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  • richardmitnick 1:27 pm on August 2, 2019 Permalink | Reply
    Tags: "We Have Already Entered The Sixth And Final Era Of Our Universe", , , , , , , Inflation   

    From Ethan Siegel: “We Have Already Entered The Sixth And Final Era Of Our Universe” 

    From Ethan Siegel
    Aug 2, 2019

    Timeline of the Inflationary Universe WMAP

    From the inflationary state that preceded the Big Bang to our cold, lonely, dark energy-dominated fate, the Universe goes through six different eras. We’re living in the last one already.

    The Universe is not the same today as it was yesterday. With each moment that goes by, a number of subtle but important changes occur, even if many of them are imperceptible on measurable, human timescales. The Universe is expanding, which means that the distances between the largest cosmic structures are increasing with time.

    A second ago, the Universe was slightly smaller; a second from now, the Universe will be slightly larger. But those subtle changes both build up over large, cosmic timescales, and affect more than just distances. As the Universe expands, the relative importance of radiation, matter, neutrinos, and dark energy all change. The temperature of the Universe changes. And what you’d see in the sky would change dramatically as well. All told, there are six different eras we can break the Universe into, and we’re already in the final one.

    2
    How matter (top), radiation (middle), and a cosmological constant (bottom) all evolve with time in an expanding Universe. As the Universe expands, the matter density dilutes, but the radiation also becomes cooler as its wavelengths get stretched to longer, less energetic states. Dark energy’s density, on the other hand, will truly remain constant if it behaves as is currently thought: as a form of energy intrinsic to space itself. (E. SIEGEL / BEYOND THE GALAXY)

    The reason for this can be understood from the graph above. Everything that exists in our Universe has a certain amount of energy in it: matter, radiation, dark energy, etc. As the Universe expands, the volume that these forms of energy occupy changes, and each one will have its energy density evolve differently. In particular, if we define the observable horizon by the variable a, then:

    matter will have its energy density evolve as 1/a³, since (for matter) density is just mass over volume, and mass can easily be converted to energy via E = mc²,
    radiation will have its energy density evolve as 1/a⁴, since (for radiation) the number density is the number of particles divided by volume, and the energy of each individual photon stretches as the Universe expands, adding an additional factor of 1/a relative to matter,
    and dark energy is a property of space itself, so its energy density remains constant (1/a⁰), irrespective of the Universe’s expansion or volume.

    3
    A visual history of the expanding Universe includes the hot, dense state known as the Big Bang and the growth and formation of structure subsequently. The full suite of data, including the observations of the light elements and the cosmic microwave background, leaves only the Big Bang as a valid explanation for all we see. As the Universe expands, it also cools, enabling ions, neutral atoms, and eventually molecules, gas clouds, stars, and finally galaxies to form. (NASA / CXC / M. WEISS)

    A Universe that has been around longer, therefore, will have expanded more. It will be cooler in the future and was hotter in the past; it was gravitationally more uniform in the past and is clumpier now; it was smaller in the past and will be much, much larger in the future.

    By applying the laws of physics to the Universe, and comparing the possible solutions with the observations and measurements we’ve obtained, we can determine both where we came from and where we’re headed. We can extrapolate our past history all the way back to the beginning of the hot Big Bang and even before, to a period of cosmic inflation. We can extrapolate our current Universe into the far distant future as well, and foresee the ultimate fate that awaits everything that exists.

    4
    Our entire cosmic history is theoretically well-understood, but only because we understand the theory of gravitation that underlies it, and because we know the Universe’s present expansion rate and energy composition. Light will always continue to propagate through this expanding Universe, and we will continue to receive that light arbitrarily far into the future, but it will be limited in time as far as what reaches us. We will need to probe to fainter brightnesses and longer wavelengths to continue to see the objects presently visible, but those are technological, not physical, limitations. (NICOLE RAGER FULLER / NATIONAL SCIENCE FOUNDATION)

    When we draw the dividing lines based on how the Universe behaves, we find that there are six different eras that will come to pass.

    Inflationary era: which preceded and set up the hot Big Bang.
    Primordial Soup era: from the start of the hot Big Bang until the final transformative nuclear & particle interactions occur in the early Universe.
    Plasma era: from the end of non-scattering nuclear and particle interactions until the Universe cools enough to stably form neutral matter.
    Dark Ages era: from the formation of neutral matter until the first stars and galaxies reionize the intergalactic medium of the Universe completely.
    Stellar era: from the end of reionization until the gravity-driven formation and growth of large-scale structure ceases, when the dark energy density dominates over the matter density.
    Dark Energy era: the final stage of our Universe, where the expansion accelerates and disconnected objects speed irrevocably and irreversibly away from one another.

    We already entered this final era billions of years ago. Most of the important events that will define our Universe’s history have already occurred.

    5
    Fluctuations in spacetime itself at the quantum scale get stretched across the Universe during inflation, giving rise to imperfections in both density and gravitational waves. Whether inflation arose from an eventual singularity or not is unknown, but the signatures of whether it occurred are accessible in our observable Universe. (E. SIEGEL, WITH IMAGES DERIVED FROM ESA/PLANCK AND THE DOE/NASA/ NSF INTERAGENCY TASK FORCE ON CMB RESEARCH)

    1.) Inflationary era. Prior to the hot Big Bang, the Universe wasn’t filled with matter, antimatter, dark matter or radiation. It wasn’t filled with particles of any type. Instead, it was filled with a form of energy inherent to space itself: a form of energy that caused the Universe to expand both extremely rapidly and relentlessly, in an exponential fashion.

    Inflation

    4
    Alan Guth, from 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

    Alan Guth’s notes:

    Alan Guth’s original notes on inflation

    It stretched the Universe, from whatever geometry it once had, into a state indistinguishable from spatially flat.
    It expanded a small, causally connected patch of the Universe to one much larger than our presently visible Universe: larger than the current causal horizon.
    It took any particles that may have been present and expanded the Universe so rapidly that none of them are left inside a region the size of our visible Universe.
    And the quantum fluctuations that occurred during inflation created the seeds of structure that gave rise to our vast cosmic web today.

    And then, abruptly, some 13.8 billion years ago, inflation ended. All of that energy, once inherent to space itself, got converted into particles, antiparticles, and radiation. With this transition, the inflationary era ended, and the hot Big Bang began.

    CMB per ESA/Planck

    ESA/Planck 2009 to 2013

    6
    At the high temperatures achieved in the very young Universe, not only can particles and photons be spontaneously created, given enough energy, but also antiparticles and unstable particles as well, resulting in a primordial particle-and-antiparticle soup. Yet even with these conditions, only a few specific states, or particles, can emerge. (BROOKHAVEN NATIONAL LABORATORY)

    2.) Primordial Soup era. Once the expanding Universe is filled with matter, antimatter and radiation, it’s going to cool. Whenever particles collide, they’ll produce whatever particle-antiparticle pairs are allowed by the laws of physics. The primary restriction comes only from the energies of the collisions involved, as the production is governed by E = mc².

    As the Universe cools, the energy drops, and it becomes harder and harder to create more massive particle-antiparticle pairs, but annihilations and other particle reactions continue unabated. 1-to-3 seconds after the Big Bang, the antimatter is all gone, leaving only matter behind. 3-to-4 minutes after the Big Bang, stable deuterium can form, and nucleosynthesis of the light elements occurs. And after some radioactive decays and a few final nuclear reactions, all we have left is a hot (but cooling) ionized plasma consisting of photons, neutrinos, atomic nuclei and electrons.

    7
    At early times (left), photons scatter off of electrons and are high-enough in energy to knock any atoms back into an ionized state. Once the Universe cools enough, and is devoid of such high-energy photons (right), they cannot interact with the neutral atoms, and instead simply free-stream, since they have the wrong wavelength to excite these atoms to a higher energy level. (E. SIEGEL / BEYOND THE GALAXY)

    3.) Plasma era. Once those light nuclei form, they’re the only positively (electrically) charged objects in the Universe, and they’re everywhere. Of course, they’re balanced by an equal amount of negative charge in the form of electrons. Nuclei and electrons form atoms, and so it might seem only natural that these two species of particle would find one another immediately, forming atoms and paving the way for stars.

    Unfortunately for them, they’re vastly outnumbered — by more than a billion to one — by photons. Every time an electron and a nucleus bind together, a high-enough energy photon comes along and blasts them apart. It isn’t until the Universe cools dramatically, from billions of degrees to just thousands of degrees, that neutral atoms can finally form. (And even then, it’s only possible because of a special atomic transition.)

    At the beginning of the Plasma era, the Universe’s energy content is dominated by radiation. By the end, it’s dominated by normal and dark matter.

    Fritz Zwicky discovered Dark Matter when observing the movement of the Coma Cluster., Vera Rubin a Woman in STEM denied the Nobel, did most of the work on Dark Matter.

    Fritz Zwicky from http:// palomarskies.blogspot.com

    Coma cluster via NASA/ESA Hubble

    Astronomer Vera Rubin at the Lowell Observatory in 1965, worked on Dark Matter (The Carnegie Institution for Science)


    Vera Rubin measuring spectra, worked on Dark Matter (Emilio Segre Visual Archives AIP SPL)


    Vera Rubin, with Department of Terrestrial Magnetism (DTM) image tube spectrograph attached to the Kitt Peak 84-inch telescope, 1970. https://home.dtm.ciw.edu

    This third phase takes us to 380,000 years after the Big Bang.

    9
    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, with some regions achieving full reionization earlier and others later. The first major waves of reionization begin happening at around 250 million years of age, while a few fortunate stars may form just 50-to-100 million years after the Big Bang. With the right tools, like the James Webb Space Telescope, we may begin to reveal the earliest galaxies.(S. G. DJORGOVSKI ET AL., CALTECH DIGITAL MEDIA CENTER)

    4.) Dark Ages era. Filled with neutral atoms, at last, gravitation can begin the process of forming structure in the Universe. But with all these neutral atoms around, what we presently know as visible light would be invisible all throughout the sky.

    Why’s that? Because neutral atoms, particularly in the form of cosmic dust, are outstanding at blocking visible light.

    In order to end these dark ages, the intergalactic medium needs to be reionized. That requires enormous amounts of star-formation and tremendous numbers of ultraviolet photons, and that requires time, gravitation, and the start of the cosmic web. The first major regions of reionization take place 200–250 million years after the Big Bang, but reionization doesn’t complete, on average, until the Universe is 550 million years old. At this point, the star-formation rate is still increasing, and the first massive galaxy clusters are just beginning to form.

    10
    The galaxy cluster Abell 370, shown here, was one of the six massive galaxy clusters imaged in the Hubble Frontier Fields program. Since other great observatories were also used to image this region of sky, thousands of ultra-distant galaxies were revealed. By observing them again with a new scientific goal, Hubble’s BUFFALO (Beyond Ultra-deep Frontier Fields And Legacy Observations) program will obtain distances to these galaxies, enabling us to better understand how galaxies formed, evolved, and grew up in our Universe. When combined with intracluster light measurements, we could gain an even greater understanding, via multiple lines of evidence of the same structure, of the dark matter inside. (NASA, ESA, A. KOEKEMOER (STSCI), M. JAUZAC (DURHAM UNIVERSITY), C. STEINHARDT (NIELS BOHR INSTITUTE), AND THE BUFFALO TEAM)

    5.) Stellar era. Once the dark ages are over, the Universe is now transparent to starlight. The great recesses of the cosmos are now accessible, with stars, star clusters, galaxies, galaxy clusters, and the great, growing cosmic web all waiting to be discovered. The Universe is dominated, energy-wise, by dark matter and normal matter, and the gravitationally bound structures continue to grow larger and larger.

    The star-formation rate rises and rises, peaking about 3 billion years after the Big Bang. At this point, new galaxies continue to form, existing galaxies continue to grow and merge, and galaxy clusters attract more and more matter into them. But the amount of free gas within galaxies begins to drop, as the enormous amounts of star-formation have used up a large amount of it. Slowly but steadily, the star-formation rate drops.

    As time goes forward, the stellar death rate will outpace the birth rate, a fact made worse by the following surprise: as the matter density drops with the expanding Universe, a new form of energy — dark energy — begins to appear and dominate. 7.8 billion years after the Big Bang, distant galaxies stop slowing down in their recession from one another, and begin speeding up again. The accelerating Universe is upon us. A little bit later, 9.2 billion years after the Big Bang, dark energy becomes the dominant component of energy in the Universe. At this point, we enter the final era.

    10
    The different possible fates of the Universe, with our actual, accelerating fate shown at the right. After enough time goes by, the acceleration will leave every bound galactic or supergalactic structure completely isolated in the Universe, as all the other structures accelerate irrevocably away. We can only look to the past to infer dark energy’s presence and properties, which require at least one constant, but its implications are larger for the future.(NASA & ESA)

    6.) Dark Energy age.

    Dark Energy Survey


    Dark Energy Camera [DECam], built at FNAL


    NOAO/CTIO Victor M Blanco 4m Telescope which houses the DECam at Cerro Tololo, Chile, housing DECam at an altitude of 7200 feet

    Timeline of the Inflationary Universe WMAP

    The Dark Energy Survey (DES) is an international, collaborative effort to map hundreds of millions of galaxies, detect thousands of supernovae, and find patterns of cosmic structure that will reveal the nature of the mysterious dark energy that is accelerating the expansion of our Universe. DES began searching the Southern skies on August 31, 2013.

    According to Einstein’s theory of General Relativity, gravity should lead to a slowing of the cosmic expansion. Yet, in 1998, two teams of astronomers studying distant supernovae made the remarkable discovery that the expansion of the universe is speeding up. To explain cosmic acceleration, cosmologists are faced with two possibilities: either 70% of the universe exists in an exotic form, now called dark energy, that exhibits a gravitational force opposite to the attractive gravity of ordinary matter, or General Relativity must be replaced by a new theory of gravity on cosmic scales.

    DES is designed to probe the origin of the accelerating universe and help uncover the nature of dark energy by measuring the 14-billion-year history of cosmic expansion with high precision. More than 400 scientists from over 25 institutions in the United States, Spain, the United Kingdom, Brazil, Germany, Switzerland, and Australia are working on the project. The collaboration built and is using an extremely sensitive 570-Megapixel digital camera, DECam, mounted on the Blanco 4-meter telescope at Cerro Tololo Inter-American Observatory, high in the Chilean Andes, to carry out the project.

    Over six years (2013-2019), the DES collaboration used 758 nights of observation to carry out a deep, wide-area survey to record information from 300 million galaxies that are billions of light-years from Earth. The survey imaged 5000 square degrees of the southern sky in five optical filters to obtain detailed information about each galaxy. A fraction of the survey time is used to observe smaller patches of sky roughly once a week to discover and study thousands of supernovae and other astrophysical transients.

    Once dark energy takes over, something bizarre happens: the large-scale structure in the Universe ceases to grow. The objects that were gravitationally bound to one another before dark energy’s takeover will remain bound, but those that were not yet bound by the onset of the dark energy age will never become bound. Instead, they will simply accelerate away from one another, leading lonely existences in the great expanse of nothingness.

    The individual bound structures, like galaxies and groups/clusters of galaxies, will eventually merge to form one giant elliptical galaxy. The existing stars will die; new star formation will slow down to a trickle and then stop; gravitational interactions will eject most of the stars into the intergalactic abyss. Planets will spiral into their parent stars or stellar remnants, owing to decay by gravitational radiation. Even black holes, with extraordinarily long lifetimes, will eventually decay from Hawking radiation.

    In the end, only black dwarf stars and isolated masses too small to ignite nuclear fusion will remain, sparsely populated and disconnected from one another in this empty, ever-expanding cosmos. These final-state corpses will exist even googols of years onward, continuing to persist as dark energy remains the dominant factor in our Universe.

    This last era, of dark energy domination, has already begun. Dark energy became important for the Universe’s expansion 6 billion years ago, and began dominating the Universe’s energy content around the time our Sun and Solar System were being born. The Universe may have six unique stages, but for the entirety of Earth’s history, we’ve already been in the final one. Take a good look at the Universe around us. It will never be this rich — or this easy to access — ever again.

    See the full article here .

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    Please help promote STEM in your local schools.

    Stem Education Coalition

    “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 11:18 am on May 7, 2019 Permalink | Reply
    Tags: "A universe is born", , , , , , , , , Inflation, , , , The Planck epoch   

    From Symmetry: “A universe is born” 

    Symmetry Mag
    From Symmetry

    05/07/19
    Diana Kwon

    Take a (brief) journey through the early history of our cosmos.

    Timeline of the Inflationary Universe WMAP

    The universe was a busy place during the first three minutes. The cosmos we see today expanded from a tiny speck to much closer to its current massive size; the elementary particles appeared; and protons and neutrons combined into the first nuclei, filling the universe with the precursors of elements.

    By developing clever theories and conducting experiments with particle colliders, telescopes and satellites, physicists have been able to wind the film of the universe back billions of years—and glimpse the details of the very first moments in the history of our cosmic home.

    Take an abridged tour through this history:

    The Planck epoch
    Time: < 10^-43 seconds

    The Planck Epoch https:// http://www.slideshare.net ericgolob the-big-bang-10535251

    Welcome to the Planck epoch, named after the smallest scale of measurements possible in particle physics today. This is currently the closet scientists can get to the beginning of time.

    Theoretical physicists don’t know much about the earliest moments of the universe. After the Big Bang theory gained popularity, scientists thought that in the first moments, the cosmos was at its hottest and densest and that all four fundamental forces—electromagnetic, weak, strong and gravitational—were combined into a single, unified force. But the current leading theoretical framework for our universe’s beginning doesn’t necessarily require these conditions.

    The universe expands
    Time: From 10^-43 seconds to about 10^-36 seconds

    In this stage, which began either at Planck time or shortly after it, scientists think the universe underwent superfast, exponential expansion in a process known as inflation.

    Inflation

    4
    Alan Guth, from 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

    Alan Guth’s notes:
    5

    Physicists first proposed the theory of inflation in the 1980s to address the shortcomings of the Big Bang theory, which, despite its popularity, could not explain why the universe was so flat and uniform, and why its different parts began expanding simultaneously.

    During inflation, quantum fluctuations could have stretched out to produce a pattern that later determined the locations of galaxies. It might have been only after this period of inflation the universe became a hot, dense fireball as described in the Big Bang theory.

    The elementary particles are born
    Time: ~10^-36 seconds

    When the universe was still very hot, the cosmos was like a gigantic accelerator, much more powerful than the Large Hadron Collider, running at extremely high energies. In it, the elementary particles we know today were born.

    Scientists think that first came exotic particles, followed by more familiar ones, such as electrons, neutrinos and quarks. It could be that dark matter particles came about during this time.

    3
    Quarks APS/Alan Stonebraker

    The quarks soon combined, forming the familiar protons and neutrons, which are collectively known as baryons. Neutrinos were able to escape this plasma of charged particles and began traveling freely through space, while photons continued to be trapped by the plasma.

    Standard Model of Particle Physics

    The first nuclei emerge
    Time: ~1 second to 3 minutes

    Scientists think that when the universe cooled enough for violent collisions to subside, protons and neutrons clumped together into nuclei of the light elements—hydrogen, helium and lithium—in a process known as Big Bang nucleosynthesis.

    Protons are more stable than neutrons, due to their lower mass. In fact, a free neutron decays with a 15-minute half-life, while protons may not decay at all, as far as we know.

    So as the particles combined, many protons remained unpaired. As a result, hydrogen—protons that never found a partner—make up around 74% of the mass of “normal” matter in our cosmos. The second most abundant element is helium, which makes up approximately 24%, followed by trace amounts of deuterium, lithium, and helium-3 (helium with a three-baryon core).

    Periodic table Sept 2017. Wikipedia

    Scientists have been able to accurately measure the density of baryons in our universe. Most of those measurements line up with theorists’ estimations of what the quantities ought to be, but there is one lingering issue: Lithium calculations are off by a factor of three. It could be that the measurements are off, but it could also be that something we don’t yet know about happened during this time period to change the abundance of lithium.

    The cosmic microwave background becomes visible
    Time: 380,000 years

    Hundreds of thousands of years after inflation, the particle soup had cooled enough for electrons to bind to nuclei to form electrically neutral atoms. Through this process, which is also known as recombination, photons became free to traverse the universe, creating the cosmic microwave background.

    CMB per ESA/Planck


    ESA/Planck 2009 to 2013

    Today, the CMB is one of the most valuable tools for cosmologists, who probe its depths in search of answers for many of the universe’s lingering secrets, including the nature of inflation and the cause of matter-antimatter asymmetry.

    Shortly after the CMB became detectable, neutral hydrogen particles formed into a gas that filled the universe. Without any objects emitting high-energy photons, the cosmos was plunged into the dark ages for millions of years.

    Dark Energy Camera Enables Astronomers a Glimpse at the Cosmic Dawn. CREDIT National Astronomical Observatory of Japan

    The earliest stars shine
    Time: ~100 million years

    The dark ages ended with the formation of the first stars and the occurrence of reionization, a process through which highly energetic photons stripped electrons off neutral hydrogen atoms.

    Reionization era and first stars, Caltech

    Scientists think that the vast majority of the ionizing photons emerged from the earliest stars. But other processes, such as collisions between dark matter particles, may have also played a role.

    At this time, matter began to form the first galaxies. Our own galaxy, the Milky Way, contains stars that were born when the universe was only several hundred million years old.

    Milky Way NASA/JPL-Caltech /ESO R. Hurt

    Our sun is born
    Time: 9.2 billion years

    3
    NASA

    The sun is one of a few hundred billion stars in the Milky Way. Scientists think it formed from a giant cloud of gas that consisted mostly hydrogen and helium.

    Today
    Time: 13.8 billion years

    Today, our cosmos sits at a cool 2.7 Kelvin (minus 270.42 degrees Celsius). The universe is expanding at an increasing rate, in a manner similar to (but many orders of magnitude slower than) inflation.

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

    Physicists think that dark energy—a mysterious repulsive force that currently accounts for about 70% of the energy in our universe—is most likely driving that accelerated expansion.

    Dark energy depiction. Image: Volker Springle/Max Planck Institute for Astrophysics/SP)

    See the full article here .


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    Please help promote STEM in your local schools.


    Stem Education Coalition

    Symmetry is a joint Fermilab/SLAC publication.


     
  • richardmitnick 12:10 pm on April 23, 2019 Permalink | Reply
    Tags: "Falsifiability and physics", , , , , , , Inflation, Karl Popper (1902-1994) "The Logic of Scientific Discovery", , ,   

    From Symmetry: “Falsifiability and physics” 

    Symmetry Mag
    From Symmetry

    04/23/19
    Matthew R. Francis

    1
    Illustration by Sandbox Studio, Chicago with Corinne Mucha

    Can a theory that isn’t completely testable still be useful to physics?

    What determines if an idea is legitimately scientific or not? This question has been debated by philosophers and historians of science, working scientists, and lawyers in courts of law. That’s because it’s not merely an abstract notion: What makes something scientific or not determines if it should be taught in classrooms or supported by government grant money.

    The answer is relatively straightforward in many cases: Despite conspiracy theories to the contrary, the Earth is not flat. Literally all evidence is in favor of a round and rotating Earth, so statements based on a flat-Earth hypothesis are not scientific.

    In other cases, though, people actively debate where and how the demarcation line should be drawn. One such criterion was proposed by philosopher of science Karl Popper (1902-1994), who argued that scientific ideas must be subject to “falsification.”

    Popper wrote in his classic book The Logic of Scientific Discovery that a theory that cannot be proven false—that is, a theory flexible enough to encompass every possible experimental outcome—is scientifically useless. He wrote that a scientific idea must contain the key to its own downfall: It must make predictions that can be tested and, if those predictions are proven false, the theory must be jettisoned.

    When writing this, Popper was less concerned with physics than he was with theories like Freudian psychology and Stalinist history. These, he argued, were not falsifiable because they were vague or flexible enough to incorporate all the available evidence and therefore immune to testing.

    But where does this falsifiability requirement leave certain areas of theoretical physics? String theory, for example, involves physics on extremely small length scales unreachable by any foreseeable experiment.

    String Theory depiction. Cross section of the quintic Calabi–Yau manifold Calabi yau.jpg. Jbourjai (using Mathematica output)

    Cosmic inflation, a theory that explains much about the properties of the observable universe, may itself be untestable through direct observations.

    Some critics believe these theories are unfalsifiable and, for that reason, are of dubious scientific value.

    At the same time, many physicists align with philosophers of science who identified flaws in Popper’s model, saying falsification is most useful in identifying blatant pseudoscience (the flat-Earth hypothesis, again) but relatively unimportant for judging theories growing out of established paradigms in science.

    “I think we should be worried about being arrogant,” says Chanda Prescod-Weinstein of the University of New Hampshire. “Falsifiability is important, but so is remembering that nature does what it wants.”

    Prescod-Weinstein is both a particle cosmologist and researcher in science, technology, and society studies, interested in analyzing the priorities scientists have as a group. “Any particular generation deciding that they’ve worked out all that can be worked out seems like the height of arrogance to me,” she says.

    Tracy Slatyer of MIT agrees, and argues that stringently worrying about falsification can prevent new ideas from germinating, stifling creativity. “In theoretical physics, the vast majority of all the ideas you ever work on are going to be wrong,” she says. “They may be interesting ideas, they may be beautiful ideas, they may be gorgeous structures that are simply not realized in our universe.”

    Particles and practical philosophy

    Take, for example, supersymmetry. SUSY is an extension of the Standard Model in which each known particle is paired with a supersymmetric partner.

    Standard Model of Supersymmetry via DESY

    The theory is a natural outgrowth of a mathematical symmetry of spacetime, in ways similar to the Standard Model itself. It’s well established within particle physics, even though supersymmetric particles, if they exist, may be out of scientists’ experimental reach.

    SUSY could potentially resolve some major mysteries in modern physics. For one, all of those supersymmetric particles could be the reason the mass of the Higgs boson is smaller than quantum mechanics says it should be.

    CERN CMS Higgs Event


    CERN ATLAS Higgs Event

    “Quantum mechanics says that [the Higgs boson] mass should blow up to the largest mass scale possible,” says Howard Baer of the University of Oklahoma. That’s because masses in quantum theory are the result of contributions from many different particles involved in interactions—and the Higgs field, which gives other particles mass, racks up a lot of these interactions. But the Higgs mass isn’t huge, which requires an explanation.

    “Something else would have to be tuned to a huge negative [value] in order to cancel [the huge positive value of those interactions] and give you the observed value,” Baer says. That level of coincidence, known as a “fine-tuning problem,” makes physicists itchy. “It’s like trying to play the lottery. It’s possible you might win, but really you’re almost certain to lose.”

    If SUSY particles turn up in a certain mass range, their contributions to the Higgs mass “naturally” solve this problem, which has been an argument in favor of the theory of supersymmetry. So far, the Large Hadron Collider has not turned up any SUSY particles in the range of “naturalness.”

    LHC

    CERN map


    CERN LHC Tunnel

    CERN LHC particles

    However, the broad framework of supersymmetry can accommodate even more massive SUSY particles, which may or may not be detectable using the LHC. In fact, if naturalness is abandoned, SUSY doesn’t provide an obvious mass scale at all, meaning SUSY particles might be out of range for discovery with any earthly particle collider. That point has made some critics queasy: If there’s no obvious mass scale at which colliders can hunt for SUSY, is the theory falsifiable?

    A related problem confronts dark matter researchers: Despite strong indirect evidence for a large amount of mass invisible to all forms of light, particle experiments have yet to find any dark matter particles. It could be that dark matter particles are just impossible to directly detect. A small but vocal group of researchers has argued that we need to consider alternative theories of gravity instead.

    Fritz Zwicky, the Father of Dark Matter research.No image credit after long search

    Astronomer Vera Rubin at the Lowell Observatory in 1965, worked on Dark Matter (The Carnegie Institution for Science)

    U Washington ADMX Axion Dark Matter Experiment

    DEAP Dark Matter detector, The DEAP-3600, suspended in the SNOLAB deep in Sudbury’s Creighton Mine

    Dark Side-50 Dark Matter Experiment at Gran Sasso

    Slatyer, whose research involves looking for dark matter, considers the criticism partly as a problem of language. “When you say ‘dark matter,’ [you need] to distinguish dark matter from specific scenarios for what dark matter could be,” she says. “The community has not always done that well.”

    In other words, specific models for dark matter can stand or fall, but the dark matter paradigm as a whole has withstood all tests so far. But as Slatyer points out, no alternative theory of gravity can explain all the phenomena that a simple dark matter model can, from the behavior of galaxies to the structure of the cosmic microwave background.

    Prescod-Weinstein argues that we’re a long way from ruling out all dark matter possibilities. “How will we prove that the dark matter, if it exists, definitively doesn’t interact with the Standard Model?” she says. “Astrophysics is always a bit of a detective game. Without laboratory [detection of] dark matter, it’s hard to make definitive statements about its properties. But we can construct likely narratives based on what we know about its behavior.”

    Similarly, Baer thinks that we haven’t exhausted all the SUSY possibilities yet. “People say, ‘you’ve been promising supersymmetry for 20 or 30 years,’ but it was based on overly optimistic naturalness calculations,” he says. “I think if one evaluates the naturalness properly, then you find that supersymmetry is still even now very natural. But you’re going to need either an energy upgrade of LHC or an ILC [International Linear Collider] in order to discover it.”

    ILC schematic, being planned for the Kitakami highland, in the Iwate prefecture of northern Japan

    Beyond falsifiability of dark matter or SUSY, physicists are motivated by more mundane concerns. “Even if these individual scenarios are in principle falsifiable, how much money would [it] take and how much time would it take?” Slatyer says. In other words, rather than try to demonstrate or rule out SUSY as a whole, physicists focus on particle experiments that can be performed within a certain number of budgetary cycles. It’s not romantic, but it’s true nevertheless.

    2
    Illustration by Sandbox Studio, Chicago with Corinne Mucha

    Is it science? Who decides?

    Historically, sometimes theories that seem untestable turn out to just need more time. For example, 19th century physicist Ludwig Boltzmann and colleagues showed they could explain many results in thermal physics and chemistry if everything were made up of “atoms”—what we call particles, atoms, and molecules today—governed by Newtonian physics.

    Since atoms were out of reach of experiments of the day, prominent philosophers of science argued that the atomic hypothesis was untestable in principle, and therefore unscientific.

    However, the atomists eventually won the day: J. J. Thompson demonstrated the existence of electrons, while Albert Einstein showed that water molecules could make grains of pollen dance on a pond’s surface.

    Atoms provide a case study for how falsifiability proved to be the wrong criterion. Many other cases are trickier.

    For instance, Einstein’s theory of general relativity is one of the best-tested theories in all of science. At the same time, it allows for physically unrealistic “universes,” such as a “rotating” cosmos where movement back and forth in time is possible, which are contradicted by all observations of the reality we inhabit.

    General relativity also makes predictions about things that are untestable by definition, like how particles move inside the event horizon of a black hole: No information about these trajectories can be determined by experiment.

    The first image of a black hole, Messier 87 Credit Event Horizon Telescope Collaboration, via NSF 4.10.19

    Yet no knowledgeable physicist or philosopher of science would argue that general relativity is unscientific. The success of the theory is due to enough of its predictions being testable.

    Eddington/Einstein exibition of gravitational lensing solar eclipse of 29 May 1919

    Another type of theory may be mostly untestable, but have important consequences. One such theory is cosmic inflation, which (among other things) explains why we don’t see isolated magnetic monopoles and why the universe is a nearly uniform temperature everywhere we look.

    The key property of inflation—the extremely rapid expansion of spacetime during a tiny split second after the Big Bang—cannot be tested directly. Cosmologists look for indirect evidence for inflation, but in the end it may be difficult or impossible to distinguish between different inflationary models, simply because scientists can’t get the data. Does that mean it isn’t scientific?

    Inflation

    4
    Alan Guth, from 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

    Alan Guth’s notes:
    5

    “A lot of people have personal feelings about inflation and the aesthetics of physical theories,” Prescod-Weinstein says. She’s willing to entertain alternative ideas which have testable consequences, but inflation works well enough for now to keep it around. “It’s also the case that the majority of the cosmology community continues to take inflation seriously as a model, so I have to shrug a little when someone says it’s not science.”

    On that note, Caltech cosmologist Sean M. Carroll argues that many very useful theories have both falsifiable and unfalsifiable predictions. Some aspects may be testable in principle, but not by any experiment or observation we can perform with existing technology. Many particle physics models fall into that category, but that doesn’t stop physicists from finding them useful. SUSY as a concept may not be falsifiable, but many specific models within the broad framework certainly are. All the evidence we have for the existence of dark matter is indirect, which won’t go away even if laboratory experiments never find dark matter particles. Physicists accept the concept of dark matter because it works.

    Slatyer is a practical dark matter hunter. “The questions I’m most interested asking are not even just questions that are in principle falsifiable, but questions that in principle can be tested by data on the timescale of less than my lifetime,” she says. “But it’s not only problems that can be tested by data on a timescale of ‘less than Tracy’s lifetime’ are good scientific questions!”

    Prescod-Weinstein agrees, and argues for keeping an open mind. “There’s a lot we don’t know about the universe, including what’s knowable about it. We are a curious species, and I think we should remain curious.”

    See the full article here .


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  • richardmitnick 10:40 am on February 23, 2019 Permalink | Reply
    Tags: "Supernovas show the universe expands at the same rate in all directions", , , , , Inflation,   

    From Science News: “Supernovas show the universe expands at the same rate in all directions” 

    From Science News

    February 22, 2019
    Emily Conover

    An analysis of exploding stars reveals the universe’s symmetry.

    1
    STRETCH OUT The universe is expanding at the same rate no matter what direction you look, a new study of supernovas finds. Credit: sripfoto/Shutterstock

    Standard Candles to measure age and distance of the universe NASA

    The cosmos doesn’t care whether you’re looking up or down, left or right: In all directions, the universe is expanding at the same clip. When compared across large swaths of the sky, expansion rates agree to better than 1 percent, researchers report in a paper in press in Physical Review Letters.

    Observations of exploding stars, or supernovas, indicate that the universe is not only expanding, but that expansion is accelerating over time (SN Online: 1/16/18).

    2
    In August of 2011, researchers discovered SN 2011fe, a type 1a supernova 21 million light-years away in galaxy M101 (images show the galaxy before and after the supernova, with the supernova circled at right). Studies using type 1a supernovas as “standard candles” to measure how fast the universe expands (the Hubble constant) produce a result in conflict with other data used to infer the cosmic growth rate. Credit: NASA, Swift, Peter Brown, Univ. of Utah

    NASA Neil Gehrels Swift Observatory

    To check if that expansion proceeds apace in different parts of the sky, the scientists studied more than 1,000 exploding stars called type 1a supernovas, which detonate with a known brightness. By measuring how much the supernovas’ light is stretched as space expands, researchers can estimate how fast the universe spreads out, and see if the data differ from what would be expected for a uniform expansion.

    Even in a perfectly evenly expanding universe, there’s bound to be a bit of noise — a sort of random jitter — in the data. Previous analyses have relied on computer simulations to estimate the expected noise. But “it’s a real pain to get simulations to have all the right bells and whistles” necessary for fully reliable results, says theoretical cosmologist Dragan Huterer of the University of Michigan in Ann Arbor. So Huterer and colleagues instead used a scrambled version of the supernova data to gauge how much noise to expect, a technique Huterer says is more reliable.

    The results confirm cosmologists’ understanding of the universe. According to the theory of inflation, the universe expanded extremely rapidly just after the Big Bang (SN: 7/28/12, p.20).

    Inflation

    4
    Alan Guth, from 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

    Alan Guth’s notes:
    5

    That expansion is thought to have proceeded equally in all directions, setting up the universe’s uniform spreading today.

    See the full article here .


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  • richardmitnick 9:44 pm on December 3, 2014 Permalink | Reply
    Tags: , , , , , , , Inflation,   

    From Ethan Siegel: “The Moment of Truth for BICEP2” 

    Starts with a bang
    Starts with a Bang

    Dec 2, 2014
    Ethan Siegel

    “The paradigm of physics — with its interplay of data, theory and prediction — is the most powerful in science.” -Geoffrey West

    Earlier this year, the BICEP2 experiment shook up the world of cosmology, announcing that they had detected gravitational waves originating from before the Big Bang! Not only did they announce this, but they announced that they had done so with a signal in excess of 5σ, which is regarded as the gold standard for a detection in physics.

    BICEP 2
    BICEP 2 interior
    BICEP2 (With South Pole Telescope

    1
    Image credit: BICEP2 Collaboration — P. A. R. Ade et al, 2014 (R).

    But this may all turn out — despite the hoopla — to be absolutely nothing. Or, as it were, nothing more than a phantasm, as the observed signal may have originated from a source as mundane as our own galaxy, and have nothing to do with anything from billions of years ago!

    How did we get into this mess, and how do we get out of it? The answer to both questions is “science,” and it’s a great illustration of how the process and the body of knowledge actually evolves. Put your preconceptions of how it ought to work aside, and let’s dive in!

    2
    Image credit: ESA and the Planck Collaboration.

    This is a snapshot of the cosmic microwave background (CMB), the leftover glow from the Big Bang, as viewed by the Planck satellite. Planck has the best resolution of any all-sky map of the CMB, getting down to resolutions smaller than one tenth of a degree. The temperature fluctuations are minuscule: on the order of just a few tens of microKelvin, less than 0.01% of the actual CMB temperature.

    3
    Image credit: Wikimedia Commons user SuperManu.

    But buried in this signal is another, even more subtle one: the signal of photon polarization.

    4
    Image credit:the BICEP2 collaboration, via http://www.cfa.harvard.edu/news/2014-05.

    Basically, when photons pass through electrically charged particles in certain configurations, their polarizations — or how their electric and magnetic fields are oriented — are affected. If we look at how the two types of polarization, the E-modes and B-modes, are affected on a variety of angular scales, we ought to be able to reconstruct what caused these signals.

    2
    7
    Images credit: Amanda Yoho [Upper]; http://b-pol.org/ [Lower], of an E-mode polarization pattern at left and a B-mode pattern at right.

    A portion of this signal, in addition to charged particles, could also originate from gravitational waves created in the early Universe. There are two main classes of models of inflation that give us a Universe consistent with what we observe in all ways: new inflation, which was actually the second model (and first viable model) ever proposed, and chaotic inflation, which was the third model (and second viable one).

    9
    o
    Images credit: two inflation potentials, with chaotic inflation [Upper] and new inflation [Lower] shown. Chaotic inflation generates very large gravitational waves, while new inflation generates tiny ones. Generated by me, using google graph.

    These two models of inflation make vastly different predictions for gravitational radiation: new inflation predicts gravitational waves (and primordial B-modes) that are extraordinarily tiny, and well beyond the reach of any current or even planned experiment or observatory, while chaotic inflation predicts huge B-modes, some of the largest ones allowable. These signatures have a characteristic frequency spectrum and affect all wavelengths of light identically, so it should be an easy signal to find if our equipment is sensitive to it.

    And that’s where BICEP2 comes in.

    y
    Image credit: Sky and Telescope / Gregg Dinderman, via http://www.skyandtelescope.com/news/First-Direct-Evidence-of-Big-Bang-Inflation-250681381.html.

    Rather than measuring the whole sky, BICEP2 measured just a tiny fraction of the sky — about three fingers held together at arm’s length worth — but were able to tease out both the E-mode and B-mode polarization signals. And based on their analysis of the B-modes, which was very careful and very good, mind you, they claimed the greater-than-5σ detection.

    What this means is that they had enough data so that the odds that what they were seeing was a “fluke” of having observed just a serendipitous patch of sky was tiny, or a one in 1.7 million chance. Flukes happen all the time at the one-in-100 level or the one-in-1,000, but one-in-1.7 million flukes… well, let’s just say you don’t win the lotto jackpot very often.

    But there’s another type of error that they didn’t report. Not a statistical error, which is the kind you can improve on by taking more data, but a systematic error, which could be an effect that causes what you think is your signal, but is actually due to some other source! This type of error normally goes undetected because if you knew about it you’d account for it!

    This is exactly what happened a couple of years ago, if you remember the “faster-than-light-neutrino” business. An experiment at CERN had reported the early arrival by just a few nanoseconds of thousands upon thousands of neutrinos, meaning that they would have exceeded the speed of light by something like 0.003%, a small but meaningful amount. As it turned out, the neutrinos weren’t arriving early; there was a loose cable that accounted for the error!

    f
    Image credit: ESA / Planck Collaboration, via http://www.mpa-garching.mpg.de/mpa/institute/news_archives/news1101_planck/news1101_planck-en-print.html.

    Well, one of the things the BICEP2 team didn’t measure was the galactic foreground emission. Polarized light — including light that contains these B-modes — gets emitted by the Milky Way galaxy, and that can contaminate your signal. The BICEP2 team used a very clever trick to try and eliminate this, by interpolating unreleased Planck data about galactic foregrounds, but when the Planck team actually released their data, the foregrounds were significantly different from what BICEP2 had anticipated. And with the new Planck data, the announcement of a “discovery” needed to be walked back; the evidence was now something like a one-in-200 chance of being a fluke.

    l
    Image credit: John Kovac, viahttp://cosmo2014.uchicago.edu/depot/invited-talk-kovac-john.pdf.

    In other words, although gravitational waves could have caused this signal, so could other, far more mundane sources, including just our boring old galaxy!

    Sometime later this month, the Planck team will release their all-sky polarization results, and either at that moment or shortly thereafter, we’ll find out whether there really are gravitational waves from inflation that can be detected with our current generation of telescopes, satellites and observatories. We’ll find out whether chaotic inflation is right, or whether we need to keep searching for the gravitational wave signal from before the Big Bang. We already have the density fluctuation signal, so we can be confident that inflation happened. It’s just a question of which type.

    n
    Image credit: Bock et al. (2006, astro-ph/0604101); modifications by me.

    Stay curious, stay hungry for more knowledge, but always demand that your scientific claims be independently verified, that your possible systematic errors be checked, and that you have overwhelming evidence before believing the extraordinary claims. It’s easy to make a bold statement; it’s hard to start a bona fide scientific revolution!

    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.

     
  • richardmitnick 2:00 pm on November 20, 2014 Permalink | Reply
    Tags: , Inflation, , ,   

    From phys.org: “Gravity may have saved the universe after the Big Bang, say researchers” 

    physdotorg
    phys.org

    Nov 18, 2014
    No Writer Credit

    New research by a team of European physicists could explain why the universe did not collapse immediately after the Big Bang.

    Studies of the Higgs particle – discovered at CERN in 2012 and responsible for giving mass to all particles – have suggested that the production of Higgs particles during the accelerating expansion of the very early universe (inflation) should have led to instability and collapse.

    in
    Time Line of the Universe. Credit: NASA/WMAP Science Team

    Scientists have been trying to find out why this didn’t happen, leading to theories that there must be some new physics that will help explain the origins of the universe that has not yet been discovered. Physicists from Imperial College London, and the Universities of Copenhagen and Helsinki, however, believe there is a simpler explanation.

    In a new study in Physical Review Letters, the team describe how the spacetime curvature – in effect, gravity – provided the stability needed for the universe to survive expansion in that early period. The team investigated the interaction between the Higgs particles and gravity, taking into account how it would vary with energy.

    They show that even a small interaction would have been enough to stabilise the universe against decay.

    “The Standard Model of particle physics, which scientists use to explain elementary particles and their interactions, has so far not provided an answer to why the universe did not collapse following the Big Bang,” explains Professor Arttu Rajantie, from the Department of Physics at Imperial College London.

    sm
    The Standard Model of elementary particles, with the three generations of matter, gauge bosons in the fourth column, and the Higgs boson in the fifth.

    “Our research investigates the last unknown parameter in the Standard Model – the interaction between the Higgs particle and gravity. This parameter cannot be measured in particle accelerator experiments, but it has a big effect on the Higgs instability during inflation. Even a relatively small value is enough to explain the survival of the universe without any new physics!”

    The team plan to continue their research using cosmological observations to look at this interaction in more detail and explain what effect it would have had on the development of the early universe. In particular, they will use data from current and future European Space Agency missions measuring cosmic microwave background radiation and gravitational waves.

    “Our aim is to measure the interaction between gravity and the Higgs field using cosmological data,” says Professor Rajantie. “If we are able to do that, we will have supplied the last unknown number in the Standard Model of particle physics and be closer to answering fundamental questions about how we are all here.”

    See the full article here.

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    Phys.org™ (formerly Physorg.com) is a leading web-based science, research and technology news service which covers a full range of topics. These include physics, earth science, medicine, nanotechnology, electronics, space, biology, chemistry, computer sciences, engineering, mathematics and other sciences and technologies. Launched in 2004, Phys.org’s readership has grown steadily to include 1.75 million scientists, researchers, and engineers every month. Phys.org publishes approximately 100 quality articles every day, offering some of the most comprehensive coverage of sci-tech developments world-wide. Quancast 2009 includes Phys.org in its list of the Global Top 2,000 Websites. Phys.org community members enjoy access to many personalized features such as social networking, a personal home page set-up, RSS/XML feeds, article comments and ranking, the ability to save favorite articles, a daily newsletter, and other options.

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  • richardmitnick 3:44 pm on November 10, 2014 Permalink | Reply
    Tags: , , Inflation, , ,   

    From Quanta: “Multiverse Collisions May Dot the Sky” 

    Quanta Magazine
    Quanta Magazine

    November 10, 2014
    Jennifer Ouellette

    Like many of her colleagues, Hiranya Peiris, a cosmologist at University College London, once largely dismissed the notion that our universe might be only one of many in a vast multiverse. It was scientifically intriguing, she thought, but also fundamentally untestable. She preferred to focus her research on more concrete questions, like how galaxies evolve.

    Then one summer at the Aspen Center for Physics, Peiris found herself chatting with the Perimeter Institute’s Matt Johnson, who mentioned his interest in developing tools to study the idea. He suggested that they collaborate.

    At first, Peiris was skeptical. “I think as an observer that any theory, however interesting and elegant, is seriously lacking if it doesn’t have testable consequences,” she said. But Johnson convinced her that there might be a way to test the concept. If the universe that we inhabit had long ago collided with another universe, the crash would have left an imprint on the cosmic microwave background (CMB), the faint afterglow from the Big Bang. And if physicists could detect such a signature, it would provide a window into the multiverse.

    Cosmic Background Radiation Planck
    Cosmic Microwave Background per ESA/Planck

    Erick Weinberg, a physicist at Columbia University, explains this multiverse by comparing it to a boiling cauldron, with the bubbles representing individual universes — isolated pockets of space-time. As the pot boils, the bubbles expand and sometimes collide. A similar process may have occurred in the first moments of the cosmos.

    In the years since their initial meeting, Peiris and Johnson have studied how a collision with another universe in the earliest moments of time would have sent something similar to a shock wave across our universe. They think they may be able to find evidence of such a collision in data from the Planck space telescope, which maps the CMB.

    The project might not work, Peiris concedes. It requires not only that we live in a multiverse but also that our universe collided with another in our primal cosmic history. But if physicists succeed, they will have the first improbable evidence of a cosmos beyond our own.

    When Bubbles Collide

    Multiverse theories were once relegated to science fiction or crackpot territory. “It sounds like you’ve gone to crazy land,” said Johnson, who holds joint appointments at the Perimeter Institute of Theoretical Physics and York University. But scientists have come up with many versions of what a multiverse might be, some less crazy than others.

    The multiverse that Peiris and her colleagues are interested in is not the controversial “many worlds” hypothesis that was first proposed in the 1950s and holds that every quantum event spawns a separate universe. Nor is this concept of a multiverse related to the popular science-fiction trope of parallel worlds, new universes that pinch off from our space-time and become separate realms. Rather, this version arises as a consequence of inflation, a widely accepted theory of the universe’s first moments.

    Inflation holds that our universe experienced a sudden burst of rapid expansion an instant after the Big Bang, blowing up from a infinitesimally small speck to one spanning a quarter of a billion light-years in mere fractions of a second.

    Yet inflation, once started, tends to never completely stop. According to the theory, once the universe starts expanding, it will end in some places, creating regions like the universe we see all around us today. But elsewhere inflation will simply keep on going eternally into the future.

    This feature has led cosmologists to contemplate a scenario called eternal inflation. In this picture, individual regions of space stop inflating and become “bubble universes” like the one in which we live. But on larger scales, exponential expansion continues forever, and new bubble universes are continually being created. Each bubble is deemed a universe in its own right, despite being part of the same space-time, because an observer could not travel from one bubble to the next without moving faster than the speed of light. And each bubble may have its own distinct laws of physics. “If you buy eternal inflation, it predicts a multiverse,” Peiris said.

    In 2012, Peiris and Johnson teamed up with Anthony Aguirre and Max Wainwright — both physicists at the University of California, Santa Cruz — to build a simulated multiverse with only two bubbles. They studied what happened after the bubbles collided to determine what an observer would see. The team concluded that a collision of two bubble universes would appear to us as a disk on the CMB with a distinctive temperature profile.

    bubble
    Olena Shmahalo/Quanta Magazine; source: S. M. Freeney et. al., Physical Review Letters

    An ancient collision with a bubble universe would have altered the temperature of the cosmic microwave background (left), creating a faint disk in the sky (right) that could potentially be observed.

    To guard against human error — we tend to see the patterns we want to see — they devised a set of algorithms to automatically search for these disks in data from the Wilkinson Microwave Anisotropy Probe (WMAP), a space-based observatory. The program identified four potential regions with temperature fluctuations consistent with what could be a signature of a bubble collision. When data from the Planck satellite becomes available later this year, researchers should be able to improve on that earlier analysis.

    WMAP
    WMAP

    ESA Planck
    ESA/Planck

    Yet detecting convincing signatures of the multiverse is tricky. Simply knowing what an encounter might look like requires a thorough understanding of the dynamics of bubble collisions — something quite difficult to model on a computer, given the complexity of such interactions.

    When tackling a new problem, physicists typically find a good model that they already understand and adapt it by making minor tweaks they call “perturbations.” For instance, to model the trajectory of a satellite in space, a physicist might use the classical laws of motion outlined by Isaac Newton in the 17th century and then make small refinements by calculating the effects of other factors that might influence its motion, such as pressure from the solar wind. For simple systems, there should be only small discrepancies from the unperturbed model. Try to calculate the airflow patterns of a complex system like a tornado, however, and those approximations break down. Perturbations introduce sudden, very large changes to the original system instead of smaller, predictable refinements.

    Modeling bubble collisions during the inflationary period of the early universe is akin to modeling a tornado. By its very nature, inflation stretches out space-time at an exponential rate — precisely the kind of large jumps in values that make calculating the dynamics so challenging.

    “Imagine you start with a grid, but within an instant, the grid has expanded to a massive size,” Peiris said. With her collaborators, she has used techniques like adaptive mesh refinement — an iterative process of winnowing out the most relevant details in such a grid at increasingly finer scales — in her simulations of inflation to deal with the complexity. Eugene Lim, a physicist at King’s College London, has found that an unusual type of traveling wave might help simplify matters even further.

    Waves of Translation

    In August 1834, a Scottish engineer named John Scott Russell was conducting experiments along Union Canal with an eye toward improving the efficiency of the canal boats. One boat being drawn by a team of horses stopped suddenly, and Russell noted a solitary wave in the water that kept rolling forward at a constant speed without losing its shape. The behavior was unlike typical waves, which tend to flatten out or rise to a peak and topple quickly. Intrigued, Russell tracked the wave on horseback for a couple of miles before it finally dissipated in the channel waters. This was the first recorded observation of a soliton.

    Russell was so intrigued by the indomitable wave that he built a 30-foot wave tank in his garden to further study the phenomenon, noting key characteristics of what he called “the wave of translation.” Such a wave could maintain size, shape and speed over longer distances than usual. The speed depended on the wave’s size, and the width depended on the depth of the water. And if a large solitary wave overtook a smaller one, the larger, faster wave would just pass right through.

    Russell’s observations were largely dismissed by his peers because his findings seemed to contradict what was known about water wave physics at the time. It wasn’t until the mid-1960s that such waves were dubbed solitons and physicists realized their usefulness in modeling problems in diverse areas such as fiber optics, biological proteins and DNA. Solitons also turn up in certain configurations of quantum field theory. Poke a quantum field and you will create an oscillation that usually dissipates outward, but configure things in just the right way and that oscillation will maintain its shape — just like Russell’s wave of translation.

    Because solitons are so stable, Lim believes they could work as a simplified toy model for the dynamics of bubble collisions in the multiverse, providing physicists with better predictions of what kinds of signatures might show up in the CMB. If his hunch is right, the expanding walls of our bubble universe are much like solitons.

    However, while it is a relatively straightforward matter to model a solitary standing wave, the dynamics become vastly more complicated and difficult to calculate when solitons collide and interact, forcing physicists to rely on computer simulations instead. In the past, researchers have used a particular class of soliton with an exact mathematical solution and tweaked that model to suit their purposes. But this approach only works if the target system under study is already quite similar to the toy model; otherwise the changes are too large to calculate.

    To get around that hurdle, Lim devised a neat trick based on a quirky feature of soliton collisions. When imagining two objects colliding, we naturally assume that the faster they are moving, the greater the impact and the more complicated the dynamics. Two cars ramming each other at high speeds, for instance, will produce scattered debris, heat, noise and other effects. The same is true for colliding solitons — at least initially. Collide two solitons very slowly, and there will be very little interaction, according to Lim. As the speed increases, the solitons interact more strongly.

    But Lim found that as the speed continues to increase, the pattern eventually reverses: The soliton interaction begins to decrease. By the time they are traveling at the speed of light, there is no interaction at all. “They just fly right past each other,” Lim said. “The faster you collide two solitons, the simpler they become.” The lack of interactions makes it easier to model the dynamics of colliding solitons, as well as colliding bubble universes with solitons as their “edges,” since the systems are roughly similar.

    According to Johnson, Lim has uncovered a very simple rule that can be applied broadly: Multiverse interactions are weak during high-speed collisions, making it easier to simulate the dynamics of those encounters. One can simply create a new model of the multiverse, use solitons as a tool to map the new model’s expected signatures onto cosmic microwave data, and rule out any theories that don’t match what researchers see. This process would help physicists identify the most viable models for the multiverse, which — while still speculative — would be consistent both with the latest observational data and with inflationary theory.

    The Multiverse’s Case for String Theory

    One reason that more physicists are taking the idea of the multiverse seriously is that certain such models could help resolve a significant challenge in string theory. One of the goals of string theory has been to unify quantum mechanics and general relativity, two separate “rule books” in physics that govern very different size scales, into a single, simple solution.

    But around 10 years ago, “the dream of string theory kind of exploded,” Johnson said — and not in a good way. Researchers began to realize that string theory doesn’t provide a unique solution. Instead, it “gives you the theory of a vast number of worlds,” Weinberg said. A common estimate — one that Weinberg thinks is conservative — is 10500 possibilities. This panoply of worlds implies that string theory can predict every possible outcome.

    The multiverse would provide a possible means of incorporating all the different worlds predicted by string theory. Each version could be realized in its own bubble universe. “Everything depends on which part of the universe you live in,” Lim said.

    Peiris acknowledges that this argument has its critics. “It can predict anything, and therefore it’s not valid,” Peiris said of the reasoning typically used to dismiss the notion of a multiverse as a tautology, rather than a true scientific theory. “But I think that’s the wrong way to think about it.” The theory of evolution, Peiris argues, also resembles a tautology in certain respects — “an organism exists because it survived” — yet it holds tremendous explanatory power. It is a simple model that requires little initial input to produce the vast diversity of species we see today.

    A multiverse model tied to eternal inflation could have the same kind of explanatory power. In this case, the bubble universes function much like speciation. Those universes that happen to have the right laws of physics will eventually “succeed” — that is, they will become home to conscious observers like ourselves. If our universe is one of many in a much larger multiverse, our existence seems less unlikely.

    Uncertain Signals

    Ultimately, however, Peiris’ initial objection still stands: Without some means of gathering experimental evidence, the multiverse hypothesis will be untestable by definition. As such, it will lurk on the fringes of respectable physics — hence the strong interest in detecting bubble collision signatures in the CMB.

    Of course, “just because these bubble collisions can leave a signature doesn’t mean they do leave a signature,” Peiris emphasized. “We need nature to be kind to us.” An observable signal could be a rare find, given how quickly space expanded during inflation. The collisions may not have been rare, but subsequent inflation “tends to dilute away the effects of the collision just like it dilutes away all other prior ‘structure’ in the early universe, leaving you with a small chance of seeing a signal in the CMB sky,” Peiris said.

    “My own feeling is you need to adjust the numbers rather finely to get it to work,” Weinberg said. The rate of formation of the bubble universes is key. If they had formed slowly, collisions would not have been possible because space would have expanded and driven the bubbles apart long before any collision could take place. Alternatively, if the bubbles had formed too quickly, they would have merged before space could expand sufficiently to form disconnected pockets. Somewhere in between is the Goldilocks rate, the “just right” rate at which the bubbles would have had to form for a collision to be possible.

    Researchers also worry about finding a false positive. Even if such a collision did happen and evidence was imprinted on the CMB, spotting the telltale pattern would not necessarily constitute evidence of a multiverse. “You can get an effect and say it will be consistent with the calculated predictions for these [bubble] collisions,” Weinberg said. “But it might well be consistent with lots of other things.” For instance, a distorted CMB might be evidence of theoretical entities called cosmic strings. These are like the cracks that form in the ice when a lake freezes over, except here the ice is the fabric of space-time. Magnetic monopoles are another hypothetical defect that could affect the CMB, as could knots or twists in space-time called textures.

    Weinberg isn’t sure it would even be possible to tell the difference between these different possibilities, especially because many models of eternal inflation exist. Without knowing the precise details of the theory, trying to make a positive identification of the multiverse would be like trying to distinguish between the composition of two meteorites that hit the roof of a house solely by the sound of the impacts, without knowing how the house is constructed and with what materials.

    Should a signature for a bubble collision be confirmed, Peiris doesn’t see a way to study another bubble universe any further because by now it would be entirely out of causal contact with ours. But it would be a stunning validation that the notion of a multiverse deserves a seat at the testable physics table.

    And should that signal turn out to be evidence for cosmic strings or magnetic monopoles instead, it would still constitute exciting new physics at the frontier of cosmology. In that respect, “the cosmic microwave background radiation is the underpinning of modern cosmology,” Peiris said. “It’s the gift that keeps on giving.”

    See the full article, with video, here.

    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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  • richardmitnick 3:32 pm on April 6, 2014 Permalink | Reply
    Tags: , , , , , Inflation,   

    From Symmetry: “Inflation” 

    January 01, 2005
    Logbook

    In 1978 Alan Guth heard about the “flatness problem” of the universe while attending a talk on cosmology—a field he was only marginally curious about. A year later, Guth found a solution.

    Alan Guth

    At the beginning of the big bang, for an incredibly small fraction of a second, the universe could have expanded exponentially fast, rapidly transforming curved space into flat one. Quickly running out of energy, the expansion would slow down, eventually reaching today’s sluggish pace. Such an initial explosive rush, which Guth later called inflation, could solve a number of cosmic paradoxes (see Growth of Inflation).

    Although scientists still debate the driving force behind inflation—Guth soon realized his original idea of “supercooling” wouldn’t work—the concept of inflation has become the leading theme and the crux of modern cosmology.

    inflation
    Courtesy of Alan Guth and the Adler Planetarium and Astronomy Museum in Chicago

    Guth’s notebook is now part of a permanent exhibit at the Adler Planetarium and Astronomy Museum in Chicago.

    See the full article here.

    Symmetry is a joint Fermilab/SLAC publication.



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  • richardmitnick 9:24 am on March 20, 2014 Permalink | Reply
    Tags: , , , , , , Inflation   

    From M.I.T.: “3 Questions: Alan Guth on new insights into the ‘Big Bang’” 

    March 19, 2014
    Steve Bradt, MIT News Office

    Earlier this week, scientists announced that a telescope observing faint echoes of the so-called “Big Bang” had found evidence of the universe’s nearly instantaneous expansion from a mere dot into a dense ball containing more than 1090 particles. This discovery, using the BICEP2 telescope at the South Pole, provides the first strong evidence of “cosmic inflation” at the birth of our universe, when it expanded billions of times over.

    BICEP Telescope
    BICEP2 Telescope at South Pole

    The theory of cosmic inflation was first proposed in 1980 by Alan Guth, now the Victor F. Weisskopf Professor of Physics at MIT. Inflation has become a cornerstone of Big Bang cosmology, but until now it had remained a theory without experimental support. Guth discussed the significance of the new BICEP2 results with MIT News.

    ag
    Dr. Alan Guth

    Q: Can you explain the theory of cosmic inflation that you first put forth in 1980?

    A: I usually describe inflation as a theory of the “bang” of the Big Bang: It describes the propulsion mechanism that drove the universe into the period of tremendous expansion that we call the Big Bang. In its original form, the Big Bang theory never was a theory of the bang. It said nothing about what banged, why it banged, or what happened before it banged.

    The original Big Bang theory was really a theory of the aftermath of the bang. The universe was already hot and dense, and already expanding at a fantastic rate. The theory described how the universe was cooled by the expansion, and how the expansion was slowed by the attractive force of gravity.

    Inflation proposes that the expansion of the universe was driven by a repulsive form of gravity. According to [Isaac] Newton, gravity is a purely attractive force, but this changed with [Albert] Einstein and the discovery of general relativity. General relativity describes gravity as a distortion of spacetime, and allows for the possibility of repulsive gravity.

    Modern particle theories strongly suggest that at very high energies, there should exist forms of matter that create repulsive gravity. Inflation, in turn, proposes that at least a very small patch of the early universe was filled with this repulsive-gravity material. The initial patch could have been incredibly small, perhaps as small as 10-24 centimeter, about 100 billion times smaller than a single proton. The small patch would then start to exponentially expand under the influence of the repulsive gravity, doubling in size approximately every 10-37 second. To successfully describe our visible universe, the region would need to undergo at least 80 doublings, increasing its size to about 1 centimeter. It could have undergone significantly more doublings, but at least this number is needed.

    During the period of exponential expansion, any ordinary material would thin out, with the density diminishing to almost nothing. The behavior in this case, however, is very different: The repulsive-gravity material actually maintains a constant density as it expands, no matter how much it expands! While this appears to be a blatant violation of the principle of the conservation of energy, it is actually perfectly consistent.

    This loophole hinges on a peculiar feature of gravity: The energy of a gravitational field is negative. As the patch expands at constant density, more and more energy, in the form of matter, is created. But at the same time, more and more negative energy appears in the form of the gravitational field that is filling the region. The total energy remains constant, as it must, and therefore remains very small.

    It is possible that the total energy of the entire universe is exactly zero, with the positive energy of matter completely canceled by the negative energy of gravity. I often say that the universe is the ultimate free lunch, since it actually requires no energy to produce a universe.

    At some point the inflation ends because the repulsive-gravity material becomes metastable. The repulsive-gravity material decays into ordinary particles, producing a very hot soup of particles that form the starting point of the conventional Big Bang. At this point the repulsive gravity turns off, but the region continues to expand in a coasting pattern for billions of years to come. Thus, inflation is a prequel to the era that cosmologists call the Big Bang, although it of course occurred after the origin of the universe, which is often also called the Big Bang.

    Q: What is the new result announced this week, and how does it provide critical support for your theory?

    A: The stretching effect caused by the fantastic expansion of inflation tends to smooth things out — which is great for cosmology, because an ordinary explosion would presumably have left the universe very splotchy and irregular. The early universe, as we can see from the afterglow of the cosmic microwave background (CMB) radiation, was incredibly uniform, with a mass density that was constant to about one part in 100,000.

    CMB Planck ESA
    Cosmic Microwave Background

    ESA Planck
    ESA/Planck

    The tiny nonuniformities that did exist were then amplified by gravity: In places where the mass density was slightly higher than average, a stronger-than-average gravitational field was created, which pulled in still more matter, creating a yet stronger gravitational field. But to have structure form at all, there needed to be small nonuniformities at the end of inflation.

    In inflationary models, these nonuniformities — which later produce stars, galaxies, and all the structure of the universe — are attributed to quantum theory. Quantum field theory implies that, on very short distance scales, everything is in a state of constant agitation. If we observed empty space with a hypothetical, and powerful, magnifying glass, we would see the electric and magnetic fields undergoing wild oscillations, with even electrons and positrons popping out of the vacuum and then rapidly disappearing. The effect of inflation, with its fantastic expansion, is to stretch these quantum fluctuations to macroscopic proportions.

    The temperature nonuniformities in the cosmic microwave background were first measured in 1992 by the COBE satellite, and have since been measured with greater and greater precision by a long and spectacular series of ground-based, balloon-based, and satellite experiments. They have agreed very well with the predictions of inflation. These results, however, have not generally been seen as proof of inflation, in part because it is not clear that inflation is the only possible way that these fluctuations could have been produced.

    NASA COBE satellite
    NASA/COBE

    The stretching effect of inflation, however, also acts on the geometry of space itself, which according to general relativity is flexible. Space can be compressed, stretched, or even twisted. The geometry of space also fluctuates on small scales, due to the physics of quantum theory, and inflation also stretches these fluctuations, producing gravity waves in the early universe.

    The new result, by John Kovac and the BICEP2 collaboration, is a measurement of these gravity waves, at a very high level of confidence. They do not see the gravity waves directly, but instead they have constructed a very detailed map of the polarization of the CMB in a patch of the sky. They have observed a swirling pattern in the polarization (called “B modes”) that can be created only by gravity waves in the early universe, or by the gravitational lensing effect of matter in the late universe.

    But the primordial gravity waves can be separated, because they tend to be on larger angular scales, so the BICEP2 team has decisively isolated their contribution. This is the first time that even a hint of these primordial gravity waves has been detected, and it is also the first time that any quantum properties of gravity have been directly observed.

    Q: How would you describe the significance of these new findings, and your reaction to them?

    A: The significance of these new findings is enormous. First of all, they help tremendously in confirming the picture of inflation. As far as we know, there is nothing other than inflation that can produce these gravity waves. Second, it tells us a lot about the details of inflation that we did not already know. In particular, it determines the energy density of the universe at the time of inflation, which is something that previously had a wide range of possibilities.

    By determining the energy density of the universe at the time of inflation, the new result also tells us a lot about which detailed versions of inflation are still viable, and which are no longer viable. The current result is not by itself conclusive, but it points in the direction of the very simplest inflationary models that can be constructed.

    Finally, and perhaps most importantly, the new result is not the final story, but is more like the opening of a new window. Now that these B modes have been found, the BICEP2 collaboration and many other groups will continue to study them. They provide a new tool to study the behavior of the early universe, including the process of inflation.

    When I (and others) started working on the effect of quantum fluctuations in the early 1980s, I never thought that anybody would ever be able to measure these effects. To me it was really just a game, to see if my colleagues and I could agree on what the fluctuations would theoretically look like. So I am just astounded by the progress that astronomers have made in measuring these minute effects, and particularly by the new result of the BICEP2 team. Like all experimental results, we should wait for it to be confirmed by other groups before taking it as truth, but the group seems to have been very careful, and the result is very clean, so I think it is very likely that it will hold up.

    See the full article here.


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  • richardmitnick 12:17 pm on March 19, 2014 Permalink | Reply
    Tags: , , , , , , Inflation   

    From Fermilab: “From quantum to cosmos” 


    Fermilab is an enduring source of strength for the US contribution to scientific research world wide.

    Wednesday, March 19, 2014

    ch
    Craig Hogan, head of the Center for Particle Astrophysics, wrote this column.

    On Monday morning, cosmologists around the world felt a wave of ecstasy as they learned of a breathtaking discovery: a particular pattern of light coming from the early universe, imprinted on the cosmic expansion during its first moments. It feels like a love letter from Mother Nature has invited us to share her deepest secrets.

    CMB Planck ESA
    Cosmic Background from ESA/Planck

    All forms of matter and energy come in quanta — the “particles” of particle physics. For the first time, we have now detected a quantum behavior of space and time. The new result invokes an interplay among all the scales of physical universe, from the smallest to largest, from the beginning to the present day. It spectacularly confirms many of the “inner space/outer space” connections pioneered over several decades by Fermilab’s astrophysics theory group. This includes the amazing idea that quantum fluctuations can be amplified to enormous size by cosmic expansion and lead not only to gravitational waves, but ultimately to the formation of all cosmic structures, including galaxies, stars, planets and life.

    The now discovered polarization of cosmic background light displays a faint but distinctive pattern of swirls that can be created only by an extraordinarily exotic process known as inflation, a stretching of space-time (gravitational waves), caused by its own subatomic, quantum fluctuations. This unique signature reaches us intact across all the vast stretches of space since the beginning of time and can now be studied in precise detail.

    The discovery, published in this paper, came sooner than anyone expected. Theorists, including Fermilab’s Albert Stebbins, proposed long ago the possibility of isolating the distinctive swirling signature used to make the discovery, but everyone was surprised this week that the signal in the real universe is so strong. The implications for cosmology are immediate and profound. We now know far more reliably what conditions were during the cosmic inflation that created our expansion; for example, the new data directly measures how fast things were expanding back then. We can now delve much more concretely into the new physics that governs cosmic origins and how it connects to the unification of the Standard Model particles and forces studied at the Tevatron and the LHC. Cosmic polarization experiments may even provide real data addressing the quantum system underlying unification of the Standard Model with gravity — the “theory of everything.”

    sm
    The Standard Model of elementary particles, with the three generations of matter, gauge bosons in the fourth column, and the Higgs boson in the fifth.

    Fermilab Tevatron
    Tevatron

    CERN LHC
    Inside the LHC

    The discovery was inspired by theory but propelled in recent years by new transformational technology, in particular, a new generation of sensors being developed at Argonne, Berkeley, Jet Propulsion Laboratory and NIST. Large focal plane arrays of antennas are fabricated on silicon wafers, together with superconducting detectors that achieve quantum-noise-limited performance. In experiments, they are deployed in advanced telescopes at the world’s best site for peering deep into space, the South Pole.

    The newly discovered effect is strong enough to confirm soon with other experiments, perhaps even using data already obtained. The next step will be to improve the quality of the measurements with a larger area of sky, more frequency bands and higher angular resolution. That will require larger focal planes with more detection elements and a larger telescope.

    We are already developing this next-generation experiment. It will use the world’s leading facility for cosmic background studies, the South Pole Telescope (SPT). As part of a new joint effort with Argonne, the University of Chicago and other partners, Fermilab is playing a central role in developing and building the new SPT-3G cryogenic camera system, an order of magnitude more capable than that currently deployed. Over the next two years, the system will be assembled, integrated and tested at Fermilab by a team led by Brad Benson, using many of the facilities previously developed for the Dark Energy Camera and the QUIET polarization experiment, before being shipped to the South Pole.

    South Pole Telescope
    South Pole Telescope

    Fermilab DECam
    DECam

    Plans are also under way for an even more ambitious fourth-generation cosmic microwave background polarization experiment, by a larger consortium of national labs and universities. A recent APS Community Summer Study (“Snowmass“) report, co-led by Fermilab’s Scott Dodelson, identified such an experiment, in synergy with other surveys, as a unique opportunity to study many aspects of new physics, including neutrino masses, new relativistic species (so-called dark radiation) and dark energy. A study group proposes to expand CMB polarization capabilities by another order of magnitude beyond SPT-3G, including the addition of more telescopes to access more of the sky not visible from the South Pole. The new discovery extends and enriches the science reach of this enterprise to a new and deeper level — one we had hardly dared to dream about until this week.

    See the full article here.

    Fermilab Campus

    Fermi National Accelerator Laboratory (Fermilab), located just outside Batavia, Illinois, near Chicago, is a US Department of Energy national laboratory specializing in high-energy particle physics.


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