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  • richardmitnick 1:50 pm on October 18, 2018 Permalink | Reply
    Tags: , , , , , Gravity, , ,   

    From Symmetry: “Five mysteries the Standard Model can’t explain” 

    Symmetry Mag
    From Symmetry

    10/18/18
    Oscar Miyamoto Gomez

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


    Standard Model of Particle Physics from Symmetry Magazine

    Our best model of particle physics explains only about 5 percent of the universe.

    The Standard Model is a thing of beauty. It is the most rigorous theory of particle physics, incredibly precise and accurate in its predictions. It mathematically lays out the 17 building blocks of nature: six quarks, six leptons, four force-carrier particles, and the Higgs boson. These are ruled by the electromagnetic, weak and strong forces.

    “As for the question ‘What are we?’ the Standard Model has the answer,” says Saúl Ramos, a researcher at the National Autonomous University of Mexico (UNAM). “It tells us that every object in the universe is not independent, and that every particle is there for a reason.”

    For the past 50 years such a system has allowed scientists to incorporate particle physics into a single equation that explains most of what we can see in the world around us.

    Despite its great predictive power, however, the Standard Model fails to answer five crucial questions, which is why particle physicists know their work is far from done.

    1
    Illustration by Sandbox Studio, Chicago with Ana Kova

    1. Why do neutrinos have mass?

    Three of the Standard Model’s particles are different types of neutrinos. The Standard Model predicts that, like photons, neutrinos should have no mass.

    However, scientists have found that the three neutrinos oscillate, or transform into one another, as they move. This feat is only possible because neutrinos are not massless after all.

    “If we use the theories that we have today, we get the wrong answer,” says André de Gouvêa, a professor at Northwestern University.

    The Standard Model got neutrinos wrong, but it remains to be seen just how wrong. After all, the masses neutrinos have are quite small.

    Is that all the Standard Model missed, or is there more that we don’t know about neutrinos? Some experimental results have suggested, for example, that there might be a fourth type of neutrino called a sterile neutrino that we have yet to discover.

    2
    Illustration by Sandbox Studio, Chicago with Ana Kova

    2. What is dark matter?

    Scientists realized they were missing something when they noticed that galaxies were spinning much faster than they should be, based on the gravitational pull of their visible matter. They were spinning so fast that they should have torn themselves apart. Something we can’t see, which scientists have dubbed “dark matter,” must be giving additional mass—and hence gravitional pull—to these galaxies.

    Dark matter is thought to make up 27 percent of the contents of the universe. But it is not included in the Standard Model.

    Scientists are looking for ways to study this mysterious matter and identify its building blocks. If scientists could show that dark matter interacts in some way with normal matter, “we still would need a new model, but it would mean that new model and the Standard Model are connected,” says Andrea Albert, a researcher at the US Department of Energy’s SLAC National Laboratory who studies dark matter, among other things, at the High-Altitude Water Cherenkov Observatory in Mexico. “That would be a huge game changer.”

    HAWC High Altitude Cherenkov Experiment, located on the flanks of the Sierra Negra volcano in the Mexican state of Puebla at an altitude of 4100 meters(13,500ft), at WikiMiniAtlas 18°59′41″N 97°18′30.6″W. searches for cosmic rays

    3
    Illustration by Sandbox Studio, Chicago with Ana Kova

    3. Why is there so much matter in the universe?

    Whenever a particle of matter comes into being—for example, in a particle collision in the Large Hadron Collider or in the decay of another particle—normally its antimatter counterpart comes along for the ride. When equal matter and antimatter particles meet, they annihilate one another.

    Scientists suppose that when the universe was formed in the Big Bang, matter and antimatter should have been produced in equal parts. However, some mechanism kept the matter and antimatter from their usual pattern of total destruction, and the universe around us is dominated by matter.

    The Standard Model cannot explain the imbalance. Many different experiments are studying matter and antimatter in search of clues as to what tipped the scales.

    4
    Illustration by Sandbox Studio, Chicago with Ana Kova

    4. Why is the expansion of the universe accelerating?

    Before scientists were able to measure the expansion of our universe, they guessed that it had started out quickly after the Big Bang and then, over time, had begun to slow. So it came as a shock that, not only was the universe’s expansion not slowing down—it was actually speeding up.

    The latest measurements by the Hubble Space Telescope and the European Space Agency observatory Gaia indicate that galaxies are moving away from us at 45 miles per second. That speed multiplies for each additional megaparsec, a distance of 3.2 million light years, relative to our position.

    This rate is believed to come from an unexplained property of space-time called dark energy, which is pushing the universe apart. It is thought to make up around 68 percent of the energy in the universe. “That is something very fundamental that nobody could have anticipated just by looking at the Standard Model,” de Gouvêa says.

    5
    Illustration by Sandbox Studio, Chicago with Ana Kova

    5. Is there a particle associated with the force of gravity?

    The Standard Model was not designed to explain gravity. This fourth and weakest force of nature does not seem to have any impact on the subatomic interactions the Standard Model explains.

    But theoretical physicists think a subatomic particle called a graviton might transmit gravity the same way particles called photons carry the electromagnetic force.

    “After the existence of gravitational waves was confirmed by LIGO, we now ask: What is the smallest gravitational wave possible? This is pretty much like asking what a graviton is,” says Alberto Güijosa, a professor at the Institute of Nuclear Sciences at UNAM.

    More to explore

    These five mysteries are the big questions of physics in the 21st century, Ramos says. Yet, there are even more fundamental enigmas, he says: What is the source of space-time geometry? Where do particles get their spin? Why is the strong force so strong while the weak force is so weak?

    There’s much left to explore, Güijosa says. “Even if we end up with a final and perfect theory of everything in our hands, we would still perform experiments in different situations in order to push its limits.”

    “It is a very classic example of the scientific method in action,” Albert says. “With each answer come more questions; nothing is ever done.”

    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.


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  • richardmitnick 11:08 am on September 8, 2018 Permalink | Reply
    Tags: , , , , Dark matter is our leading theory for a reason, , Gravity, Modified Gravity, Modified Gravity Could Soon Be Ruled Out Says New Research On Dwarf Galaxies, New detailed studies of the smallest galaxies could kill off the most studied alternative, Newton’s law of gravity   

    From Ethan Siegel: “Modified Gravity Could Soon Be Ruled Out, Says New Research On Dwarf Galaxies” 

    From Ethan Siegel
    Sep 7, 2018

    Dark matter is our leading theory for a reason. New, detailed studies of the smallest galaxies could kill off the most studied alternative.

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    Only approximately 1000 stars are present in the entirety of dwarf galaxies Segue 1 and Segue 3, which has a gravitational mass of 600,000 Suns. The stars making up the dwarf satellite Segue 1 are circled here. If new research is correct, then dark matter will obey a different distribution depending on how star formation, over the galaxy’s history, has heated it. (MARLA GEHA AND KECK OBSERVATORIES)


    Keck Observatory, Maunakea, Hawaii, USA.4,207 m (13,802 ft), above sea level, showing also NASA’s IRTF and NAOJ Subaru

    When you look out at the Universe, there are a few things you’d rationally expect. You’d expect that the same things that made up everything we saw — like atoms and light — made up everything there was. You’d expect that the fundamental laws would apply equally well everywhere you looked, from small scales to large scales. And you’d expect that if you had multiple ways of measuring the same physical quantity, they’d give you the same answer.

    Which is why the dark matter problem is such a puzzle. There are a huge variety of measurements we can make that indicate that about 5/6ths of the Universe, by mass, isn’t made up of any of the known particles. It doesn’t interact with normal matter or light. And if you measure the mass of a galaxy directly, from its light, it doesn’t match the mass you infer from gravity.

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    According to models and simulations, all galaxies should be embedded in dark matter halos, whose densities peak at the galactic centers. On long enough timescales, of perhaps a billion years, a single dark matter particle from the outskirts of the halo will complete one orbit. The effects of gas, feedback, star formation, supernovae, and radiation all complicate this environment, making it extremely difficult to extract universal dark matter predictions. (NASA, ESA, AND T. BROWN AND J. TUMLINSON (STSCI))

    NASA/ESA Hubble Telescope

    Traditionally, the way to approach this problem has been to add a single ingredient: dark matter.

    Dark matter cosmic web and the large-scale structure it forms The Millenium Simulation, V. Springel et al


    Caterpillar Project A Milky-Way-size dark-matter halo and its subhalos circled, an enormous suite of simulations . Griffen et al. 2016

    If you assume that the Universe isn’t simply made up of the matter we can directly detect, but that there’s an additional component, you wouldn’t expect that those two mass measurements would line up. If there’s something besides protons, neutrons, and electrons making up the Universe, their gravitational effects would show themselves without necessarily leaving a visible light signature.

    But another option would be to modify the law of gravity. If you simply add in an additional term to Newton’s law of gravity that defines a minimum acceleration scale, you can explain how galaxies rotate to a superior degree to the dark matter idea. The great hope of modified gravity is to reproduce the entire observable Universe without adding in dark matter.

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    Individual galaxies could, in principle, be explained by either dark matter or a modification to gravity, but they are not the best evidence we have for what the Universe is made of, or how it got to be the way it is today. (STEFANIA.DELUCA OF WIKIMEDIA COMMONS)

    While attempts to make a modification to gravity that explain all the cosmic observations have proved elusive thus far, this remains the best option to explain how galaxies (and smaller objects) behave. Without a direct detection of a theoretical particle that could be responsible for dark matter, the door must remained open for alternatives. Despite the overwhelming cosmological evidence pointing to dark matter, other options deserve consideration, too.

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    Our galaxy is thought to be embedded in an enormous, diffuse dark matter halo, indicating that there must be dark matter flowing through the solar system. But it isn’t very much, density-wise, and that makes it extremely difficult to detect locally. (ROBERT CALDWELL & MARC KAMIONKOWSKI NATURE 458, 587–589 (2009))[Not made available]

    In science, the way you decide which ideas are admissible versus which ones are no longer possible is to put them to the test against one another. Dark matter and modified gravity have a hard time going head-to-head on galactic scales because there are a number of confounding elements involved. For galaxies, star formation, feedback between gas, radiation, and dark matter, as well as stellar winds and complicated merger scenarios make universal predictions difficult on these small scales. Modified gravity might give you much cleaner predictions on these small scales, but fail catastrophically when you attempt to extend these modifications to larger ones, where dark matter achieves its greatest successes.

    5
    The X-ray (pink) and overall matter (blue) maps of various colliding galaxy clusters show a clear separation between normal matter and gravitational effects, some of the strongest evidence for dark matter. Alternative theories now need to be so contrived that they are considered by many to be quite ridiculous. But dark matter and modified gravity are both contenders for explaining the Universe on small (galactic) scales. (X-RAY: NASA/CXC/ECOLE POLYTECHNIQUE FEDERALE DE LAUSANNE, SWITZERLAND/D.HARVEY NASA/CXC/DURHAM UNIV/R.MASSEY; OPTICAL/LENSING MAP: NASA, ESA, D. HARVEY (ECOLE POLYTECHNIQUE FEDERALE DE LAUSANNE, SWITZERLAND) AND R. MASSEY (DURHAM UNIVERSITY, UK))

    NASA/Chandra X-ray Telescope

    But there’s a new paper out [MNRAS] that has devised a brilliant, head-to-head test for dark matter against modified gravity. If the law of gravity is truly different from Einstein’s General Relativity, then it should apply equally well to all galaxies under all conditions.

    If we can find two galaxies with the same mass profiles — where they’re not only the same overall mass, but have the same mass-as-a-function-of-radius as one another — we’d expect them to exhibit the same internal motions as one another. If there’s no dark matter, but just the matter we observe, the force of gravity, even if it’s a modified force of gravity, would have to be the same.

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    Some galaxies are observed, if we try to fit them with dark matter, to have a ‘core’ in the center where the density is low, while others have a ‘cusp’ where the density is high. If dark matter gets heated based on the galaxy’s star formation history, this mystery could at last be solved. (J. I. READ, M. G. WALKER, P. STEGER; ARXIV:1808.06634 [above])

    So if we look at two galaxies and see that they don’t match, either at least one of the galaxies must be out of equilibrium, meaning it’s in a state of change, or modified gravity can’t explain it.

    On the other hand, there is a tremendously powerful explanation that dark matter offers that could explain it all, even if both galaxies are in equilibrium. The reason? Because galaxies could have formed stars at different times or different rates, and star-formation history affects not just the normal matter, but the dark matter as well.

    The Illustris Simulation

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    While the web of dark matter (purple) might seem to determine cosmic structure formation on its own, the feedback from normal matter (red) can severely impact galactic scales. Even small galaxies are subject to these effects, and if dark matter heats up from star formation, the effect can be quite severe. (ILLUSTRIS COLLABORATION / ILLUSTRIS SIMULATION)

    While it’s true that only normal matter interacts (i.e., scatters) with photons, both normal matter and dark matter should respond to radiation pressure. If a galaxy formed stars only a very long time ago, and not for many billions of years, there should be plenty of dark matter that now populates the inner reaches of a galaxy. But if there has been a lot of recent star formation occurring in multiple bursts, it should evacuate the mass from the galactic center. With less mass there, the orbits of the dark matter particles changes, lowering the inner density of dark matter in the innermost regions. (There was a nice review of this back in 2014 [Nature].) As Justin Read explained in a conversation with him:

    “…radiation pressure, stellar winds and supernovae push the gas (via the usual electromagnetic interaction) and dark matter then responds to the altered central gravitational potential.”

    The best laboratory to test this is with small, dwarf galaxies, where these effects should be the largest.

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    Dwarf galaxy NGC 5477 is one of many irregular dwarf galaxies. The blue regions are indicative of new star formation, but many such galaxies have formed no new stars in many billions of years. Even with the same light profiles, their mass profiles appear to be different, a challenge for modified theories of gravity. (NASA/ESA Hubble)

    If the galaxies all demonstrate the same gravitational behavior, it would be a victory for modified gravity. But if we can trace out the star-formation histories of these galaxies — which we can do by examining the stellar populations found inside them — and if these galaxies exhibit different gravitational behaviors because of them, that would be a victory for dark matter, and a blow to the theories of modified gravity that make contrary predictions.

    The number of galaxies we’ve found and examined to test this is small, but in a new paper [https://arxiv.org/abs/1808.06634v1 (above)] led by Justin Read, they look at 16 such galaxies, and find that the dark matter “heating” explanation appears to work!

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    The dwarf ‘twins’ Carina and Draco: a challenge for alternative gravity explanations for DM. The solid and dashed black and purple lines show predictions for Draco and Carina in MOND, which clearly fare poorly. Despite their similarities in terms of light, stellar kinematics imply that Draco is substantially denser than Carina. (FIG 7. FROM J. I. READ, M. G. WALKER, P. STEGER; ARXIV:1808.06634 [above])

    They looked at 8 dwarf spheroidal and 8 dwarf irregular galaxies, and found that there were two populations: one where star formation hasn’t occurred for the past 6 billion years, and one where it has. The ones where star formation didn’t occur recently are consistent with lots of dark mass in the center (no recent heating), and the ones where it did occur recently show far less dark matter in their centers (evidence for recent heating). It’s an indication that there is dark matter, it is cold and collisionless, and that it can be heated up by recent star formation.

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    The Draco dwarf spheroidal galaxy is one of the 16 galaxies examined in the Read et al. paper, and displays extremely different mass profiles from its gravitational effects than the Carina galaxy, which otherwise appears extremely similar except for a different star formation history. (BERNHARD HUBL / ASTROPHOTON.COM)

    Carina Nebula. 1.5-m Danish telescope at ESO’s La Silla Observatory


    ESO Danish 1.54 meter telescope at La Silla, 600 km north of Santiago de Chile at an altitude of 2400 metres.

    In particular, two of the galaxies (Draco and Carina) have almost the same masses and normal mass profiles, but widely different gravitational effects.

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    The Carina dwarf galaxy, very similar in size, star distribution, and morphology to the Draco dwarf galaxy, exhibits a very different gravitational profile from Draco. This can be cleanly explained with dark matter if it can be heated up by star formation, but not by modified gravity. (ESO/G. BONO & CTIO)

    The authors note:

    These two galaxies require different dynamical mass profiles for almost the same radial light profile. This is a challenge not only for MOND, but for any weak-field gravity theory that seeks to fully explain DM.

    Mordehai Milgrom, MOND theorist, is an Israeli physicist and professor in the department of Condensed Matter Physics at the Weizmann Institute in Rehovot, Israel

    The fact that these two galaxies exhibit such different gravitational effects tell us that either something is very funny with one of them (something must be out-of-equilibrium), or that dark matter gets heated up by star formation and modified gravity cannot explain this. As always, more data, additional galaxies, and further research will be required to solve this mystery, but at long last, we’re looking at a viable way to prove modified gravity wrong on galaxy scales. Even without directly detecting a particle, dark matter might just achieve a knockout blow over its greatest competing alternative.

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    five-ways-keep-your-child-safe-school-shootings

    Please help promote STEM in your local schools.

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    See the full article here .

    “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 9:05 am on August 23, 2018 Permalink | Reply
    Tags: , , , Bouncing barrier, , , Gravity, , NASA Researchers Find Evidence of Planet-Building Clumps, Planetesimal formation   

    From NASA Ames: “NASA Researchers Find Evidence of Planet-Building Clumps” 

    NASA Ames Icon

    From NASA AMES

    Aug. 21, 2018
    Darryl Waller
    NASA Ames Research Center, Silicon Valley
    650-604-2675
    darryl.e.waller@nasa.gov

    Noah Michelsohn
    NASA Johnson Space Center, Houston
    281-483-5111
    noah.j.michelsohn@nasa.gov

    1
    False-color image of Allendale meteorite showing the apparent golf ball size clumps. Credits: NASA/J. Simon and J. Cuzzi

    NASA scientists have found the first evidence supporting a theory that golf ball-size clumps of space dust formed the building blocks of our terrestrial planets.

    A new paper from planetary scientists at the Astromaterials Research and Exploration Science Division (ARES) at NASA’s Johnson Space Center in Houston, Texas, and NASA’s Ames Research Center in Silicon Valley, California, provides evidence for an astrophysical theory called “pebble accretion” where golf ball-sized clumps of space dust came together to form tiny planets, called planetesimals, during the early stages of planetary formation.

    “This is very exciting because our research provides the first direct evidence supporting this theory,” said Justin Simon, a planetary researcher in ARES. “There have been a lot of theories about planetesimal formation, but many have been stymied by a factor called the ‘bouncing barrier.’”

    “The bouncing barrier principle stipulates that planets cannot form directly through the accumulation of small dust particles colliding in space because the impact would knock off previously attached aggregates, stalling growth. Astrophysicists had hypothesized that once the clumps grew to the size of a golf ball, any small particle colliding with the clump would knock other material off. Yet, if the colliding objects were not the size of a particle, but much larger – for example, clumps of dust the size of a golf ball – that they could exhibit enough gravity to hold themselves together in clusters to form larger bodies.”

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    Mosaic photograph of the ancient Northwest Africa 5717 ordinary chondrite with clusters of particles. Credits: NASA/J. Simon and J. Cuzzi

    The research provides evidence of a common, possibly universal, dust sticking process from studying two ancient meteorites – Allende and Northwest Africa 5717 – that formed in the pre-planetary period of the Solar System and have remained largely unaltered since that time. Scientists know through dating methods that these meteorites are older than Earth, Moon, and Mars, which means they have remained unaltered since the birth of the Solar System. The meteorites studied for this research are so old that they are often used to date the Solar System itself.

    The meteorites were analyzed using electron microscope images and high-resolution photomicrographs that showed particles within the meteorite slices appeared to concentrate together in three to four-centimeter clumps. The existence of the clumps demonstrates that the meteorites themselves were produced by the clustering of golf ball-sized objects, providing strong evidence that the process was possible for other bodies as well.

    The research, titled “Particle size distributions in chondritic meteorites: Evidence for pre-planetesimal histories,” was published in the journal Earth and Planetary Science Letters in July. The publication culminated six years of research that was led by planetary scientists Simon at Johnson and Jeffrey Cuzzi at Ames.

    Dig up more about how NASA studies meteorites, visit:

    https://ares.jsc.nasa.gov/

    See the full article here .

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    Please help promote STEM in your local schools.

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    Ames Research Center, one of 10 NASA field Centers, is located in the heart of California’s Silicon Valley. For over 60 years, Ames has led NASA in conducting world-class research and development. With 2500 employees and an annual budget of $900 million, Ames provides NASA with advancements in:
    Entry systems: Safely delivering spacecraft to Earth & other celestial bodies
    Supercomputing: Enabling NASA’s advanced modeling and simulation
    NextGen air transportation: Transforming the way we fly
    Airborne science: Examining our own world & beyond from the sky
    Low-cost missions: Enabling high value science to low Earth orbit & the moon
    Biology & astrobiology: Understanding life on Earth — and in space
    Exoplanets: Finding worlds beyond our own
    Autonomy & robotics: Complementing humans in space
    Lunar science: Rediscovering our moon
    Human factors: Advancing human-technology interaction for NASA missions
    Wind tunnels: Testing on the ground before you take to the sky

    NASA image

     
  • richardmitnick 4:33 pm on August 20, 2018 Permalink | Reply
    Tags: , Anomalies, Bosons and fermions, Branes, , , , Gravity, Murray Gell-Mann, Parity violation, , , , , , , The second superstring revolution, Theorist John Schwarz   

    From Caltech: “Long and Winding Road: A Conversation with String Theory Pioneer” John Schwarz 

    Caltech Logo

    From Caltech

    08/20/2018

    Whitney Clavin
    (626) 395-1856
    wclavin@caltech.edu

    John Schwarz discusses the history and evolution of superstring theory.

    1
    John Schwarz. Credit: Seth Hansen for Caltech

    The decades-long quest for a theory that would unify all the known forces—from the microscopic quantum realm to the macroscopic world where gravity dominates—has had many twists and turns. The current leading theory, known as superstring theory and more informally as string theory, grew out of an approach to theoretical particle physics, called S-matrix theory, which was popular in the 1960s. Caltech’s John H. Schwarz, the Harold Brown Professor of Theoretical Physics, Emeritus, began working on the problem in 1971, while a junior faculty member at Princeton University. He moved to Caltech in 1972, where he continued his research with various collaborators from other universities. Their studies in the 1970s and 1980s would dramatically shift the evolution of the theory and, in 1984, usher in what’s known as the first superstring revolution.

    Essentially, string theory postulates that our universe is made up, at its most fundamental level, of infinitesimal tiny vibrating strings and contains 10 dimensions—three for space, one for time, and six other spatial dimensions curled up in such a way that we don’t perceive them in everyday life or even with the most sensitive experimental searches to date. One of the many states of a string is thought to correspond to the particle that carries the gravitational force, the graviton, thereby linking the two pillars of fundamental physics—quantum mechanics and the general theory of relativity, which includes gravity.

    We sat down with Schwarz to discuss the history and evolution of string theory and how the theory itself might have moved past strings.

    What are the earliest origins of string theory?

    The first study often regarded as the beginning of string theory came from an Italian physicist named Gabriele Veneziano in 1968. He discovered a mathematical formula that had many of the properties that people were trying to incorporate in a fundamental theory of the strong nuclear force [a fundamental force that holds nuclei together]. This formula was kind of pulled out of the blue, and ultimately Veneziano and others realized, within a couple years, that it was actually describing a quantum theory of a string—a one-dimensional extended object.

    How did the field grow after this paper?

    In the early ’70s, there were several hundred people worldwide working on string theory. But then everything changed when quantum chromodynamics, or QCD—which was developed by Caltech’s Murray Gell-Mann [Nobel Laureate, 1969] and others—became the favored theory of the strong nuclear force. Almost everyone was convinced QCD was the right way to go and stopped working on string theory. The field shrank down to just a handful of people in the course of a year or two. I was one of the ones who remained.

    How did Gell-Mann become interested in your work?

    Gell-Mann is the one who brought me to Caltech and was very supportive of my work. He took an interest in studies I had done with a French physicist, André Neveu, when we were at Princeton. Neveu and I introduced a second string theory. The initial Veneziano version had many problems. There are two kinds of fundamental particles called bosons and fermions, and the Veneziano theory only described bosons. The one I developed with Neveu included fermions. And not only did it include fermions but it led to the discovery of a new kind of symmetry that relates bosons and fermions, which is called supersymmetry. Because of that discovery, this version of string theory is called superstring theory.

    When did the field take off again?

    A pivotal change happened after work I did with another French physicist, Joël Scherk, whom Gell-Mann and I had brought to Caltech as a visitor in 1974. During that period, we realized that many of the problems we were having with string theory could be turned into advantages if we changed the purpose. Instead of insisting on constructing a theory of the strong nuclear force, we took this beautiful theory and asked what it was good for. And it turned out it was good for gravity. Neither of us had worked on gravity. It wasn’t something we were especially interested in but we realized that this theory, which was having trouble describing the strong nuclear force, gives rise to gravity. Once we realized this, I knew what I would be doing for the rest of my career. And I believe Joël felt the same way. Unfortunately, he died six years later. He made several important discoveries during those six years, including a supergravity theory in 11 dimensions.

    Surprisingly, the community didn’t respond very much to our papers and lectures. We were generally respected and never had a problem getting our papers published, but there wasn’t much interest in the idea. We were proposing a quantum theory of gravity, but in that era physicists who worked on quantum theory weren’t interested in gravity, and physicists who worked on gravity weren’t interested in quantum theory.

    That changed after I met Michael Green [a theoretical physicist then at the University of London and now at the University of Cambridge], at the CERN cafeteria in Switzerland in the summer of 1979. Our collaboration was very successful, and Michael visited Caltech for several extended visits over the next few years. We published a number of papers during that period, which are much cited, but our most famous work was something we did in 1984, which had to do with a problem known as anomalies.

    What are anomalies in string theory?

    One of the facts of nature is that there is what’s called parity violation, which means that the fundamental laws are not invariant under mirror reflection. For example, a neutrino always spins clockwise and not counterclockwise, so it would look wrong viewed in a mirror. When you try to write down a fundamental theory with parity violation, mathematical inconsistencies often arise when you take account of quantum effects. This is referred to as the anomaly problem. It appeared that one couldn’t make a theory based on strings without encountering these anomalies, which, if that were the case, would mean strings couldn’t give a realistic theory. Green and I discovered that these anomalies cancel one another in very special situations.

    When we released our results in 1984, the field exploded. That’s when Edward Witten [a theoretical physicist at the Institute for Advanced Study in Princeton], probably the most influential theoretical physicist in the world, got interested. Witten and three collaborators wrote a paper early in 1985 making a particular proposal for what to do with the six extra dimensions, the ones other than the four for space and time. That proposal looked, at the time, as if it could give a theory that is quite realistic. These developments, together with the discovery of another version of superstring theory, constituted the first superstring revolution.

    Richard Feynman was here at Caltech during that time, before he passed away in 1988. What did he think about string theory?

    After the 1984 to 1985 breakthroughs in our understanding of superstring theory, the subject no longer could be ignored. At that time it acquired some prominent critics, including Richard Feynman and Stephen Hawking. Feynman’s skepticism of superstring theory was based mostly on the concern that it could not be tested experimentally. This was a valid concern, which my collaborators and I shared. However, Feynman did want to learn more, so I spent several hours explaining the essential ideas to him. Thirty years later, it is still true that there is no smoking-gun experimental confirmation of superstring theory, though it has proved its value in other ways. The most likely possibility for experimental support in the foreseeable future would be the discovery of supersymmetry particles. So far, they have not shown up.

    What was the second superstring revolution about?

    The second superstring revolution occurred 10 years later in the mid ’90s. What happened then is that string theorists discovered what happens when particle interactions become strong. Before, we had been studying weakly interacting systems. But as you crank up the strength of the interaction, a 10th dimension of space can emerge. New objects called branes also emerge. Strings are one dimensional; branes have all sorts of dimensions ranging from zero to nine. An important class of these branes, called D-branes, was discovered by the late Joseph Polchinski [BS ’75]. Strings do have a special role, but when the system is strongly interacting, then the strings become less fundamental. It’s possible that in the future the subject will get a new name but until we understand better what the theory is, which we’re still struggling with, it’s premature to invent a new name.

    What can we say now about the future of string theory?

    It’s now over 30 years since a large community of scientists began pooling their talents, and there’s been enormous progress in those 30 years. But the more big problems we solve, the more new questions arise. So, you don’t even know the right questions to ask until you solve the previous questions. Interestingly, some of the biggest spin-offs of our efforts to find the most fundamental theory of nature are in pure mathematics.

    Do you think string theory will ultimately unify the forces of nature?

    Yes, but I don’t think we’ll have a final answer in my lifetime. The journey has been worth it, even if it did take some unusual twists and turns. I’m convinced that, in other intelligent civilizations throughout the galaxy, similar discoveries will occur, or already have occurred, in a different sequence than ours. We’ll find the same result and reach the same conclusions as other civilizations, but we’ll get there by a very different route.

    See the full article here .

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    The California Institute of Technology (commonly referred to as Caltech) is a private research university located in Pasadena, California, United States. Caltech has six academic divisions with strong emphases on science and engineering. Its 124-acre (50 ha) primary campus is located approximately 11 mi (18 km) northeast of downtown Los Angeles. “The mission of the California Institute of Technology is to expand human knowledge and benefit society through research integrated with education. We investigate the most challenging, fundamental problems in science and technology in a singularly collegial, interdisciplinary atmosphere, while educating outstanding students to become creative members of society.”

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  • richardmitnick 11:11 am on August 17, 2018 Permalink | Reply
    Tags: , , Gravity, Is Gravity Quantum?, , , ,   

    From Scientific American: “Is Gravity Quantum?” 

    Scientific American

    From Scientific American

    August 14, 2018
    Charles Q. Choi

    1
    Artist’s rendition of gravitational waves generated by merging neutron stars. The primordial universe is another source of gravitational waves, which, if detected, could help physicists devise a quantum theory of gravity. Credit: R. Hurt, Caltech-JPL

    All the fundamental forces of the universe are known to follow the laws of quantum mechanics, save one: gravity. Finding a way to fit gravity into quantum mechanics would bring scientists a giant leap closer to a “theory of everything” that could entirely explain the workings of the cosmos from first principles. A crucial first step in this quest to know whether gravity is quantum is to detect the long-postulated elementary particle of gravity, the graviton. In search of the graviton, physicists are now turning to experiments involving microscopic superconductors, free-falling crystals and the afterglow of the big bang.

    Quantum mechanics suggests everything is made of quanta, or packets of energy, that can behave like both a particle and a wave—for instance, quanta of light are called photons. Detecting gravitons, the hypothetical quanta of gravity, would prove gravity is quantum. The problem is that gravity is extraordinarily weak. To directly observe the minuscule effects a graviton would have on matter, physicist Freeman Dyson famously noted, a graviton detector would have to be so massive that it collapses on itself to form a black hole.

    “One of the issues with theories of quantum gravity is that their predictions are usually nearly impossible to experimentally test,” says quantum physicist Richard Norte of Delft University of Technology in the Netherlands. “This is the main reason why there exist so many competing theories and why we haven’t been successful in understanding how it actually works.”

    In 2015 [Physical Review Letters], however, theoretical physicist James Quach, now at the University of Adelaide in Australia, suggested a way to detect gravitons by taking advantage of their quantum nature. Quantum mechanics suggests the universe is inherently fuzzy—for instance, one can never absolutely know a particle’s position and momentum at the same time. One consequence of this uncertainty is that a vacuum is never completely empty, but instead buzzes with a “quantum foam” of so-called virtual particles that constantly pop in and out of existence. These ghostly entities may be any kind of quanta, including gravitons.

    Decades ago, scientists found that virtual particles can generate detectable forces. For example, the Casimir effect is the attraction or repulsion seen between two mirrors placed close together in vacuum. These reflective surfaces move due to the force generated by virtual photons winking in and out of existence. Previous research suggested that superconductors might reflect gravitons more strongly than normal matter, so Quach calculated that looking for interactions between two thin superconducting sheets in vacuum could reveal a gravitational Casimir effect. The resulting force could be roughly 10 times stronger than that expected from the standard virtual-photon-based Casimir effect.

    Recently, Norte and his colleagues developed a microchip to perform this experiment. This chip held two microscopic aluminum-coated plates that were cooled almost to absolute zero so that they became superconducting. One plate was attached to a movable mirror, and a laser was fired at that mirror. If the plates moved because of a gravitational Casimir effect, the frequency of light reflecting off the mirror would measurably shift. As detailed online July 20 in Physical Review Letters, the scientists failed to see any gravitational Casimir effect. This null result does not necessarily rule out the existence of gravitons—and thus gravity’s quantum nature. Rather, it may simply mean that gravitons do not interact with superconductors as strongly as prior work estimated, says quantum physicist and Nobel laureate Frank Wilczek of the Massachusets Institute of Technology, who did not participate in this study and was unsurprised by its null results. Even so, Quach says, this “was a courageous attempt to detect gravitons.”

    Although Norte’s microchip did not discover whether gravity is quantum, other scientists are pursuing a variety of approaches to find gravitational quantum effects. For example, in 2017 two independent studies suggested that if gravity is quantum it could generate a link known as “entanglement” between particles, so that one particle instantaneously influences another no matter where either is located in the cosmos. A tabletop experiment using laser beams and microscopic diamonds might help search for such gravity-based entanglement. The crystals would be kept in a vacuum to avoid collisions with atoms, so they would interact with one another through gravity alone. Scientists would let these diamonds fall at the same time, and if gravity is quantum the gravitational pull each crystal exerts on the other could entangle them together.

    The researchers would seek out entanglement by shining lasers into each diamond’s heart after the drop. If particles in the crystals’ centers spin one way, they would fluoresce, but they would not if they spin the other way. If the spins in both crystals are in sync more often than chance would predict, this would suggest entanglement. “Experimentalists all over the world are curious to take the challenge up,” says quantum gravity researcher Anupam Mazumdar of the University of Groningen in the Netherlands, co-author of one of the entanglement studies.

    Another strategy to find evidence for quantum gravity is to look at the cosmic microwave background [CMB] radiation, the faint afterglow of the big bang, says cosmologist Alan Guth of M.I.T.

    Cosmic Background Radiation per ESA/Planck

    ESA/Planck 2009 to 2013

    Quanta such as gravitons fluctuate like waves, and the shortest wavelengths would have the most intense fluctuations. When the cosmos expanded staggeringly in size within a sliver of a second after the big bang, according to Guth’s widely supported cosmological model known as inflation, these short wavelengths would have stretched to longer scales across the universe.

    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

    This evidence of quantum gravity could be visible as swirls in the polarization, or alignment, of photons from the cosmic microwave background radiation.

    However, the intensity of these patterns of swirls, known as B-modes, depends very much on the exact energy and timing of inflation. “Some versions of inflation predict that these B-modes should be found soon, while other versions predict that the B-modes are so weak that there will never be any hope of detecting them,” Guth says. “But if they are found, and the properties match the expectations from inflation, it would be very strong evidence that gravity is quantized.”

    One more way to find out whether gravity is quantum is to look directly for quantum fluctuations in gravitational waves, which are thought to be made up of gravitons that were generated shortly after the big bang. The Laser Interferometer Gravitational-Wave Observatory (LIGO) first detected gravitational waves in 2016, but it is not sensitive enough to detect the fluctuating gravitational waves in the early universe that inflation stretched to cosmic scales, Guth says.


    VIRGO Gravitational Wave interferometer, near Pisa, Italy

    Caltech/MIT Advanced aLigo Hanford, WA, USA installation


    Caltech/MIT Advanced aLigo detector installation Livingston, LA, USA

    Cornell SXS, the Simulating eXtreme Spacetimes (SXS) project

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

    ESA/eLISA the future of gravitational wave research

    1
    Skymap showing how adding Virgo to LIGO helps in reducing the size of the source-likely region in the sky. (Credit: Giuseppe Greco (Virgo Urbino group)

    A gravitational-wave observatory in space, such as the Laser Interferometer Space Antenna (eLISA, just above), could potentially detect these waves, Wilczek adds.

    In a paper recently accepted by the journal Classical and Quantum Gravity, however, astrophysicist Richard Lieu of the University of Alabama, Huntsville, argues that LIGO should already have detected gravitons if they carry as much energy as some current models of particle physics suggest. It might be that the graviton just packs less energy than expected, but Lieu suggests it might also mean the graviton does not exist. “If the graviton does not exist at all, it will be good news to most physicists, since we have been having such a horrid time in developing a theory of quantum gravity,” Lieu says.

    Still, devising theories that eliminate the graviton may be no easier than devising theories that keep it. “From a theoretical point of view, it is very hard to imagine how gravity could avoid being quantized,” Guth says. “I am not aware of any sensible theory of how classical gravity could interact with quantum matter, and I can’t imagine how such a theory might work.”

    See the full article here .


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    Scientific American, the oldest continuously published magazine in the U.S., has been bringing its readers unique insights about developments in science and technology for more than 160 years.

     
  • richardmitnick 2:21 pm on September 21, 2017 Permalink | Reply
    Tags: But quantum mechanics doesn’t really define what a measurement is, Gravity, Gravity at its most fundamental comes in indivisible parcels called quanta, GRW model-Ghirardi–Rimini–Weber theory, In quantum theory the state of a particle is described by its wave function, Much like the electromagnetic force comes in quanta called photons, , ,   

    From New Scientist: “Gravity may be created by strange flashes in the quantum realm” 

    NewScientist

    New Scientist

    20 September 2017
    Anil Ananthaswamy

    1
    Gravity comes about in a flash. Emma Johnson/Getty

    HOW do you reconcile the two pillars of modern physics: quantum theory and gravity? One or both will have to give way. A new approach says gravity could emerge from random fluctuations at the quantum level, making quantum mechanics the more fundamental of the two theories.

    Of our two main explanations of reality, quantum theory governs the interactions between the smallest bits of matter. And general relativity deals with gravity and the largest structures in the universe. Ever since Einstein, physicists have been trying to bridge the gap between the two, with little success.

    Part of the problem is knowing which strands of each theory are fundamental to our understanding of reality.

    One approach towards reconciling gravity with quantum mechanics has been to show that gravity at its most fundamental comes in indivisible parcels called quanta, much like the electromagnetic force comes in quanta called photons. But this road to a theory of quantum gravity has so far proved impassable.

    Now Antoine Tilloy at the Max Planck Institute of Quantum Optics in Garching, Germany, has attempted to get at gravity by tweaking standard quantum mechanics.

    In quantum theory, the state of a particle is described by its wave function. The wave function lets you calculate, for example, the probability of finding the particle in one place or another on measurement. Before the measurement, it is unclear whether the particle exists and if so, where. Reality, it seems, is created by the act of measurement, which “collapses” the wave function.

    But quantum mechanics doesn’t really define what a measurement is. For instance, does it need a conscious human? The measurement problem leads to paradoxes like Schrödinger’s cat, in which a cat can be simultaneously dead and alive inside a box, until someone opens the box to look.

    One solution to such paradoxes is a so-called GRW model that was developed in the late 1980s. It incorporates “flashes”, which are spontaneous random collapses of the wave function of quantum systems. The outcome is exactly as if there were measurements being made, but without explicit observers.

    Tilloy has modified this model to show how it can lead to a theory of gravity. In his model, when a flash collapses a wave function and causes a particle to be in one place, it creates a gravitational field at that instant in space-time. A massive quantum system with a large number of particles is subject to numerous flashes, and the result is a fluctuating gravitational field.

    It turns out that the average of these fluctuations is a gravitational field that one expects from Newton’s theory of gravity (arxiv.org/abs/1709.03809). This approach to unifying gravity with quantum mechanics is called semiclassical: gravity arises from quantum processes but remains a classical force. “There is no real reason to ignore this semiclassical approach, to having gravity being classical at the fundamental level,” says Tilloy.

    “I like this idea in principle,” says Klaus Hornberger at the University of Duisburg-Essen in Germany. But he points out that other problems need to be tackled before this approach can be a serious contender for unifying all the fundamental forces underpinning the laws of physics on scales large and small. For example, Tilloy’s model can be used to get gravity as described by Newton’s theory, but the maths still has to be worked out to see if it is effective in describing gravity as governed by Einstein’s general relativity.

    Tilloy agrees. “This is very hard to generalise to relativistic settings,” he says. He also cautions that no one knows which of the many tweaks to quantum mechanics is the correct one.

    Nonetheless, his model makes predictions that can be tested. For example, it predicts that gravity will behave differently at the scale of atoms from how it does on larger scales. Should those tests find that Tilloy’s model reflects reality and gravity does indeed originate from collapsing quantum fluctuations, it would be a big clue that the path to a theory of everything would involve semiclassical gravity.

    See the full article here .

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  • richardmitnick 6:57 am on May 16, 2017 Permalink | Reply
    Tags: , , EPR paradox, , Gravity, , ,   

    From COSMOS: “Using Einstein’s ‘spooky action at a distance’ to hear ripples in spacetime” 

    Cosmos Magazine bloc

    COSMOS

    16 May 2017
    Cathal O’Connell

    1
    The new technique will aid in the detection of gravitational waves caused by colliding black holes. Henze / NASA

    In new work that connects two of Albert Einstein’s ideas in a way he could scarcely have imagined, physicists have proposed a way to improve gravitational wave detectors, using the weirdness of quantum physics.

    The new proposal, published in Nature Physics, could double the sensitivity of future detectors listening out for ripples in spacetime caused by catastrophic collisions across the universe.

    When the advanced Laser Interferometer Gravitational-Wave Observatory (LIGO) detected gravitational waves in late 2015 it was the first direct evidence of the gravitational waves Einstein had predicted a century before.


    Caltech/MIT Advanced aLigo Hanford, WA, USA installation


    Caltech/MIT Advanced aLigo detector installation Livingston, LA, USA

    Cornell SXS, the Simulating eXtreme Spacetimes (SXS) project


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

    Now it another of Einstein’s predictions – one he regarded as a failure – could potentially double the sensitivity of LIGOs successors.

    The story starts with his distaste for quantum theory – or at least for the fundamental fuzziness of all things it seemed to demand.

    Einstein thought the universe would ultimately prove predictable and exact, a clockwork universe rather than one where God “plays dice”. In 1935 he teamed up with Boris Podolsky and Nathan Rosen to publish a paper they thought would be a sort of reductio ad absurdum. They hoped to disprove quantum mechanics by following it to its logical, ridiculous conclusion. Their ‘EPR paradox’ (named for their initials) described the instantaneous influence of one particle on another, what Einstein called “spooky action at a distance” because it seemed at first to be impossible.

    Yet this sally on the root of quantum physics failed, as the EPR effect turned out not to be a paradox after all. Quantum entanglement, as it’s now known, has been repeatedly proven to exist, and features in several proposed quantum technologies, including quantum computation and quantum cryptography.

    2
    Artistic rendering of the generation of an entangled pair of photons by spontaneous parametric down-conversion as a laser beam passes through a nonlinear crystal. Inspired by an image in Dance of the Photons by Anton Zeilinger. However, this depiction is from a different angle, to better show the “figure 8” pattern typical of this process, clearly shows that the pump beam continues across the entire image, and better represents that the photons are entangled.
    Date 31 March 2011
    Source Entirely self-generated using computer graphics applications.
    Author J-Wiki at English Wikipedia

    Now we can add gravity wave detection to the list.

    LIGO works by measuring the minute wobbling of mirrors as a gravitational wave stretches and squashes spacetime around them. It is insanely sensitive – able to detect wobbling down to 10,000th the width of a single proton.

    At this level of sensitivity the quantum nature of light becomes a problem. This means the instrument is limited by the inherent fuzziness of the photons bouncing between its mirrors — this quantum noise washes out weak signals.

    To get around this, physicists plan to use so-called squeezed light to dial down the level of quantum noise near the detector (while increasing it elsewhere).

    The new scheme aids this by adding two new, entangled laser beams to the mix. Because of the ‘spooky’ connection between the two entangled beams, their quantum noise is correlated – detecting one allows the prediction of the other.

    This way, the two beams can be used to probe the main LIGO beam, helping nudge it into a squeezed light state. This reduces the noise to a level that standard quantum theory would deem impossible.

    The authors of the new proposal write that it is “appropriate for all future gravitational-wave detectors for achieving sensitivities beyond the standard quantum limit”.

    Indeed, the proposal could as much as double the sensitivity of future detectors.

    Over the next 30 years, astronomers aim to improve the sensitivity of the detectors, like LIGO, by 30-fold. At that level, we’d be able to hear all black hole mergers in the observable universe.

    ESA/eLISA, the future of gravitational wave research

    However, along with improved sensitivity, the proposed system would also increase the number of photons lost in the detector. Raffaele Flaminio, a physicist at the National Astronomical Observatory of Japan, points out in a perspective piece for Nature Physics [no link], Flaminio that the team need to do more work to understand how this will affect ultimate performance.

    “But the idea of using Einstein’s most famous (mistaken) paradox to improve the sensitivity of gravitational-wave detectors, enabling new tests of his general theory of relativity, is certainly intriguing,” Flaminio writes. “Einstein’s ideas – whether wrong or right – continue to have a strong influence on physics and astronomy.”

    See the full article here .

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  • richardmitnick 9:54 am on April 23, 2017 Permalink | Reply
    Tags: , , , Computer modelling, , , , Gravity, Modified Newtonian Dynamics, or MOND, , Simulating galaxies,   

    From Durham: “Simulated galaxies provide fresh evidence of dark matter” 

    Durham U bloc

    Durham University

    21 April 2017
    No writer credit

    1
    A simulated galaxy is pictured, showing the main ingredients that make up a galaxy: the stars (blue), the gas from which the stars are born (red), and the dark matter halo that surrounds the galaxy (light grey). No image credit.

    Further evidence of the existence of dark matter – the mysterious substance that is believed to hold the Universe together – has been produced by Cosmologists at Durham University.

    Using sophisticated computer modelling techniques, the research team simulated the formation of galaxies in the presence of dark matter and were able to demonstrate that their size and rotation speed were linked to their brightness in a similar way to observations made by astronomers.

    One of the simulations is pictured, showing the main ingredients that make up a galaxy: the stars (blue), the gas from which the stars are born (red), and the dark matter halo that surrounds the galaxy (light grey).

    Alternative theories

    Until now, theories of dark matter have predicted a much more complex relationship between the size, mass and brightness (or luminosity) of galaxies than is actually observed, which has led to dark matter sceptics proposing alternative theories that are seemingly a better fit with what we see.

    The research led by Dr Aaron Ludlow of the Institute for Computational Cosmology, is published in the academic journal, Physical Review Letters.

    Most cosmologists believe that more than 80 per cent of the total mass of the Universe is made up of dark matter – a mysterious particle that has so far not been detected but explains many of the properties of the Universe such as the microwave background measured by the Planck satellite.

    CMB per ESA/Planck

    ESA/Planck

    Convincing explanations

    Alternative theories include Modified Newtonian Dynamics, or MOND. While this does not explain some observations of the Universe as convincingly as dark matter theory it has, until now, provided a simpler description of the coupling of the brightness and rotation velocity, observed in galaxies of all shapes and sizes.

    The Durham team used powerful supercomputers to model the formation of galaxies of various sizes, compressing billions of years of evolution into a few weeks, in order to demonstrate that the existence of dark matter is consistent with the observed relationship between mass, size and luminosity of galaxies.

    Long-standing problem resolved

    Dr Ludlow said: “This solves a long-standing problem that has troubled the dark matter model for over a decade. The dark matter hypothesis remains the main explanation for the source of the gravity that binds galaxies. Although the particles are difficult to detect, physicists must persevere.”

    Durham University collaborated on the project with Leiden University, Netherlands; Liverpool John Moores University, England and the University of Victoria, Canada. The research was funded by the European Research Council, the Science and Technology Facilities Council, Netherlands Organisation for Scientific Research, COFUND and The Royal Society.

    See the full article here .

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    Durham U campus

    Durham University is distinctive – a residential collegiate university with long traditions and modern values. We seek the highest distinction in research and scholarship and are committed to excellence in all aspects of education and transmission of knowledge. Our research and scholarship affect every continent. We are proud to be an international scholarly community which reflects the ambitions of cultures from around the world. We promote individual participation, providing a rounded education in which students, staff and alumni gain both the academic and the personal skills required to flourish.

     
  • richardmitnick 8:38 am on December 29, 2016 Permalink | Reply
    Tags: , , , , DDM hypothesis, , , Gravity, Institute for Nuclear Research in Moscow, , The Universe is losing dark matter and researchers have finally measured how much   

    From Science Alert: “The Universe is losing dark matter, and researchers have finally measured how much” 

    ScienceAlert

    Science Alert

    28 DEC 2016
    JOSH HRALA

    1
    MIPT

    Researchers from Russia have, for the first time, been able to measure the amount of dark matter the Universe has lost since the Big Bang some 13.7 billion years ago, and calculate that as much as 5 percent of dark matter could have deteriorated.

    The finding could explain one of the biggest mysteries in physics – why our Universe appears to function in a slightly different way than it did in the years just after the Big Bang, and it could also shed insight into how it might continue to evolve in future.

    “The discrepancy between the cosmological parameters in the modern Universe and the Universe shortly after the Big Bang can be explained by the fact that the proportion of dark matter has decreased,” said co-author Igor Tkachev, from the Institute for Nuclear Research in Moscow.

    “We have now, for the first time, been able to calculate how much dark matter could have been lost, and what the corresponding size of the unstable component would be.”

    The mystery surrounding dark matter was first brought up way back in the 1930s, when astrophysicists and astronomers observed that galaxies moved in weird ways, appearing to be under the effect of way more gravity than could be explained by the visible matter and energy in the Universe.

    This gravitational pull has to come from somewhere. So, researchers came up with a new type of ‘dark matter’ to describe the invisible mass responsible for the things they were witnessing.

    As of right now, the current hypothesis states that the Universe is made up of 4.9 percent normal matter – the stuff we can see, such as galaxies and stars – 26.8 percent dark matter, and 68.3 percent dark energy, a hypothetical type of energy that’s spread throughout the Universe, and which might be responsible for the Universe’s expansion.

    But even though the majority of matter predicted to be in the Universe is actually dark, very little is known about dark matter – in fact, scientists still haven’t been able to prove that it actually exists.

    One of the ways scientists study dark matter is by examining the cosmic microwave background (CMB), which some call the ‘echo of the Big Bang’.

    CMB per ESA/Planck
    CMB per ESA/Planck

    The CMB is the thermal radiation left over from the Big Bang, making it somewhat of an astronomical time capsule that researchers can use to understand the early, newly born Universe.

    The problem is that the cosmological parameters that govern how our Universe works – such as the speed of light and the way gravity works – appear to differ ever so slightly in the CMB compared to the parameters we know to exist in the modern Universe.

    “This variance was significantly more than margins of error and systematic errors known to us,” Tkachev explains. “Therefore, we are either dealing with some kind of unknown error, or the composition of the ancient universe is considerably different to the modern Universe.”

    One of the hypotheses that might explain why the early Universe was so different is the ‘decaying dark matter‘ [Nature] (DDM) hypothesis – the idea that dark matter has slowly been disappearing from the Universe.

    And that’s exactly what Tkachev and his colleagues set out to analyse on a mathematical level, looking for just how much dark matter might have decayed since the creation of the Universe.

    The study’s lead author, Dmitry Gorbunov, also from the Institute for Nuclear Research, explains:

    “Let us imagine that dark matter consists of several components, as in ordinary matter (protons, electrons, neutrons, neutrinos, photons). And one component consists of unstable particles with a rather long lifespan.

    In the era of the formation of hydrogen, hundreds of thousands of years after the Big Bang, they are still in the Universe, but by now (billions of years later), they have disappeared, having decayed into neutrinos or hypothetical relativistic particles. In that case, the amount of dark matter in the era of hydrogen formation and today will be different.”

    To come up with a figure, the team analysed data taken from the Planck Telescope observations on the CMB, and compared it to different dark matter models like DDM.

    ESA/Planck
    ESA/Planck

    They found that the DDM model accurately depicts the observational data found in the modern Universe over other possible explanations for why our Universe looks so different today compared to straight after the Big Bang.

    The team was able to take the study a step further by comparing the CMB data to the modern observational studies of the Universe and error-correcting for various cosmological effects – such as gravitational lensing, which can amplify regions of space thanks to the way gravity can bend light.

    In the end, they suggest that the Universe has lost somewhere between 2 and 5 percent of its dark matter since the Big Bang, as a result of these hypothetical dark matter particles decaying over time.

    “This means that in today’s Universe, there is 5 percent less dark matter than in the recombination era,” Tkachev concludes.

    “We are not currently able to say how quickly this unstable part decayed; dark matter may still be disintegrating even now, although that would be a different and considerably more complex model.”

    These findings suggest that dark matter decays over time, making the Universe move in different ways than it had in the past, though the findings call for more outside research before anything is said for certain.

    Even so, this research is another step closer to potentially understanding the nature of dark matter, and solving one of science’s greatest mysteries – why the Universe looks the way it does, and how it will evolve in the future.

    The team’s work was published in Physical Review D.

    See the full article here .

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