—have shown, surprisingly, that inflation can begin much more easily than anyone ever thought. In fact, you don’t need a smooth, flat patch—even lumpy, messy regions will start inflating. “This has been one of the biggest leaps forward,” Kaiser says.

    “The problem of the initial smooth patch has disappeared, in our opinion,” says Leonardo Senatore, a Stanford physicist who worked on the simulations and signed the letter to Scientific American. According to the work, only very few barriers to inflation remain. This “reversed the problem of the probability to start inflation,” says Senatore. “Right now, it seems unlikely for it not to start.” “The problem of initial conditions was solved in 2015,” Linde agrees. “This criticism shows a total ignorance of what is going on.”

    So what about the second criticism, that inflation is too flexible to be tested? It’s true that while the idea behind inflation is simple, its parameters can be tweaked in seemingly endless ways: You can change the energy scale at which inflation begins, the features of the scalar field that drives its expansion, or the number of times spacetime doubles in size before inflation rolls to an end. Turning the dial on any of those parameters leads to different patterns in the microwave background radiation, different arrangements of stars and galaxies and different amplitudes of gravitational waves. In other words, the critics say, go out and measure almost anything and someone will say, “hey, that’s evidence for inflation.” Theories that can predict anything predict nothing. Inflation, they say, isn’t science.

    But supporters argue that this shows a fundamental misunderstanding of what inflation is. It’s not a single model, they insist—it’s a class of models, a sweeping principle, a paradigm from which individual models can be derived and then tested. The key is to figure out which model of inflation is right—if any—and not to prove or falsify all of them all in one fell swoop. “Each model makes specific predictions, and can be tested with precision by the traditional methods of empirical science,” says Guth, now at MIT. Darwin’s concept of evolution by natural selection does not predict exactly which species of animal you’ll see outside your window; Einstein’s concept that gravity is the curvature of spacetime does not predict the actual spacetime geometry of our universe; and inflation doesn’t predict every last bump and wiggle in the microwave background radiation.

    It’s fashionable to refer to the philosopher Karl Popper’s falsification criterion as a kind of litmus test to determine what is and is not science—Susskind has called those who are quick to cry Popper “the Popperazzi.” But in the case of inflation, invoking Popper doesn’t quite add up. “I don’t think that Popper, or any other responsible thinker, has ever advocated that to decide if a theory is scientific, one should ask what class of theories does that theory belong to, and then ask if there is an experimental finding that would rule out that entire class at once,” Guth says. “That would make no sense.”

    Philosophers of science have long known that falsification is not as black and white as it sounds. For every theoretical prediction we put to the test, there are scores of auxiliary assumptions implicit in the entire experimental set-up—observations are “theory-laden,” as the philosophers say. If a prediction doesn’t pan out, is it the theory we’re testing that’s wrong, or one of the auxiliary assumptions? They’re impossible to disentangle. Historians of science, meanwhile, know that science has never proceeded by falsification anyway. If a theory has real explanatory power, scientists don’t abandon it just because it seems too flexible or even if it’s disproven by an anomalous observation. They abandon it if a better theory comes along.

    Take Newtonian gravity. It was a remarkably elegant idea—connecting the motions of falling apples with the motions of orbiting planets—but it had its problems. One of them was that Mercury’s orbit did not conform to its equations. Mercury, by Popperazzi standards, had rendered Newtonian gravity falsified. Wash your hands of it, take it out with the trash falsified. But no one tossed the theory aside, nor should they have. It was only when general relativity came along—which could accurately predict Mercury’s orbit, yes, but also had more elegance, more explanatory power—that Newtonian gravity was declared wrong. “Wrong” is too strong a term, really—when velocities are small and gravity is weak, Einstein’s equations reduce to Newton’s. Newtonian gravity was right—it just wasn’t the whole picture.

    Inflation produces a remarkably accurate account of the geometry of the observable universe and explains why distant patches of sky that would never otherwise have come into contact are in near perfect thermal equilibrium. Like Newtonian gravity, inflation isn’t the whole picture. It can’t be—because it isn’t formulated using quantum gravity, the complete “theory of everything” that will unify quantum mechanics with general relativity. “In my opinion, the problems with inflation are probably going to be solved by a deeper theory of quantum gravity, or by some other process that occurred before slow roll (non-eternal) inflation started in our patch of the universe,” says co-signer Juan Maldacena of the Institute for Advanced Study. “The problems with the hot Big Bang theory were not removed by discarding the Big Bang theory. They were solved by another type of evolution prior to the big bang phase (namely, inflation). Similarly, I think that the problems with inflation will not be solved by ditching inflation but by some other theory of what happened before.”

    And here’s the kicker: Inflationary cosmology itself provides the best hope we have for getting an empirical look at quantum gravity in action. After all, the whole point of inflation is that it takes the quantum-scale dynamics of spacetime itself and magnifies them to cosmic proportions. In doing so, it allows us to study physics at energies 10 billion times greater than the energy scales reached by the Large Hadron Collider at CERN. We will never find a better magnifying glass to peer at the most intricate details of reality. The horizon of a ginormous black hole would make a decent one, but it would still be nothing compared to inflation. “This may be the closest we will ever get to seeing the combined effects of quantum mechanics and general relativity,” Susskind says. One can begin to understand, then, why physicists from fields ranging from the most concrete experimental astrophysics to the most abstract M-theory signed the letter supporting inflation.

    Now for the third and final criticism leveraged by Steinhardt, Ijjas, and Loeb. Back in the early days of inflation, Linde realized that thanks to quantum uncertainty, inflation won’t stop at precisely the same time everywhere. So as our universe exits the inflationary phase and begins its ordinary subluminal expansion, a little patch of scalar field remains. That little patch inflates into a whole new universe, which in turn leaves behind yet another patch, creating yet another universe, ad infinitum. Each universe can contain different fields with different particles with different masses with different behavior. Linde called it the eternal self-reproducing universe. Today, physicists call it the multiverse.

    The multiverse was yet another aspect of inflation that seemed kind of crazy at first, but then the rest of physics caught up. String theory, for instance, turned out to describe a near-infinite number of universes, all of which cried out for a physical mechanism that might produce them. Meanwhile, observations like the value of dark energy were turning up that seemed improbably fine-tuned for the existence of biological life, crying out for an explanation. In each case, eternal inflation came to the rescue.

    Still, not everyone likes the idea of a multiverse. Steinhardt, Ijjas, and Loeb refer to it as “the multimess.” They dislike the fact that various features of physics can differ from one universe to the next, because that reduces the features of our little universe to mere happenstance. “I believe it is a great advantage of the theory,” Linde says, “but they believe it’s a crime.” But not liking the implications of a theory doesn’t make it wrong. “It does seem to make some people uncomfortable—but the comfort zone is not necessarily where we want to be,” Susskind says.

    It does seem like universes forever beyond our reach might also lie beyond the reach of experiment. But there’s an interesting way that eternal inflation can be put to the test. Energy is always conserved in physics—you can’t create more stuff than what you started with. And eternal inflation seems, on glance, to violate this with abandon. There’s another loophole, though: Gravity is a form of negative energy. As inflation creates more universes, it simultaneously creates more gravity, and the two work to perfectly cancel out. Energy—which adds up to zero—is conserved. What about other conservation laws? Angular momentum is conserved—but if you tally up the angular momentum of all the galaxies in the universe, it sums to zero. Electric charge is conserved, but the universe is electrically neutral; it adds up to zero, too. Eternal inflation can make universes like ours precisely because it doesn’t have to violate any conservation laws to do it. It’s making more and more of nothing.

    Again, inflation has taken what seems like a loophole in physics and exposes it to be a profound glimpse into the nature of reality: You can create a universe from nothing—you can create infinite universes from nothing—as long as they all add up to nothing. Not only is that a deep insight, it also creates a testable prediction. “Eternal inflation certainly predicts that the average density of all conserved quantities should be zero,” Guth says. “So if we ever became convinced that the universe has a nonzero density of electric charge or angular momentum, eternal inflation would no longer be an option.”

    Still, infinity makes Steinhardt, Ijjas, and Loeb uneasy because it makes it extremely difficult to define probabilities for the outcomes of measurements. In an infinite multiverse, anything that can happen will happen—an infinite number of times. Such a situation requires what’s called a probability measure to assign different likelihoods to different events, even when there an infinite number of them. But different measures yield different probabilities, and it hasn’t been clear which measure is the right one. That leaves us in a situation where we can’t predict anything, the critics say.

    That might be true—but all it means is that physicists have to figure out which measure to use, not that the multiverse is a free-for-all. “Even the rarest of events will occur an infinite number of times,” Guth explains. “But such events will remain extremely rare. The claim that no outcome will be preferred over any other has absolutely no basis in logic. Mathematicians have known for nearly a century that probabilities can be rigorously defined on infinite spaces, and it is certainly not the case that all events must be equally likely. So, unfortunately, even if there is a multiverse, the chances of my winning the Massachusetts lottery tomorrow will remain incredibly small.”

    “The problem of the measure is very complicated,” Linde says. “It’s very similar to what happened with the interpretation of quantum mechanics. Everyone knows that quantum mechanics works, but people have been debating its interpretation for 100 years. The multiverse is somewhat similar. It is a deep problem, putting a quantum mechanical description of gravity and cosmology all together.”

    A deep problem, indeed—and far subtler than critics of the multiverse imply. (Steinhardt, Ijjas, and Loeb, it should be noted, are far from the only multiverse critics.)The infinite universes that comprise the multiverse are separated from one another by event horizons which, because they are described by both general relativity and quantum mechanics, are strange objects, and not always what they seem. In the last few decades, physicists have discovered that fundamental assumptions about the nature of reality come into question whenever an event horizon lurks. Thought experiments and paradoxes concerning the fate of information that falls behind the horizon of a black hole led Susskind to introduce the notion of “horizon complementarity”: The two sides of a horizon should not be thought of as distinct regions of spacetime, but more like two different descriptions of the very same reality. If horizon complementarity holds in the multiverse, it’s possible that the multiverse is a kind of vast redundancy, and the fundamental theory of the universe may well be written in terms of a single observable universe, rendering the measure problem moot. All the metaphysical baggage of the multiverse, and the distaste some people have for it, might be nothing more than an artifact of all-too-classical thinking. “You have to interpret the multiverse carefully,” says Nomura. “Our world is quantum mechanical.”

    Hawking—who signed the letter—has been working on a model he calls top-down cosmology. In his view, in light of quantum mechanics, it doesn’t make sense to talk about the origin and evolution of the universe as if it followed a single unique trajectory. Instead, he argues, we ought to use the universe as we observe it right now—coupled with the assumption that it arose from nothing—and take a quantum superposition of every possible history that could have led from nothing to now. It’s not that we don’t know which history really occurred—it’s that they all occurred. Rather than a multiverse with a single history, you have a single universe with multiple histories. When Hawking takes the sum of these histories to determine the most probable path, it is—voila!—a history in which the early universe went through inflation. Inflation pops out on its own, from a theory that doesn’t involve a multiverse.

    Many roads, it seems, lead back to inflation and inflation in turn leads to unexpected places. Steinhardt, Ijjas and Loeb are standing by their criticisms of the theory, and have made a website to reiterate them. But the 33 leading physicists who signed the letter—and countless others—are more confident in inflation than ever, exploring its strange territory, optimistic that it will eventually lead them to its own replacement: a more complete theory of the universe’s origin. That, certainly, is science. And it is pretty spectacular.

    See the full article here .

    Please help promote STEM in your local schools.

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    Welcome to Nautilus. We are delighted you joined us. We are here to tell you about science and its endless connections to our lives. Each month we choose a single topic. And each Thursday we publish a new chapter on that topic online. Each issue combines the sciences, culture and philosophy into a single story told by the world’s leading thinkers and writers. We follow the story wherever it leads us. Read our essays, investigative reports, and blogs. Fiction, too. Take in our games, videos, and graphic stories. Stop in for a minute, or an hour. Nautilus lets science spill over its usual borders. We are science, connected.

     
  • richardmitnick 8:25 am on May 25, 2017 Permalink | Reply
    Tags: , , , , Cosmic Inflation, , ,   

    From Nautilus: “The Origin of the Universe” 

    Nautilus

    Nautilus

    April 2017
    John Carlstrom

    1
    The current South Pole telescope measuring small variations in the cosmic microwave background radiation that permeates the universe. Multiple telescopes with upgraded detectors could unlock additional secrets about the origins of the universe. Jason Gallicchio

    Measuring tiny variations in the cosmic microwave background will enable major discoveries about the origin of the universe.

    CMB per ESA/Planck


    ESA/Planck

    How is it possible to know in detail about things that happened nearly 14 billion years ago? The answer, remarkably, could come from new measurements of the cosmic microwave radiation that today permeates all space, but which was emitted shortly after the universe was formed.

    Previous measurements of the microwave background showed that the early universe was remarkably uniform, but not perfectly so: There are small variations in the intensity (or temperature) and polarization of the background radiation. These faint patterns show close agreement with predictions from what is now the standard theoretical model of how the universe began. That model describes an extremely energetic event—the Big Bang—followed within a tiny fraction of a second by a period of very accelerated expansion of the universe called cosmic inflation.

    4
    Alan Guth, Highland Park High School, NJ, USA and M.I.T., who first proposed cosmic inflation

    HPHS Owls

    Lambda-Cold Dark Matter, Accelerated Expansion of the Universe, Big Bang-Inflation (timeline of the universe) Date 2010 Credit: Alex MittelmannColdcreation

    5
    Alan Guth’s notes. http://www.bestchinanews.com/Explore/4730.html

    During this expansion, small quantum fluctuations were stretched to astrophysical scales, becoming the seeds that gave rise to the galaxies and other large-scale structures of the universe visible today.

    Universe map Sloan Digital Sky Survey (SDSS) 2dF Galaxy Redshift Survey

    After the cosmic inflation ended, the expansion began to slow and the primordial plasma of radiation and high-energy sub-atomic particles began to cool. Within a few hundred thousand years, the plasma had cooled sufficiently for atoms to form, for the universe to become transparent to light, and for the first light to be released. That first light has since been shifted—its wavelengths stretched 1,000-fold into the microwave spectrum by the continuing expansion of the universe—and is what we now measure as the microwave background [see above].

    Inflationary Universe. NASA/WMAP

    Recently the development of new superconducting detectors and more powerful telescopes are providing the tools to conduct an even more detailed study of the microwave background. And the payoff could be immense, including additional confirmation that cosmic inflation actually occurred, when it occurred, and how energetic it was, in addition to providing new insights into the quantum nature of gravity. Specifically the new research we propose can address a wide range of fundamental questions:

    1. The accelerated expansion of the universe in the first fraction of a second of its existence, as described by the inflation model, would have created a sea of gravitational waves. These distortions in spacetime would in turn would have left a distinct pattern in the polarization of the microwave background. Detecting that pattern would thus provide a key test of the inflation model, because the level of the polarization links directly to the time of inflation and its energy scale.
    2. Investigating the cosmic gravitational wave background would build on the stunning recent discovery of gravity waves, apparently from colliding black holes, helping to further the new field of gravitational wave astronomy.
    3. These investigations would provide a valuable window on physics at unimaginably high energy scales, a trillion times more energetic than the reach of the most powerful Earth-based accelerators.
    4. The cosmic microwave background provides a backlight on all structure in the universe. Its precise measurement will illuminate the evolution of the universe to the present day, allowing unprecedented insights and constraints on its still mysterious contents and the laws that govern them.

    The origin of the universe was a fantastic event. To gain an understanding of that beginning as an inconceivably small speck of spacetime and its subsequent evolution is central to unraveling continuing mysteries such as dark matter and dark energy. It can shed light on how the two great theories of general relativity and quantum mechanics relate to each other. And it can help us gain a clearer perspective on our human place within the universe. That is the opportunity that a new generation of telescopes and detectors can unlock.

    How to Measure Variations in the Microwave Background with Unparalleled Precision

    2
    Figure 1Ultra-sensitive superconducting bolometer detectors manufactured with thin-film techniques. The project proposes to deploy 500,000 such detectors. Chrystian Posada Arbelaez.

    The time for the next generation cosmic microwave background experiment is now. Transformational improvements have been made in both the sensitivity of microwave detectors and the ability to manufacture them in large numbers at low cost. The advance stems from the development of ultra-sensitive superconducting detectors called bolometers. These devices (Figure 1) essentially eliminate thermal noise by operating at a temperature close to absolute zero, but also are designed to make sophisticated use of electrothermal feedback—adjusting the current to the detectors when incoming radiation deposits energy, so as to keep the detector at its critical superconducting transition temperature under all operating conditions. The sensitivity of these detectors is limited only by the noise of the incoming signal—they generate an insignificant amount of noise of their own.

    Equally important are the production advances. These new ultra-sensitive detectors are manufactured with thin film techniques adapted from Silicon Valley—although using exotic superconducting materials—so that they can be rapidly and uniformly produced at greatly reduced cost. That’s important, because the proposed project needs to deploy about 500,000 detectors in all—something that would not be possible with hand-assembled devices as in the past. Moreover, the manufacturing techniques allow these sophisticated detectors to automatically filter the incoming signals for the desired wavelength sensitivity.

    3
    Figure 2The current focal plane on the South Pole Telescope with seven wafers of detectors plus hand-assembled individual detectors. A single detector wafer of the advanced design proposed here would provide more sensitivity and frequency coverage than this entire focal plane; the project would deploy several hundred such wafers across 10 or more telescopes. Jason Henning.

    To deploy the detectors, new telescopes are needed that have a wide enough focal plane to accommodate a large number of detectors—about 10,000 per telescope to capture enough incoming photons and see a wide enough area of the sky (Figure 2). They need to be placed at high altitude, exceedingly dry locations, so as to minimize the water vapor in the atmosphere that interferes with the incoming photons. The plan is to build on the two sites already established for ongoing background observations, the high Antarctic plateau at the geographic South Pole, and the high Atacama plateau in Chile. Discussions are underway with the Chinese about developing a site in Tibet; Greenland is also under consideration. In all, about 10 specialized telescopes will be needed, and will need to operate for roughly 5 years to accomplish the scientific goals described above. Equally important, the science teams that have come together to do this project will need significant upgrades to their fabrication and testing capabilities.

    The resources needed to accomplish this project are estimated at $100 million over 10 years, in addition to continuation of current federal funding. The technology is already proven and the upgrade path understood. Equally important, a cadre of young, enthusiastic, and well-trained scientists are eager to move forward. Unfortunately, constraints on the federal funding situation are already putting enormous stress on the ability of existing teams just to continue, and the expanded resources to accomplish the objectives described above are not available. This is thus an extraordinary opportunity for private philanthropy—an opportunity to “see” back in time to the very beginning of the universe and to understand the phenomena that shaped our world.

    See the full article here .

    Please help promote STEM in your local schools.

    STEM Icon

    Stem Education Coalition

    Welcome to Nautilus. We are delighted you joined us. We are here to tell you about science and its endless connections to our lives. Each month we choose a single topic. And each Thursday we publish a new chapter on that topic online. Each issue combines the sciences, culture and philosophy into a single story told by the world’s leading thinkers and writers. We follow the story wherever it leads us. Read our essays, investigative reports, and blogs. Fiction, too. Take in our games, videos, and graphic stories. Stop in for a minute, or an hour. Nautilus lets science spill over its usual borders. We are science, connected.

     
  • richardmitnick 4:42 pm on May 12, 2017 Permalink | Reply
    Tags: , , , , Cosmic Inflation, , ,   

    From Ethan Siegel: “What If Cosmic Inflation Is Wrong?” 

    Ethan Siegel
    May 11, 2017

    1
    The earliest stages of the Universe, before the Big Bang, are what set up the initial conditions that everything we see today has evolved from. E. Siegel, with images derived from ESA/Planck and the DoE/NASA/ NSF interagency task force on CMB research.

    All scientific ideas, no matter how accepted or widespread they are, are susceptible to being overturned. For all the successes any idea may have, it only takes one experiment or observation to falsify it, invalidate it, or necessitate that it be revised. Beyond that, every scientific idea or model has a limitation to its range of validity: Newtonian mechanics breaks down close to the speed of light; General Relativity breaks down at singularities; evolution breaks down when you reach the origin of life. Even the Big Bang has its limitations, as there’s only so far back we can extrapolate the hot, dense, expanding state that gave rise to what we see today. Since 1980, the leading idea for describing what came before it has been cosmic inflation, for many compelling reasons. But recently, a spate of public statements has shown a deeper controversy:

    In February, a group of theorists, including one of inflation’s co-founders, claimed that inflation had failed.
    The mainstream group of inflationary cosmologists, including inflation’s inventor, Alan Guth, wrote a rebuttal.
    This prompted the original group to dig in further, denouncing the rebuttal.
    And earlier this week, a major publication and one of the rebuttal’s co-signers highlighted and gave their perspective on the debate.

    2
    The expanding Universe, full of galaxies and complex structure we see today, arose from a smaller, hotter, denser, more uniform state. C. Faucher-Giguère, A. Lidz, and L. Hernquist, Science 319, 5859 (47)

    There are three things going on here: the problems with the Big Bang that led to the development of cosmic inflation, the solution(s) that cosmic inflation provides and generic behavior, and subsequent developments, consequences, and difficulties with the idea. Is that enough to cast doubt on the entire enterprise? Let’s lay it all out for you to see.

    Ever since we first recognized that there are galaxies beyond our own Milky Way, all the indications have shown us that our Universe is expanding. Because the wavelength of light is what determines its energy and temperature, then the fabric of expanding space stretches those wavelengths to be longer, causing the Universe to cool. If the Universe is expanding and cooling as we head into the future, then that means it was closer together, denser, and hotter in the past. As we extrapolate farther and farther back, the hot, dense, uniform Universe tells us a story about its past.

    3
    The stars and galaxies we see today didn’t always exist, and the farther back we go, the closer to an apparent singularity the Universe gets, but there is a limit to that extrapolation. NASA, ESA, and A. Feild (STScI)

    We arrive at a point where galaxy clusters, individual galaxies or even stars haven’t had time to form due to the influence of gravity. We can go even earlier, where the amount of energy in particles and radiation make it impossible for neutral atoms to form; they’d immediately be blasted apart. Even earlier, and atomic nuclei are blasted apart, preventing anything more complex than a proton or neutron from forming. Even earlier, and we begin creating matter/antimatter pairs spontaneously, due to the high energies present. And if you go all the way back, as far as your equations can take you, you’d arrive at a singularity, where all the matter and energy in the entire Universe were condensed into a single point: a singular event in spacetime. That was the original idea of the Big Bang.

    4
    If these three different regions of space never had time to thermalize, share information or transmit signals to one another, then why are they all the same temperature? E. Siegel

    If that were the way things worked, there would be a number of puzzles based on the observations we had.

    Why would the Universe be the same temperature everywhere? The different regions of space from different directions wouldn’t have had time to exchange information and thermalize; there’s no reason for them to be the same temperature. Yet the Universe, everywhere we looked, had the same background 2.73 K temperature.
    Why would the Universe be perfectly spatially flat? The expansion rate and the energy density are two completely independent quantities, yet they must be equal to one part in 1024 in order to produce the flat Universe we have today.
    Why are there no leftover high-energy relics, as practically every high-energy theory predicts? There are no magnetic monopoles, no heavy, right-handed neutrinos, no relics from grand unification, etc. Why not?

    In 1979, Alan Guth had the idea that an early phase of exponential expansion preceding the hot Big Bang could solve all of these problems, and would make additional predictions about the Universe that we could go and look for. This was the big idea of cosmic inflation.

    6
    Alan Guth

    7
    In 1979, Alan Guth had a revelation that a period of exponential expansion in the Universe’s past could set up and provide the initial conditions for the Big Bang. Alan Guth’s 1979 notebook, tweeted via @SLAClab

    This type of expansion, exponential expansion, is different from what happened for the majority of the Universe’s history. When your Universe is full of matter and radiation, the energy density drops as the Universe expands. As the volume expands, the density goes down, and so the expansion rate goes down, too. But during inflation, the Universe is filled with energy inherent to space itself, so as the Universe expands, it simply creates more space, and that keeps the density the same, and prevents the expansion rate from dropping. This, all at once, solves the three puzzles as follows:

    The Universe is the same temperature everywhere today because disparate, distant regions were once connected in the distant past, before the exponential expansion drove them apart.
    The Universe is flat because inflation stretched it to be indistinguishable from flat; the part of the Universe that’s observable to us is so small relative to how much inflation stretched it that it’s unlikely to be any other way.
    And the reason there are no high-energy relics is because inflation pushed them away via the exponential expansion, and then when inflation ended and the Universe got hot again, it never achieved the ultra-high temperatures necessary to create them again.

    By the early 1980s, not only did inflation solve those puzzles, but we also began coming up with models that successfully recovered a Universe that was isotropic (the same in all directions) and homogeneous (the same in all location), consistent with all our observations.

    8
    The fluctuations in the Cosmic Microwave Background were first measured accurately by COBE in the 1990s, then more accurately by WMAP in the 2000s and Planck (above) in the 2010s. This image encodes a huge amount of information about the early Universe. ESA and the Planck Collaboration

    NASA WMAP satellite

    ESA/Planck

    These predictions are interesting, but not enough, of course. For a physical theory to go from interesting to compelling to validated, it needs to make new predictions that can then be tested. It’s important not to gloss over the fact that these early models of inflation did exactly that, making six important predictions:

    The Universe should be perfectly flat. Yes, that was one of the original motivations for it, but at the time, we had very weak constraints. 100% of the Universe could be in matter and 0% in curvature; 5% could be matter and 95% could be curvature, or anywhere in between. Inflation, quite generically, predicted that 100% needed to be “matter plus whatever else,” but curvature should be 0%. This prediction has been validated by our ΛCDM model, where 5% is matter, 27% is dark matter and 68% is dark energy; curvature is still 0%.

    9
    Lambda-Cold Dark Matter, Accelerated Expansion of the Universe, Big Bang-Inflation (timeline of the universe)
    Date 2010
    Author User:Coldcreation

    There should be an almost scale-invariant spectrum of fluctuations. If quantum physics is real, then the Universe should have experienced quantum fluctuations even during inflation. These fluctuations should be stretched, exponentially, across the Universe. When inflation ends, these fluctuations should get turned into matter and radiation, giving rise to overdense and underdense regions that grow into stars and galaxies, or great cosmic voids. Because of how inflation proceeds in the final stages, the fluctuations should be slightly greater on either small scales or large scales, depending on the model of inflation. For perfect scale invariance, a parameter we call n_s would equal 1 exactly; n_s is observed to be 0.96.

    There should be fluctuations on scales larger than light could have traveled since the Big Bang. This is another consequence of inflation, but there’s no way to get a coherent set of fluctuations on large scales like this without something stretching them across cosmic distances. The fact that we see these fluctuations in the cosmic microwave background and in the large-scale structure of the Universe — and didn’t know about them in the early 1980s — further validates inflation.

    These quantum fluctuations, which translate into density fluctuations, should be adiabatic. Fluctuations could have come in different types: adiabatic, isocurvature, or a mixture of the two. Inflation predicted that these fluctuations should have been 100% adiabatic, which should leave unique signatures in both the cosmic microwave background and the Universe’s large-scale structure. Observations bear out that yes, in fact, the fluctuations were adiabatic: of constant entropy everywhere.

    There should be an upper limit, smaller than the Planck scale, to the temperature of the Universe in the distant past. This is also a signature that shows up in the cosmic microwave background: how high a temperature the Universe reached at its hottest. Remember, if there were no inflation, the Universe should have gone up to arbitrarily high temperatures at early times, approaching a singularity. But with inflation, there’s a maximum temperature that must be at energies lower than the Planck scale (~1019 GeV). What we see, from our observations, is that the Universe achieved temperatures no higher than about 0.1% of that (~1016 GeV) at any point, further confirming inflation.

    And finally, there should be a set of primordial gravitational waves, with a particular spectrum. Just as we had an almost perfectly scale-invariant spectrum of density fluctuations, inflation predicts a spectrum of tensor fluctuations in General Relativity, which translate into gravitational waves. The magnitude of these fluctuations are model-dependent on inflation, but the spectrum has a set of unique predictions. This sixth prediction is the only one that has not been verified observationally.

    9
    The final prediction of cosmic inflation is the existence of primordial gravitational waves. It is the only prediction to not be verified by observation… yet. National Science Foundation (NASA, JPL, Keck Foundation, Moore Foundation, related) — Funded BICEP2 Program; modifications by E. Siegel

    So inflation has a tremendous number of successes to its name. But since the late 1980s, theorists have spent a lot of time cooking up a variety of inflationary models. They’ve found some incredibly odd, non-generic behavior in some of them, including exceptions that break some of the predictive rules, above. In general, the simplest inflationary models are based on a potential: you draw a line with a trough or well at the bottom, the inflationary field starts off at some point away from that bottom, and it slowly rolls down towards the bottom, resulting in inflation until it settles at its minimum. Quantum effects play a role in the field, but eventually, inflation ends, converting that field energy into matter and radiation, resulting in the Big Bang.

    10
    The Universe we see today is based on the initial conditions it began with, which are dictated, predictively, by which model of cosmic inflation you choose. Sloan Digital Sky Survey (SDSS)

    SDSS Telescope at Apache Point Observatory, NM, USA

    But you can make multi-field models, fast-roll models instead of slow-roll models, contrived models that have large departures from flatness, and so on. In other words, if you can make the models as complex as you want, you can find one that gives departures from the generic behavior described above, sometimes even resulting in departures from one or more of these six predictions.

    11
    The fluctuations in the CMB are based on primordial fluctuations produced by inflation. In particular, the ‘flat part’ on large scales (at left) have no explanation without inflation. NASA / WMAP Science Team.

    This is what the current controversy is all about! One side goes so far as to claim that because you can contrive models that can give you almost arbitrary behavior, inflation fails to rise to the standard of a scientific theory. The other side claims that inflation makes these generic, successful predictions, and that the better we measure these parameters of the Universe, the more we constrain which models are viable, and the closer we come to understanding which one(s) best describe our physical reality.

    12
    The shape of gravitational wave fluctuations is indisputable from inflation, but the magnitude of the spectrum is entirely model-dependent. Measuring this will put the debate over inflation to rest, but if the magnitude is too low to be detected over the next 25 years or so, the argument may never be settled. Planck science team.

    The facts that no one disputes are that without inflation, or something else that’s very much like inflation (stretching the Universe flat, preventing it from reaching high energies, creating the density fluctuations we see today, causing the Universe to begin at the same temperatures everywhere, etc.), there’s no explanation for the initial conditions the Universe starts off with. Alternatives to inflation have that hurdle to overcome, and right now there is no alternative that has displayed the same predictive power that the inflationary paradigm brings. That doesn’t mean that inflation is necessarily right, but there sure is a lot of good evidence for it, and many of the “possible” models that can be concocted have already been ruled out. Until an alternative model can achieve all of inflation’s successes, cosmic inflation will remain the leading idea for where our hot Big Bang came from.

    See the full article here .

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    “Starts With A Bang! is a blog/video blog about cosmology, physics, astronomy, and anything else I find interesting enough to write about. I am a firm believer that the highest good in life is learning, and the greatest evil is willful ignorance. The goal of everything on this site is to help inform you about our world, how we came to be here, and to understand how it all works. As I write these pages for you, I hope to not only explain to you what we know, think, and believe, but how we know it, and why we draw the conclusions we do. It is my hope that you find this interesting, informative, and accessible,” says Ethan

     
  • richardmitnick 7:28 am on May 12, 2017 Permalink | Reply
    Tags: , , , Cosmic Inflation, ,   

    From Stanford: “Despite a popular media story, rumors of inflationary theory’s demise is premature, Stanford researchers say” 

    Stanford University Name
    Stanford University

    May 10, 2017
    Amy Adams

    From the earliest human civilizations, people have looked to the heavens and pondered the origins of the stars and constellations above. Once, those stories involved gods and magical beings. Now, there’s science, and a large research enterprise focused on understanding how the universe came to be.

    1
    Professor Andrei Linde is among the physicists responding to a recent media story taking aim at inflationary theory. (Image credit: L.A. Cicero)

    Squarely in the center of this research enterprise is what’s known as inflationary theory. It argues that the universe was born out of an unstable, energetic vacuum-like state then expanded dramatically, spinning off entire galaxies produced by quantum fluctuations. This theory was proposed in 1980 by Alan Guth, presently at MIT.

    2
    Alan Guth: https://alchetron.com/Alan-Guth-589833-W

    A year later, this theory was improved and extended by Andrei Linde, Stanford professor of physics, who has spent a lifetime modifying and updating it as new data emerged.

    During the last 35 years, many predictions of inflationary theory have been verified by theorists and confirmed by cosmological observations. Gradually, this theory became a generally accepted description of the origin of the universe. So imagine Linde’s surprise when Scientific American published a story in February by Paul Steinhardt, a professor of physics at Princeton, and his colleagues declaring its demise.

    In response, Linde and Guth, along with their colleagues David Kaiser from MIT and Yasunori Nomura from the University of California, Berkeley, have written a letter [Scientific American] defending the inflationary theory, published in Scientific American May 10. It was signed by 33 academics who read like a Who’s Who of theoretical physicists, including Stephen Hawking of Cambridge University. In it, they take aim at the primary argument in the story: that inflationary theory isn’t really a scientific theory because it doesn’t predict anything and therefore can’t be tested.

    “As the work of several major, international collaborations has made clear, inflation is not only testable but it has been subjected to a significant number of tests and so far has passed every one,” the group wrote.

    A flat universe

    As one example, the inflationary model had predicted that if the universe is ever expanding, it would now be flat rather than open or closed. (Imagine a balloon growing infinitely large. Eventually its surface would appear completely flat.) A flat universe would be represented by a variable called Omega that is equal to 1, “Well, plus or minus a little bit because of quantum uncertainty,” Linde said.

    In fact, in the mid-’90s many astrophysicists believed that the universe was actually not flat, with an Omega closer to about 0.3. “That would be a disaster for inflation,” Linde said. He then tried to find the flaw in his own theory. However, all attempts to construct a model of inflation with Omega equal 0.3 were unsuccessful; the proposed modifications of inflationary theory were extremely complicated and unnatural, and most of them simply did not work. Fortunately, in 1998, a series of cosmological observations revealed the existence of dark energy. It turned out that the energy of a vacuum is not zero, as previously thought, and Omega was restored to 1.

    “If inflationary theory can’t predict anything, why could it appear to be dead when a prediction turned out not to be true?” Linde asked. And how could it be restored by new data that validated the prediction?

    A tense time

    A similarly dramatic situation emerged five years ago, when rumors circulated about a fairly technical issue that’s known as the Gaussianity of inflationary perturbations. The main thing to know about Gaussianity is that the discovery of a large non-Gaussianity of a specific type would rule out 99.9% of the existing inflationary models.

    In 2012 and winter 2013, there were persistent rumors that this non-Gaussianity would soon to be reported by the Planck satellite, and in fact preliminary data by the WMAP satellite indicated a possibility of a very large non-Gaussianity. If that had turned out to be true, it could be a crucial blow to the inflationary theory.

    However, the Planck data revealed no traces of non-Gaussianity. The very last sentence of the Planck paper describing that data read, “With these results, the paradigm of standard single-field slow-roll inflation has survived its most stringent tests to-date.”

    This and many other successful predictions of inflationary theory are undeniable facts, Linde said. “If we trust the arguments made in the Scientific American story, all successful predictions of inflationary cosmology are the result of pure luck, like winning the lottery,” Linde said. “One can do that once, twice, but not this many times. That is why so many leaders of modern physics signed our letter.”

    Linde added that the letters section of a popular magazine is not normally where scientific debate plays out. “A long time ago, when I was young and naive, I thought that things like that are impossible in science,” he said. Now, he just hopes people see that the opinions in the story are not shared by many of the biggest names in theoretical physics and observational cosmology.

    Linde added that he worries about the younger generation of scientists getting the wrong impression from this story. “I don’t want them to read this article and think that they are spending their time on inflationary theory in vain. But the enthusiastic support that we are receiving makes us optimistic that this is not going to happen,” he said.

    See the full article here .

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    Leland and Jane Stanford founded the University to “promote the public welfare by exercising an influence on behalf of humanity and civilization.” Stanford opened its doors in 1891, and more than a century later, it remains dedicated to finding solutions to the great challenges of the day and to preparing our students for leadership in today’s complex world. Stanford, is an American private research university located in Stanford, California on an 8,180-acre (3,310 ha) campus near Palo Alto. Since 1952, more than 54 Stanford faculty, staff, and alumni have won the Nobel Prize, including 19 current faculty members

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  • richardmitnick 3:40 pm on October 19, 2016 Permalink | Reply
    Tags: , , Cosmic Inflation, , , ,   

    From Ethan Siegel: “Where does our arrow of time come from?” 

    10.19.16
    Ethan Siegel

    1
    The history of the Universe and the arrow of time. Image credit: NASA / GSFC.

    The past is gone, the future not yet here, only the present is now. But why does it always flow the way it does for us?

    “Thus is our treaty written; thus is agreement made. Thought is the arrow of time; memory never fades. What was asked is given; the price is paid.”
    -Robert Jordan

    Every moment that passes finds us traveling from the past to the present and into the future, with time always flowing in the same direction. At no point does it ever appear to either stand still or reverse; the “arrow of time” always points forwards for us. But if we look at the laws of physics — from Newton to Einstein, from Maxwell to Bohr, from Dirac to Feynman — they appear to be time-symmetric. In other words, the equations that govern reality don’t have a preference for which way time flows. The solutions that describe the behavior of any system obeying the laws of physics, as we understand them, are just as valid for time flowing into the past as they are for time flowing into the future. Yet we know from experience that time only flows one way: forwards. So where does the arrow of time come from?

    2
    A ball in mid-bounce has its past and future trajectories determined by the laws of physics, but time will only flow into the future for us. Image credit: Wikimedia commons users MichaelMaggs and (edited by) Richard Bartz, under a c.c.a.-s.a.-3.0 license.

    Many people believe there might be a connection between the arrow of time and a quantity called entropy. While most people normally equate “disorder” with entropy, that’s a pretty lazy description that also isn’t particularly accurate. Instead, think about entropy as a measure of how much thermal (heat) energy could possibly be turned into useful, mechanical work. If you have a lot of this energy capable of potentially doing work, you have a low-entropy system, whereas if you have very little, you have a high-entropy system. The second law of thermodynamics is a very important relation in physics, and it states that the entropy of a closed (self-contained) system can only increase or stay the same over time; it can never go down. In other words, over time, the entropy of the entire Universe must increase. It’s the only law of physics that appears to have a preferred direction for time.

    3
    Still from a lecture on entropy by Clarissa Sorensen-Unruh. Image credit: C. Sorensen-Unruh of YouTube, via https://www.youtube.com/watch?v=Mz8IM7pWkok.

    So, does that mean that we only experience time the way we do because of the second law of thermodynamics? That there’s a fundamentally deep connection between the arrow of time and entropy? Some physicists think so, and it’s certainly a possibility. In an interesting collaboration between the MinutePhysics YouTube channel and physicist Sean Carroll, author of The Big Picture, From Eternity To Here and an entropy/time’s arrow fan, they attempt to answer the question of why time doesn’t flow backwards. Unsurprisingly, they point the finger squarely at entropy.


    Access mp4 video here .

    It’s true that entropy does explain the arrow of time for a number of phenomena, including why coffee and milk mix but don’t unmix, why ice melts into a warm drink but never spontaneously arises along with a warm beverage from a cool drink, and why a cooked scrambled egg never resolves back into an uncooked, separated albumen and yolk. In all of these cases, an initially lower-entropy state (with more available, capable-of-doing-work energy) has moved into a higher-entropy (and lower available energy) state as time has moved forwards. There are plenty of examples of this in nature, including of a room filled with molecules: one side full of cold, slow-moving molecules and the other full of hot, fast-moving ones. Simply give it time, and the room will be fully mixed with intermediate-energy particles, representing a large increase in entropy and an irreversible reaction.

    4
    A system set up in the initial conditions on the left and let to evolve will become the system on the right spontaneously, gaining entropy in the process. Image credit: Wikimedia Commons users Htkym and Dhollm, under a c.c.-by-2.5 license.

    Except, it isn’t irreversible completely. You see, there’s a caveat that most people forget when it comes to the second law of thermodynamics and entropy increase: it only refers to the entropy of a closed system, or a system where no external energy or changes in entropy are added or taken away. A way to reverse this reaction was first thought up by the great physicist James Clerk Maxwell way back in the 1870s: simply have an external entity that opens a divide between the two sides of the room when it allows the “cold” molecules to flow onto one side and the “hot” molecules to flow onto the other. This idea became known as Maxwell’s demon, and it enables you to decrease the energy of the system after all!

    5
    A representation of Maxwell’s demon, which can sort particles according to their energy on either side of a box. Image credit: Wikimedia Commons user Htkym, under a c.c.a.-s.a.-3.0 license.

    You can’t actually violate the second law of thermodynamics by doing this, of course. The catch is that the demon must spend a tremendous amount of energy to segregate the particles like this. The system, under the influence of the demon, is an open system; if you include the entropy of the demon itself in the total system of particles, you’ll find that the total entropy does, in fact, increase overall. But here’s the kicker: even if you lived in the box and failed to detect the existence of the demon — in other words, if all you did was live in a pocket of the Universe that saw its entropy decrease — time would still run forward for you. The thermodynamic arrow of time does not determine the direction in which we perceive time’s passage.

    So where does the arrow of time that correlates with our perception come from? We don’t know. What we do know, however, is that the thermodynamic arrow of time isn’t it. Our measurements of entropy in the Universe know of only one possible tremendous decrease in all of cosmic history: the end of cosmic inflation and its transition to the hot Big Bang. We know our Universe is headed to a cold, empty fate after all the stars burn out, after all the black holes decay, after dark energy drives the unbound galaxies apart from one another and gravitational interactions kick out the last remaining bound planetary and stellar remnants. This thermodynamic state of maximal entropy is known as the “heat death” of the Universe. Oddly enough, the state from which our Universe arose — the state of cosmic inflation — has exactly the same properties, only with a much larger expansion rate during the inflationary epoch than our current, dark energy-dominated epoch will lead to.

    6
    The quantum nature of inflation means that it ends in some “pockets” of the Universe and continues in others, but we do not yet understand either what the amount of entropy was during inflation or how it gave rise to the low-entropy state at the start of the hot Big Bang. Image credit: E. Siegel, from the book Beyond The Galaxy.

    How did inflation come to an end? How did the vacuum energy of the Universe, the energy inherent to empty space itself, get converted into a thermally hot bath of particles, antiparticles and radiation? And did the Universe go from an incredibly high-entropy state during cosmic inflation to a lower-entropy one during the hot Big Bang, or was the entropy during inflation even lower due to the eventual capacity of the Universe to do mechanical work? At this point, we have only theories to guide us; the experimental or observational signatures that would tell us the answers to these questions have not been uncovered.

    7
    From the end of inflation and the start of the hot Big Bang, entropy always increases up through the present day. Image credit: E. Siegel, with images derived from ESA/Planck and the DoE/NASA/ NSF interagency task force on CMB research. From his book, Beyond The Galaxy.

    We do understand the arrow of time from a thermodynamic perspective, and that’s an incredibly valuable and interesting piece of knowledge. But if you want to know why yesterday is in the immutable past, tomorrow will arrive in a day and the present is what you’re living right now, thermodynamics won’t give you the answer. Nobody, in fact, understands what will.

    See the full article here .

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    “Starts With A Bang! is a blog/video blog about cosmology, physics, astronomy, and anything else I find interesting enough to write about. I am a firm believer that the highest good in life is learning, and the greatest evil is willful ignorance. The goal of everything on this site is to help inform you about our world, how we came to be here, and to understand how it all works. As I write these pages for you, I hope to not only explain to you what we know, think, and believe, but how we know it, and why we draw the conclusions we do. It is my hope that you find this interesting, informative, and accessible,” says Ethan

     
  • richardmitnick 11:36 am on May 16, 2016 Permalink | Reply
    Tags: , , Cosmic Inflation, , ,   

    From NOVA: “Revealing the Universe’s Mysterious Dark Age” 

    PBS NOVA

    NOVA

    06 Apr 2016 [They just put this in social media]
    Marcus Woo

    The universe wasn’t always like this. Today it’s filled with glittering galaxies, scattered across space like city lights seen from above. But there was a time when all was dark. Really dark.

    Dark Ages Universe ESO
    Dark Ages Universe ESO

    1
    A time-lapse visualization of what the cosmic web’s emergence might have looked like. No image credit

    First, a very brief history of time: from the Big Bang, the universe burst onto the scene as a tiny but glowing inferno of energy. Immediately, it expanded and cooled, dimming into darkness as particles condensed out of the hot soup like droplets of morning dew. Electrons and protons coalesced into atoms, which formed stars, galaxies, planets, and eventually us.

    Inflationary Universe. NASA/WMAP
    Inflationary Universe. NASA/WMAP

    Universe map Sloan Digital Sky Survey (SDSS) 2dF Galaxy Redshift Survey
    Universe map Sloan Digital Sky Survey (SDSS) 2dF Galaxy Redshift Survey

    But a crucial piece still eludes scientists. It’s a gap of several hundred million years that was filled with darkness—a darkness both literal and metaphorical. Astronomers call this period the dark ages, a time that’s not just bereft of illumination, but also devoid of data.

    The Big Bang left a glowing imprint on the entire sky called the cosmic microwave background,,,

    Cosmic Microwave Background per ESA/Planck
    Cosmic Microwave Background per ESA/Planck

    ESA/Planck
    ESA/Planck
    ,,,representing the universe when it was 380,000 years old. Increasingly precise measurements of this radiation have revealed unprecedented details about the earliest cosmic moments. But from then until the emergence of galaxies big and bright enough for today’s telescopes, scientists don’t have any information. Ever mysterious, these dark ages are the final frontier of cosmology.

    And it’s a fundamental frontier. It represents the universe’s most formative years, when it matured from a primordial soup to the cosmos we recognize today.

    Even without much direct data about this era, researchers have made great strides with theory and computer models, simulating the universe through the birth of the first stars. Soon, they may be able to put those theories to the test. In a few years, a suite of new telescopes with new capabilities will start peering into the darkness, and for the first time, astronomers will reach into the unknown.

    The Final Frontier

    Considering that it’s the entire universe they’re trying to understand, cosmologists have done a pretty good job. Increasingly powerful telescopes have allowed them to peer to greater distances, and because the light takes so long to reach the telescopes, astronomers can see farther back in time, capturing snapshots of a universe only a few hundred million years old, just as it emerged from the dark ages. Given that the universe is now 13.7 billion years old, that’s like taking a picture of the cosmos as a toddler.

    That makes the cosmic microwave background, or CMB, clike a detailed ultrasound. This radiation contains the first photons that escaped the yoke of the universe’s primordial plasma. When the universe was a sea of radiation and particles, photons couldn’t travel freely because they kept running into electrons. But about 380,000 years after the Big Bang, the universe had cooled enough that protons were able to lasso electrons into an orbit to form hydrogen atoms. Without electrons in their way, the newly liberated photons could now fly through the cosmos and, more than 13 billion years later, enter the detectors of instruments like the Planck satellite, giving cosmologists the earliest picture of the universe.

    But from this point on, until the universe was a few hundred million years old—the limit of today’s telescopes—astronomers have nothing. It’s as if they have a photo album documenting a person’s entire life, with pictures of young adulthood, adolescence, childhood, and even before birth, but nothing from when the person learned to talk or walk—years of drastic changes.

    That doesn’t mean astronomers have no clue about this period. “People have thought about the first stars since the 1950s,” says Volker Bromm, a professor of astronomy at the University of Texas, Austin. “But they were very speculative because we did not know enough cosmology.” Not until the 1980s did researchers develop more accurate theories that incorporated dark matter, the still-unknown type of particle or particles that comprises about 85% of the matter in the universe. But the first key breakthrough came in 1993, when NASA’s COBE satellite measured the CMB for the first time, collecting basic but crucial data about what the universe was like at the very beginning—the so-called initial conditions of the cosmos. Theorists such as Martin Rees, now the Astronomer Royal of the United Kingdom, and Avi Loeb, a professor of astrophysics at Harvard, realized you could plug these numbers into the equations that govern how the first gas clouds and stars could form. “You could feed them into a computer simulation,” Loeb says. “It’s a well-defined problem.”

    Both Rees and Loeb would influence Bromm, then a graduate student at Yale. Rees and his early work in the 1980s, in particular, inspired Tom Abel, who was a visiting scientist during the 1990s at the University of Illinois, Urbana-Champaign. Independently, Abel and Bromm would make some of the first computer models of their kind to simulate the first stars. “That really opened the field,” Loeb says. “When I started, there were maybe one or a few people even willing to discuss this subject.”

    Theorists like Bromm and Abel, now a professor at Stanford, have since pieced together a blow-by-blow account of the dark ages. Here’s how they think it all went down.

    Then There Was Light

    In the earliest days, during the time that we see in the CMB, the entire universe was bright and as hot as the surface of the sun. But the universe kept expanding and cooling, and after nearly 15 million years, it was as cool as room temperature. “In principle, if there were planets back then, you could’ve had life on them if they had liquid water on their surface,” Loeb says. The temperature continued to fall, and the infrared radiation that suffused the universe lengthened, shifting to radio waves. “Once you cool even further, the universe became a very dark place,” Loeb says. The dark ages had officially begun.

    Meanwhile, the simulations show, things began to stir. The universe was bumpy, with regions of slightly higher and lower densities, which grew from the random quantum fluctuations that emerged in the Big Bang. These denser regions coaxed dark matter to start clumping together, forming a network of sheets and filaments that crisscrossed the universe. At the intersections, denser globs of dark matter formed. Once these roundish halos grew to about 10,000 times the mass of the Sun, Abel says—a few tens of millions of years after the Big Bang—they had enough gravity to corral hydrogen atoms into the first gas clouds.

    Those clouds could then accumulate more gas, heating up to hundreds of degrees. The heat generated enough pressure to prevent further contraction. Soon, the clouds settled into enormous, but rather dull, balls of gas about 100 light years in diameter, Abel says.

    But if the dark matter halos reached masses 100,000 times that of the sun, they could accrue enough gas that the clouds could heat up to about 1000 degrees—and that’s when things got interesting. The surplus energy allowed hydrogen atoms to merge two at a time and form hydrogen molecules—picture two balls attached with a spring. When two hydrogen molecules collide, they vibrate and emit photons that carry away energy.

    When that happens, the molecules are converting the vibrating energy that is heat into radiation that’s lost into space. These interactions cooled the gas, slowing down the molecules and allowing the clouds to collapse. As the clouds grew denser, their temperatures and pressures soared, igniting nuclear fusion. That’s how the first stars were born.

    These first stars, which formed by the time the universe was a couple hundred million years old, were much bigger than those in today’s universe. By the early 2000s, Abel’s simulations, which he says are the most realistic and advanced yet, showed that the first stars weighed about 30 to 300 times the mass of the sun. Using different techniques and algorithms, Bromm says he arrived at a similar answer. For the first time, researchers had a good idea as to what the first objects in the universe were like.

    Massive stars consume fuel like gas-guzzling SUVs. They live fast and die young, collapsing into supernovae after only a few million years.

    Supernova remnant Crab nebula. NASA/ESA Hubble
    Supernova remnant Crab nebula. NASA/ESA Hubble

    NASA/ESA Hubble Telescope
    NASA/ESA Hubble Telescope

    In cosmic timescales, that’s the blink of an eye. “You really want to think of fireworks at these early times,” Abel says. “Just flashing everywhere.”

    In general, the first stars were sparse, separated by thousands of light years. Over the next couple hundred million years, though, guided by the clustering of dark matter, the stars started grouping together to form baby galaxies. During this cosmic dawn, as astronomers call it, galaxies merged with one another and became bigger galaxies. Only after billions and billions of years would they grow into those like our own Milky Way, with hundreds of billions of stars.

    Lifting the Fog

    But there’s more to the story. The first stars shone in many wavelengths, and especially strongly in ultraviolet. The universe’s expansion would’ve stretched this light to visible and infrared wavelengths, which many of our best telescopes are designed to detect. Problem is, during the time of the first stars, a thick fog of neutral hydrogen gas blanketed the whole universe. This gas absorbed shorter-wavelength ultraviolet light, obscuring the view from telescopes. Fortunately, though, this fog would soon lift.

    “This state of affairs can’t last for very long,” says Richard Ellis, an astronomer at the European Southern Observatory in Germany.

    ESO 50 Large
    ESO

    “These ultraviolet photons have sufficient energy to break apart the hydrogen atom back into an electron and a proton.” The hydrogen was ionized, turning into a lone proton that could no longer absorb ultraviolet. The gas was now transparent.

    During this so-called period of reionization, galaxies continued to grow, producing more ultraviolet light that ionized the hydrogen surrounding them, clearing out holes in the fog. “You can imagine the hydrogen like Swiss cheese,” Loeb says. Those bubbles grew, and by the time the universe was around 800 million years old, the ultraviolet radiation ionized the hydrogen between the galaxies, leaving the entire cosmos clear and open to the gaze of telescopes. The dark ages were over, revealing a universe that looked more or less like it does today.

    Seeing into the Dark

    Of course, many details have to be worked out. Astronomers like Ellis are focusing on the latter stages of the dark ages, using the most powerful telescopes to extract clues about this reionization epoch.

    One big question has been whether the ultraviolet light from early galaxies was enough to ionize the whole universe. If it wasn’t, astronomers would have to find another exotic source—like black holes that blast powerful, ionizing jets of radiation—that would have finished the job.

    To find the answer, Ellis and a team of astronomers stretched the Hubble Space Telescope to its limits, extracting as much light as possible from one small patch of sky. These observations reached some of the most distant corners of the universe, discovering some of the earliest galaxies ever seen, during the heart of this reionization era. Their observations suggested that galaxies—large populations of small galaxies, in particular—did seem to have enough ultraviolet light to ionize the universe. Maybe nothing exotic is needed.

    NASA/ESA  Hubble Deep Field
    NASA/ESA Hubble Deep Field

    But to know exactly how it happened, astronomers need new telescopes, like the James Webb Space Telescope set for launch in 2018.

    NASA/ESA/CSA Webb Telescope annotated
    “NASA/ESA/CSA Webb Telescope annotated

    “With the current facilities, it’s just an imponderable,” Ellis says. “We don’t have the power to study these galaxies in any detail.”

    Other astronomers are focusing not on the galaxies, but the hydrogen fog itself. It turns out that the spins of a hydrogen atom’s proton and electron can flip-flop in direction. When the spins go from being aligned to unaligned, the atom releases radiation at a wavelength of 21 centimeters, or 8.27 inches, a telltale signal of neutral hydrogen that astronomers call the 21-cm line. The expanding universe would have stretched this signal to the point where it became a collection of radio waves. The more distant the source of light, the more the radiation gets stretched. By using arrays of radio telescopes to measure the extent of this stretching, astronomers can map the distribution of hydrogen at different points in time. They could then track how those holes in the gas grew and grew until the gas was all ionized.

    “It’s surveying the volume of the universe on a scale that you can’t imagine doing in any way other than through this method—it’s really quite incredible,” says Aaron Parsons, an astronomer at the University of California, Berkeley, who’s leading a project called HERA, which will consist of 352 radio antennae in South Africa.

    HERA NSF
    NSF HERA, South Africa

    Once online, the telescope could give an unprecedented view of reionization. “You can almost imagine making a movie of how the first stars galaxies formed, how they interacted, heated up, ionized, and turned into the galaxies we recognize today.”

    Other telescopes like LOFAR in the Netherlands and the Murchison Widefield Array in Australia will make similar measurements.

    ASTRON LOFAR Map
    ASTRON LOFAR Map

    ASTRON LOFAR Radio Antenna Bank
    ASTRON LOFAR Radio Antenna Bank

    SKA Murchison Widefield Array
    SKA Murchison Widefield Array

    But HERA will be more sensitive, Parsons says. And already with 19 working antennae in place, it might be closest to success, adds Loeb, who isn’t part of the HERA team. “Within a couple years, we should have the first detection of the 21-cm line from this epoch of reionization, which would be fantastic because it would allow us to see the environmental effect of ultraviolet radiation from the first stars and first galaxies on the rest of the universe.”

    This kind of data is crucial for informing computer models like the kind that Abel and Bromm have developed. But despite their successes, theorists are at the point where they need data to test whether their models are accurate.

    Unfortunately, that data won’t be pictures of the first stars. Even the most powerful telescopes won’t be able to see the brightest of them. The first galaxies contain only a few hundred stars and are just too small and faint. “We’ll come ever closer,” Abel says. “It’s very difficult to imagine we’ll actually see those in the near future, but we’ll see their brighter cousins.”

    In fact, the darkest of times, during the couple hundred million years between the CMB and the appearance of the first stars, may always remain beyond astronomers’ grasp. “We currently don’t have any idea of how you could get any direct information about that period,” he says.

    Still, new telescopes over the next few decades promise to reveal much of the dark ages and whether the story theorists are telling is true or even more fantastic than they had thought. “Even though I’m a theorist, I’m modest enough to acknowledge the fact that nature is sometimes more imaginative than we are,” Loeb says. “I’m open to surprises.”

    See the full article here .

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    NOVA is the highest rated science series on television and the most watched documentary series on public television. It is also one of television’s most acclaimed series, having won every major television award, most of them many times over.

     
  • richardmitnick 6:31 am on April 29, 2016 Permalink | Reply
    Tags: , , Cosmic Inflation,   

    From Ethan Siegel: “The biggest question about the beginning of the Universe” 

    Starts with a bang
    Starts with a Bang

    4.28.16
    Ethan Siegel

    Inflationary Universe. NASA/WMAP
    Inflationary Universe. NASA/WMAP

    “Where did it come from” is pretty high up there!

    “Space is certainly something more complicated than the average person would probably realize. Space is not just an empty background in which things happen.” -Alan Guth

    Our Universe is expanding, getting less dense and cooling today, teaching us that it was hotter and denser in the distant past. If we extrapolate backwards in time, we can reach epochs where:

    gravitation hadn’t yet had time to collapse matter into clusters, galaxies or even stars,
    the temperature of the Universe was too hot to form neutral atoms, ionizing them immediately,
    particles were so energetic that even atomic nuclei were unstable, being immediately split apart into individual protons and neutrons,
    and even to where the energy density was so high that matter/antimatter pairs were spontaneously created from pure energy.

    You might think we could go all the way back even farther, to the very birth of space and time themselves. That was, in fact, the original idea of the Big Bang, but thanks to some spectacular observations, we know that isn’t quite how our Universe began.

    Cosmic Microwave Background per ESA/Planck
    Cosmic Microwave Background per ESA/Planck

    ESA/Planck
    ESA/Planck

    Above is the earliest known “baby picture” of our Universe. When the Universe finally did cool enough to stably form neutral atoms, all the radiation from the earliest times could suddenly travel through space, in a straight line, without being absorbed, re-emitted or scattered off of a free, charged particle. This radiation then had its wavelength stretched by the expansion of the Universe, where it can now be found at microwave frequencies: the Cosmic Microwave Background (CMB), or the leftover glow from the Big Bang. When we look at the fluctuations in it — or the slight imperfections from a perfectly uniform temperature at various locations across the sky — we can use what we know about physics and astrophysics to teach us a number of very important things.

    1
    Image credit: NASA / WMAP science team.

    One of the things we can learn is that our Universe is made up of about 5% normal (atomic) matter, 27% dark matter and 68% dark energy. But no less important is this: we learn that these imperfections were initially the same on all scales, and are of such a small magnitude that the Universe couldn’t have achieved an arbitrarily high temperature in the distant past. Instead, there must have been a phase before the Universe was hot, dense and matter-and-radiation filled that set it all up. Originally conceived by Alan Guth in 1979, this phase — known today as cosmic inflation — solves a number of major problems with the Universe: stretching it flat, giving it the same temperature everywhere, eliminating high-energy relics and defects (like magnetic monopoles) from the Universe, and providing a mechanism to generate those much-needed fluctuations.

    History of the universe, National Science Foundation, E Siegel
    Image credit: National Science Foundation (NASA, JPL, Keck Foundation, Moore Foundation, related) — Funded BICEP2 Program; modifications by E. Siegel.

    The fluctuations are remarkable in particular, because two distinct types of them — density (scalar) fluctuations and gravitational wave (tensor) fluctuations — were both predicted by inflation before the evidence for either one existed. As of today, we’ve not only directly observed the scalar ones and have strict limits on the tensor ones, but we’ve measured what the spectrum of these initial fluctuations were, which tells us something about the various types of inflation that could have occurred. In general, you can visualize inflation as a ball rolling down any type of hill you can imagine, into a valley.

    3
    Image credit: E. Siegel, of three “hills-and-valleys” potentials that could describe cosmic inflation. Created with Google’s graph tool.

    In order to have enough inflation to reproduce the Universe that we see, we need for the ball to roll slowly enough down that hill so that the Universe can be stretched flat, made the same temperature everywhere and to have those quantum fluctuations (that create the density fluctuations) get stretched across the Universe. In order to determine which model of inflation is the one our Universe has — in other words, what the shape of that “hill” actually looks like — there are two things that help us out:

    1. The fluctuations can be more important on small scales or on large ones, and by measuring the full spectrum of them, we can know what the slope of that hill was when inflation came to an end.
    2. If we can measure the gravitational wave fluctuations and compare them to the density fluctuations, we can reconstruct how the slope was changing when inflation ended.

    In other words, we can “cook up” any model for inflation that we like, but only some of them will give us the right values — that match our Universe — for these two different types of fluctuations.

    4
    Various models of inflation and what they predict for the scalar (x-axis) and tensor (y-axis) fluctuations from inflation. Image credit: Planck Collaboration: P. A. R. Ade et al., 2013, A&A preprint, with additional annotations by E. Siegel.

    Thanks to the Planck spacecraft, we now have very tight restrictions on the density fluctuations, disfavoring many of the simplest models. As superior (polarization) data from projects like Planck, BICEP, POLARBEAR and others continues to come in, hope that we’ll either detect the gravitational wave signatures or set stronger limits than ever before rises even higher.

    BICEP 2
    BICEP 2

    POLARBEAR McGill Telescope located in the Atacama Desert of northern Chile in the Antofagasta Region. The POLARBEAR experiment is mounted on the Huan Tran Telescope (HTT) at the James Ax Observatory in the Chajnantor Science Reserve.
    POLARBEAR McGill Telescope located in the Atacama Desert of northern Chile in the Antofagasta Region. The POLARBEAR experiment is mounted on the Huan Tran Telescope (HTT) at the James Ax Observatory in the Chajnantor Science Reserve.

    Gravitational waves. Credit: MPI for Gravitational Physics/W.Benger-Zib
    Gravitational waves. Credit: MPI for Gravitational Physics/W.Benger-Zib

    Caltech/MIT Advanced aLIGO Hanford Washington USA installation
    Caltech/MIT Advanced aLIGO Hanford Washington USA installation

    ESA/LISA Pathfinder
    ESA/LISA Pathfinder

    People have argued for a long time that cosmic inflation has too many solutions, but the better we get at making these measurements, the more hope we have that the number of solutions will eventually be reduced to one unique one.

    Inflation to gravitational waves derived from ESAPlanck and the DoENASA NSF interagency task force on CMB research
    Inflation to gravitational waves derived from ESAPlanck and the DOE NASA NSF interagency task force on CMB research

    The Universe has a great story to tell us about its origin, to the limits of what we can conceivably measure. The better we get at actually making those measurements, the better we can understand how it all got its start. Cosmic inflation is almost definitely the answer to what happened before the Big Bang. But what was cosmic inflation like? We’re closer than ever to actually coming up with the answer.

    See the full article here .

    Please help promote STEM in your local schools.

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    “Starts With A Bang! is a blog/video blog about cosmology, physics, astronomy, and anything else I find interesting enough to write about. I am a firm believer that the highest good in life is learning, and the greatest evil is willful ignorance. The goal of everything on this site is to help inform you about our world, how we came to be here, and to understand how it all works. As I write these pages for you, I hope to not only explain to you what we know, think, and believe, but how we know it, and why we draw the conclusions we do. It is my hope that you find this interesting, informative, and accessible,” says Ethan

     
  • richardmitnick 5:02 pm on April 19, 2016 Permalink | Reply
    Tags: , , , Cosmic Inflation,   

    From Quanta: “Physicists Hunt for the Big Bang’s Triangles” 

    Quanta Magazine
    Quanta Magazine

    The story of the universe’s birth — and evidence for string theory — could be found in triangles and myriad other shapes in the sky.

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    Hannes Hummel and Olena Shmahalo/Quanta Magazine

    April 19, 2016
    Natalie Wolchover

    Once upon a time, about 13.8 billion years ago, our universe sprang from a quantum speck, ballooning to one million trillion trillion trillion trillion trillion trillion times its initial volume (by some estimates) in less than a billionth of a trillionth of a trillionth of a second. It then continued to expand at a mellower rate, in accordance with the known laws of physics.

    So goes the story of cosmic inflation, the modern version of the Big Bang theory.

    Inflationary Universe. NASA/WMAP
    Inflationary Universe. NASA/WMAP

    Universe map Sloan Digital Sky Survey (SDSS) 2dF Galaxy Redshift Survey
    Universe map Sloan Digital Sky Survey (SDSS) 2dF Galaxy Redshift Survey

    SDSS Telescope at Apache Point, NM, USA
    SDSS Telescope at Apache Point, NM, USA

    That single short, outrageous growth spurt fits all existing cosmological data well and accounts for the universe’s largeness, smoothness, flatness and lack of preferred direction.

    Cosmic Microwave Background per ESA/Planck
    Cosmic Microwave Background per ESA/Planck

    ESA/Planck
    ESA/Planck

    But as an explanation of how and why the universe began, inflation falls short. The questions it raises — why the growth spurt happened, how it happened, what (if anything) occurred beforehand — have confounded cosmologists since the theory emerged in the 1980s. “We have very strong evidence that there was this period of inflation,” said Matthew Kleban, a cosmologist at New York University. “But we have no idea — or we have many, many ideas — too many ideas — what inflation was, fundamentally.”

    To understand the origin of the universe, today’s cosmologists seek to identify the unknown driver of inflation, dubbed the “inflaton.” Often envisioned as a field of energy permeating space and driving it apart, the inflaton worked, experts say, like a clock. With each tick, it doubled the size of the universe, keeping nearly perfect time — until it stopped. Theorists like Kleban, then, are the clocksmiths, devising altogether hundreds of different models that might replicate the clockwork of the Big Bang.

    Like many cosmological clocksmiths, Kleban is an expert in string theory — the dominant candidate for a “theory of everything” that attempts to describe nature across all distances, times and energies. The known equations of physics falter when applied to the tiny, fleeting and frenzied environment of the Big Bang, in which they struggle to cram an enormous amount of energy into infinitesimal space and time. But string theory flourishes in this milieu, positing extra spatial dimensions that diffuse the energy. Familiar point particles become, at this highest energy and zoom level, one-dimensional “strings” and higher-dimensional, membranous “branes,” all of which traverse a 10-dimensional landscape. These vibrating, undulating gears may have powered the Big Bang’s clock.

    At his office on a recent afternoon, Kleban sketched his latest inflaton design on the blackboard. First, he drew a skinny cylinder to depict the string landscape. Its length represented the three spatial dimensions of macroscopic reality, and its circumference signified the six other spatial dimensions that string theory says exist, but which are too small to see. On the side of the cylinder, he drew a circle. This is Kleban’s timepiece: a membrane that bubbles into being and naturally expands. As its inflating interior forms a new universe, its energy incrementally ticks down in clocklike fashion each time the expanding circle winds around the cylinder’s circumference and overlaps itself. When the energy of the “brane” dilutes, the clock stops ticking, and inflation ends. It’s a scheme that some string cosmologists have hailed for its economy. “I think it’s pretty plausible that some version of this happens,” he said.

    2
    A sketch by the string theorist and cosmologist Matthew Kleban of his Big Bang model known as unwinding inflation. Olena Shmahalo/Quanta Magazine

    Though Kleban acknowledges that it’s too soon to tell whether he or anyone else is on to something, plans are under way to find out.

    The record of the inflaton’s breakneck ticking can be read in the distribution of galaxies, galaxy clusters and superclusters that span the cosmos. These structures (and everything in them, including you) are artifacts of “mistakes in the clock,” as Matias Zaldarriaga, a cosmologist at the Institute for Advanced Study (IAS) in Princeton, N.J., put it. That is, time is intrinsically uncertain, and so the universe inflated at slightly different rates in different places and moments, producing density variations throughout. The jitter in time can also be thought of as a jitter in energy that occurred as pairs of particles spontaneously surfaced all over an “inflaton field” and stretched apart like two points on an inflating balloon. These particles were the seeds that gravity grew into galactic structures over the course of eons. The pairs of structures spanning the largest distances in the sky today came from the earliest quantum fluctuations during inflation, while structures that are closer together were produced later. This nested distribution across all cosmic distance scales “is telling you in detail that the clock was ticking,” said Nima Arkani-Hamed, a theoretical physicist at IAS. “But it doesn’t tell you anything about what it was made of.”

    To reverse-engineer the clockwork, cosmologists are seeking a new kind of data. Their calculations indicate that galaxies and other structures are not merely randomly spread out in pairs across the sky; instead, they have a slight tendency to be arranged in more complex configurations: triangles, rectangles, pentagons and all manner of other shapes, which trace back not just to quantum jitter in the Big Bang’s clock, but to a much more meaningful turning of the gears.

    Teasing out the cosmological triangles and other shapes — which have been named “non-Gaussianities” to contrast them with the Gaussian bell curve of randomly distributed pairs of structures — will require more precise observations of the cosmos than have been made to date. And so plans are being laid for a timeline of increasingly sensitive experiments. “We’re going to have far more information than we have now, and sensitivity to far subtler effects than we can probe now,” said Marc Kamionkowski, a cosmologist at Johns Hopkins University. In the meantime, theorists are making significant progress in determining what shapes to look for and how to look for them. “There’s been a great renaissance of understanding,” said Eva Silverstein, a string cosmologist at Stanford University who devised the dimensional-winding mechanism used by Kleban, as well as many clock designs of her own.

    The rigorous study of non-Gaussianities took off in 2002, when Juan Maldacena, a revered, monklike theorist at IAS, calculated what’s known as the “gravitational floor”: the minimum number of triangles and other shapes that are guaranteed to exist in the sky, due to the unavoidable effect of gravity during cosmic inflation. Cosmologists had been struggling to calculate the gravitational floor for more than a decade, since it would provide a concrete goal for experimenters. If the floor is reached, and still no triangles are detected, Maldacena explained, “then inflation is wrong.”

    When Maldacena first calculated the gravitational floor, actually detecting it seemed a distant goal indeed. At the time, all precise knowledge of the universe’s birth came from observations of the “cosmic microwave background” — the oldest light in the sky, which illuminates a two-dimensional slice of the infant universe as it appeared 380,000 years after the Big Bang. Based on the limited number of nascent structures that appear in this 2-D snapshot, it seemed impossible that their slight propensity to be configured in triangles and other shapes could ever be detected with statistical certainty. But Maldacena’s work gave theorists the tools to calculate other, more pronounced forms of non-Gaussianity that might exist in the sky, due to stronger effects than gravity. And it motivated researchers to devise better ways to search for the signals.

    A year after Maldacena made his calculation, Zaldarriaga and collaborators showed that measuring the distribution of galaxies and groupings of galaxies that make up the universe’s “large-scale structure” would yield many more shapes than observing the cosmic microwave background. “It’s a 3-D versus 2-D argument,” said Olivier Doré, a cosmologist at NASA’s Jet Propulsion Laboratory who is working on a proposed search for non-Gaussianities in the large-scale structure. “If you start counting triangles in 3-D like you can do with galaxy surveys, there are really many more you can count.”

    The notion that counting more shapes in the sky will reveal more details of the Big Bang is implied in a central principle of quantum physics known as “unitarity.” Unitarity dictates that the probabilities of all possible quantum states of the universe must add up to one, now and forever; thus, information, which is stored in quantum states, can never be lost — only scrambled. This means that all information about the birth of the cosmos remains encoded in its present state, and the more precisely cosmologists know the latter, the more they can learn about the former.

    But how did details of the Big Bang get encoded in triangles and other shapes? According to Zaldarriaga, Maldacena’s calculation “opened up the understanding of how it comes about.” In a universe governed by quantum mechanics, all of nature’s constituents are cross-wired, morphing into and interacting with one another with varying degrees of probability. This includes the inflaton field, the gravitational field, and whatever else existed in the primordial universe: Particles arising in these fields would have morphed into and scattered with each other to produce triangles and other geometric configurations, like billiard balls scattering on a table.

    3
    Lucy Reading-Ikkanda for Quanta Magazine

    These dynamical events would be mixed in with the more mundane quantum jitter from those particle pairs that popped up in the inflaton field and engendered so-called “two-point correlations” throughout the sky. A pair of particles might, for instance, have surfaced in some other primordial field, and one member of this pair might then have decayed into two inflaton particles while the other decayed into just a single inflaton particle, yielding a three-point correlation, or triangle, in the sky. Or, two mystery particles might have collided and split into four inflaton particles, producing a four-point correlation. Rarer events would have yielded five-point, six-point and even higher-point correlations, with their numbers, sizes and interior angles encoding the types and relationships of the particles that produced them. The unitarity principle promises that by tallying the shapes ever more precisely, cosmologists will achieve an increasingly detailed account of the primordial universe, just as physicists at the Large Hadron Collider in Europe hone their theory of the known particles and look for evidence of new ones by collecting statistics on how particles morph and scatter during collisions.

    Following Maldacena’s calculation of the gravitational floor, other researchers demonstrated that even many simple inflationary models generate much more pronounced non-Gaussianity than the bare minimum. Clocksmiths like Silverstein and Kleban have since been busy working out the distinct set of triangles that their models would produce — predictions that will become increasingly testable in the coming years. Progress accelerated in 2014, when a small experiment based at the South Pole appeared to make a momentous discovery about the universe’s birth. The announcement drummed up interest in cosmological triangles, even though the supposed discovery ultimately proved a grave disappointment.

    As news began to spread on March 17, 2014, that the “smoking gun” of cosmic inflation had been detected, Stanford University’s press office posted a celebratory video on YouTube. In the footage, the cosmologist Andrei Linde, one of the decorated pioneers of inflationary cosmology, and his wife, the string and supergravity theorist and cosmologist Renata Kallosh, answer their door to find their Stanford colleague Chao-Lin Kuo on the doorstep, accompanied by a camera crew.

    “It’s five sigma, at point two,” Kuo says in the video.

    “Discovery?” Kallosh asks, after a beat. She hugs Kuo, almost melting, as Linde exclaims, “What?”

    Viewers learn that BICEP2, an experiment co-led by Kuo, has detected a swirl pattern in the cosmic microwave background that would have been imprinted by ripples in space-time known as “primordial gravitational waves.”

    BICEP 2
    BICEP 2

    And these could only have arisen during cosmic inflation, as corkscrew-like particles popped up in the gravitational field and then became stretched and permanently frozen into the shape of the universe.

    In the next scene, Linde sips champagne with his wife and their guest. In the early 1980s, Linde, Alexei Starobinsky, Alan Guth and other young cosmologists devised the theory of cosmic inflation as a patch for the broken 1930s-era Big Bang theory, which described the universe as expanding outward from a “singularity” — a nonsensical point of infinite density — and couldn’t explain why the universe hadn’t become mottled and contorted as it grew. Cosmic inflation provided a clever fix for these problems, and BICEP2’s finding suggested that the theory was conclusively proved.

    Gravitational Wave Background
    Gravitational Wave Background [?] from BICEP2

    “If this is true,” Linde says to the camera, “this is a moment of understanding of nature of such a magnitude that it just overwhelms. Let’s see. Let’s just hope that this is not a trick.”

    To many researchers, the most exciting thing about the alleged discovery was the strength of the swirl signal, measured as r = 0.2. The measurement indicated that inflation occurred at an extremely high energy scale and at the earliest moments in time, near the time-energy domain where gravity, as well as the effects of strings, branes or other exotica, would have been strong. The higher the energy scale of inflation, the more cross-wiring there would be between the inflaton and these other primordial ingredients. The result would be pronounced triangles and other non-Gaussianities in the sky.

    “After BICEP, we all stopped what we were doing and started thinking about inflation,” Arkani-Hamed said. “Inflation is like having a gigantic particle accelerator at much higher energy scales than you can get to on Earth.” The question became how such an accelerator would operate, he said, “and if there really was exotic stuff up there [near the inflation scale], how you could go about looking for it.”

    As these investigations took off, more details of BICEP2’s analysis emerged. It became clear that the discovery was indeed a trick of nature: The team’s telescope at the South Pole had picked up the swirly glow of galactic dust rather than the effect of primordial gravitational waves. A mix of anguish and anger swept through the field. Two years on, primordial gravitational waves still haven’t been detected. In January, BICEP2’s predecessor, the BICEP/Keck Array, reported that the value of r can be no more than 0.07, which lowers the ceiling on the energy scale of inflation and moves it further below the scale of strings or other exotic physics.

    Keck Array
    Keck Array

    Nonetheless, many researchers were now aware of the potential gold mine of information contained in triangles and other non-Gaussianities. It had become apparent that these fossils from inflation were worth digging for, even if they were buried deeper than BICEP2 had briefly promised. “Yeah, r went down a little bit,” Maldacena said. But it’s not so bad, in his opinion: A relatively high scale is still possible.

    In a paper last spring that drew on previous work by other researchers, Maldacena and Arkani-Hamed used symmetry arguments to show that a key feature of string theory could manifest itself in triangles. String theory predicts an infinite tower of “higher-spin states” — essentially, strings vibrating at an infinitely rising sequence of pitches. So far, no fundamental particles with a “spin” value greater than two have been discovered. Maldacena and Arkani-Hamed showed that the existence of such a higher-spin state would result in alternating peaks and troughs in the strength of the signal produced by triangles in the sky as they grow more elongated. For string theorists, this is exciting. “You can’t build a consistent interacting theory of such a particle except if you have an infinite tower of them” like the tower in string theory, explained Daniel Baumann, a theoretical cosmologist at the University of Amsterdam. Finding the oscillatory pattern in the triangles in the sky would confirm that this tower exists. “Just seeing one particle of spin greater than two would be indicative of string theory being present.”

    Other researchers are pursuing similarly general predictions. In February, Kamionkowski and collaborators reported detailed information about primordial particles that is encoded in the geometry of four-point correlations, which “get interesting,” he said, because four points can lie flat or sweep into the third dimension. Observing the signals predicted by Arkani-Hamed, Maldacena and Kamionkowski would be like striking gold, but the gold is buried deep: Their strength is probably near the gravitational floor and will require at least 1,000 times the sensitivity of current equipment to detect. Other researchers prefer to tinker with bespoke string models that predict more pronounced triangles and other shapes. “So far we’ve explored only, I think, a very small fraction of the possibilities for non-Gaussianity,” Kamionkowski said.

    Meanwhile, Linde and Kallosh are pushing in a totally different direction. Over the past three years, they’ve become enamored with a class of models called “cosmological alpha-attractors” that do not predict any non-Gaussianities above the gravitational floor at all. According to these models, cosmic inflation was completely pure, driven by a solitary inflaton field. The field is described by a Kähler manifold, which maps onto the geometric disk seen in Escher’s drawing of angels and devils. The Escherian geometry provides a continuum of possible values for the energy scale of inflation, including values so low that the inflaton’s cross-wiring to the gravitational field and other primordial fields would be extremely weak. If such a model does describe the universe, then swirls, triangles and other shapes might never be detected.

    Linde isn’t bothered by this. In supporting the alpha-attractor models, he and Kallosh are staking a position in favor of simplicity and theoretical beauty, at the expense of ever knowing for sure whether their cosmological origin story is correct. An alpha-attractor universe, Linde said, is like one of the happy families in the famous opening line of Anna Karenina. As he paraphrased Tolstoy: “Any happy family, well, they look in a sense alike. But all unhappy families — they’re unhappy for different reasons.”

    Will our universe turn out to be “happy” and completely free of distinguishing features? Baumann, who co-authored a book last year on string cosmology, argues that models like Linde’s and Kallosh’s are too simple to be plausible. “They are building these models from the bottom up,” he said. “Introducing a single field, trying to be very minimal — it would have been a beautiful model of the world.” But, he said, when you try to embed inflation into a fundamental theory of nature, it’s very hard to engineer a single field acting by itself, immune to the effects of everything else. “String theory has many of these effects; you can’t ignore them.”

    And so the search for triangles and other non-Gaussianities is under way. Between 2009 and 2013, the Planck space telescope mapped the cosmic microwave background at the highest resolution yet, and scientists have since been scouring the map for statistical excesses of triangles and other shapes. As of their most recent analysis, they haven’t found any; given the sensitivity of their instruments and their 2-D searching ground, they only ever had an outside chance of doing so. But the scientists are continuing to parse the data in new ways, with another non-Gaussianity analysis expected this year.

    Hiranya Peiris, an astrophysicist at University College London who searches for non-Gaussianities in the Planck data, said that she and her collaborators are taking cues from string cosmologists in determining which signals to look for. Peiris is keen to test a string-inflationary mechanism called axion monodromy, including variants recently developed by Silverstein and collaborators Raphael Flauger, Mehrdad Mibabayi, and Leonardo Senatore that generate an oscillatory pattern in triangles as a function of their size that can be much more pronounced than the pattern studied by Arkani-Hamed and Maldacena. To find such a signal, Peiris and her team must construct templates of the pattern and match them with the data “in a very numerically intensive and demanding analysis,” she said. “Then we have to do careful statistical tests to make sure we are not being fooled by random fluctuations in the data.”

    Some string models have already been ruled out by this data analysis. Regarding the public debate about whether string theory is too divorced from empirical testing to count as science, Silverstein said, “I find it surreal, because we are currently doing some traditional science with string theory.”

    LSST/Camera, built at SLAC
    LSST Interior
    LSST telescope, currently under construction in Chile
    LSST camera, built at SLAC, LSST telescope, currently under construction in Chile.

    Moving forward, cosmologists plan to scour ever larger volumes of the universe’s large-scale structure. Starting in 2020, the proposed SPHEREx mission could measure non-Gaussianity sensitively enough in the distribution of 300 million galaxies to determine whether inflation was driven by one clock or two cross-wired clocks (according to models of the theory known as single- and multi-field inflation, respectively).

    4

    “Just to reach this level would dramatically reduce the number of possible inflation theories,” said Doré, who is working on the SPHEREx project. A few years further out, the Large Synoptic Survey Telescope will map 20 billion cosmological structures. If the statistical presence of triangles is not detected in the universe’s large-scale structure, there is yet another, perhaps final, approach. By mapping an ultra-faint radio signal called the 21-centimeter line, which is emitted by hydrogen atoms and traces back to the creation of the first stars, cosmologists would be able to measure even more “modes,” or arrangements of structures. “It’s going to have information about the whole volume of the universe,” Maldacena said.

    If or when triangles show up, they will, one by one, reveal the nature of the inflaton clock and why it ticked. But will enough clues be gathered before we run out of sky in which to gather them?

    The promise of unitarity — that information can be scrambled but never lost — comes with a caveat.

    “If we assume we can make perfect measurements and we have an infinite sky and so on,” Maldacena said, “then in principle all the interactions and information about particles during inflation is contained in these correlators” — that is, three-point correlations, four-point correlations and so on. But perfect measurements are impossible. And worse, the sky is finite. There is a cosmic horizon: the farthest distance from which light has had time to reach us, and thus, beyond which we cannot see. During inflation, and over the entire history of the accelerating expansion of the universe since then, swirls, triangles, quadrilaterals and other shapes have been flying past this horizon and out of sight. And with them, the subtlest of signals, associated with the rarest, highest-energy processes during inflation, are lost: Cosmologists will never be able to gather enough statistics in our finite patch of sky to tease them out, precluding a complete accounting of nature’s fundamental constituents.

    In his paper with Maldacena, Arkani-Hamed initially included a discussion of this issue, but he removed most of it. He finds the possibility of a limit to knowledge “tremendously disturbing” and sees it as evidence that quantum mechanics must be extended. One possible way to do this is suggested by his work on the amplituhedron, which casts quantum mechanical probabilities (and with them, unitarity) as emergent consequences of an underlying geometry. He plans to discuss this possibility in a forthcoming paper that will relate an analogue of the amplituhedron to non-Gaussianities in the sky.

    People vary in the extent to which they are bothered by a limit to knowledge. “I’m more practical,” Zaldarriaga said. “There are, like, tens or many tens or orders of magnitude more modes that in principle we could see, that we have not been able to measure just because of technological or theoretical inability. So, these ‘in principle’ questions are interesting, but we are way before this point.”

    Kleban also feels hopeful. “Yeah, it’s a finite amount of information,” he said. “But you could say the same thing about evolution, right? There’s a limited number of fossils, and yet we have a pretty good idea of what happened, and it’s getting better and better.”

    If all goes well, enough fossils will turn up in the sky to tell a more complete story. A vast searching ground awaits.

    See the full article here .

    Please help promote STEM in your local schools.

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    Formerly known as Simons Science News, Quanta Magazine is an editorially independent online publication launched by the Simons Foundation to enhance public understanding of science. Why Quanta? Albert Einstein called photons “quanta of light.” Our goal is to “illuminate science.” At Quanta Magazine, scientific accuracy is every bit as important as telling a good story. All of our articles are meticulously researched, reported, edited, copy-edited and fact-checked.

     
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