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  • richardmitnick 9:46 am on December 14, 2016 Permalink | Reply
    Tags: , , , , Einstein’s General Relativity (GR), Emergent Gravity, , Erik Verlinde, Hubble Constant, ,   

    From astrobites: “Emergent Gravity faces its First Test in Galaxy Lensing” 

    Astrobites bloc


    Dec 13, 2016
    Gourav Khullar

    Title: First test of Verlinde’s theory of Emergent Gravity using Weak Gravitational Lensing measurements
    Authors: M. M. Brower, M.R. Visser, A Dvornik, et al.
    First Author’s Institution: Leiden Observatory, Leiden, The Netherlands
    Status: Submitted to The Monthly Notices of the Royal Astronomical Society (MNRAS), December 2016 [open access]

    Despite being a near-perfect model and explaining everything ranging from galactic rotation curves to high-redshift supernovae observations, Lambda-CDM has its problems. A lack of clear candidates for a dark matter particle and dark energy are two that certainly keep physicists up at night. This leads us towards alleys unexplored – theories that are creative, innovative and crucial to the scientific process, theories that could lead us to the eventual model of the universe with a clear explanations of all observations. One such theory that garnered some attention in the last few years is Emergent Gravity.

    Fig 1. Galaxy rotation curves observed over the last few years indicate a dominant matter halo on the outskirts of galaxies, something that’s explained concretely by dark matter.

    What is ‘Emergent’ in Emergent Gravity?

    The idea is pretty radical yet basic – gravity isn’t a manifestation of mass in spacetime as proposed by Einstein’s General Relativity (GR) or a fundamental force that fits perfectly in a four-force model of the universe. Instead, gravity is proposed to be ’emerging’ from interactions between even more fundamental particles. This is akin to seeing thermodynamical parameters like pressure and temperature arising from interactions between atoms and molecules – what’s crucial to our discussion is the macroscopic quantity. In the case here, that quantity would be gravity. This idea has been developing over the last few decades, with Theodore Jacobson, Thanu Padmanabhan and more recently, Erik Verlinde contributing heavily to its development.

    Erik Verlinde

    Fig 2. High speeds of particle collision against the walls of a container lead to higher temperature, since the system possesses more kinetic energy that gets converted to thermal energy.

    Diving deep into Entropy and Gravity

    One aspect of a theoretical model like emergent gravity (EG) is that we are allowed to derive macroscopic results without having to worry about the underlying fundamental particles that could lead to gravity ’emerging’ – at least for now. This ’emergence’ can be thought of as the result of the tendency of a physical system to increase its entropy. Early work in the field towards a ‘thermodynamics-like theory of gravity’ used something called ‘holographic scaling of entropy’, which essentially scales with surface area of an enclosed volume of spacetime. Verlinde’s new work insists that due to dark energy, we see deviations in GR at long distances that can be resolved if this entropy scaling scales as volume instead of area. Keeping details aside, this leads to a different ‘force-law’, that has additional dominant matter terms that could explain dark matter (called ‘apparent dark matter’ in this case). This and this piece are excellent sources for details on the model. It can be seen that in some sense, this model combines the origin of dark matter and dark energy in a novel way.

    Basics of Weak Gravitational Lensing

    Well, how do we test this theory? Perhaps, passing it through the same standards as GR would seem appropriate.

    The idea of gravitational lensing was one of the first tests of GR i.e. the idea that light’s path gets distorted when traveling through curved spacetime surrounding massive objects. This distortion can change the light ray received from background galaxies (and hence, apparent shape and size) due to a foreground massive object like a galaxy or a galaxy cluster, leading to weak gravitational lensing. This galaxy-galaxy lensing signal is a massive success story of GR, as observations of this phenomena in the Universe fit into the model very well.

    Fig 3. Gravitational lensing leading to a drastic distortion in light coming from background galaxies. Credit: NASA-Hubble Space Telescope.

    Since EG still gives rise to ‘apparent dark matter’, it is safe to say that the gravitational lensing formalism stays the same, since we do apply this formalim to our universe’s dark matter-dominated objects like galaxy clusters (if we believe Lambda-CDM and its predictions). This allows us to use weak lensing as a test for emergent gravity, and match observations against the predictions of this theory.

    This work

    The regime studied in this work is the low-redshift universe, or the relatively local universe, where the Hubble Constant can be treated as a constant. This is almost true because of the dominance of dark energy after redshift ~0.7-0.9. Since Verlinde’s EG isn’t evolved enough as a theory to quantify cosmology before this epoch, this work assumes a background Lambda-CDM cosmology. For studying galaxy-galaxy lensing, Brower et al. select ~33,000 galaxies from the Galaxy And Mass Assembly (GAMA) survey as ‘lenses’ and KiDS survey galaxies as background galaxies that get lensed. They model these galaxies as having a static, spherically symmetric distribution of mass- something like a point mass or an extended source resembling a point mass- because that’s what EG can handle so far.

    This work calculates the lensing effect by measuring distortions in the background galaxies’ images, termed as a ‘shear’. In the framework of GR, this quantity is comprised in something called the Extended Surface Density (ESD) profile. Brower et al. calculated the ESD for these galaxies under the many assumptions of this model, compared them with Navarror-Frenk-White (NFW) profiles of galaxies from Lambda-CDM, and found that there was general agreement in the ESD progression between the two.

    Fig 4. From the paper, a model-fit of Emergent Gravity(Point mass model), Emergent Gravity (Extended model) and Dark Matter(NFW model). The lensing signal measured in the form of an ESD is plotted for four different galactic mass bins. It can be seen that Verlinde’s Emergent Gravity model assisted by teh assumptions made by Brower et al. match NFW profile predictions very well.

    Conclusion and Summary

    So what are the assumptions? For one, EG cannot deal with evolution of the universe at the moment. Moreover, the theory isn’t developed enough to have a basic framework of what causes gravity to ’emerge’ from fundamental interactions. The paper agrees that a more ‘sophisticated implementation of both theories’ is needed to make a statement about whether apparent dark matter explains observations better than Lambda-CDM dark matter. Till then, EG shall keep on evolving and observations shall keep on being pitted against these evolving frameworks. A very exciting space to watch!

    See the full article here .

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    What do we do?

    Astrobites is a daily astrophysical literature journal written by graduate students in astronomy. Our goal is to present one interesting paper per day in a brief format that is accessible to undergraduate students in the physical sciences who are interested in active research.
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    Reading a technical paper from an unfamiliar subfield is intimidating. It may not be obvious how the techniques used by the researchers really work or what role the new research plays in answering the bigger questions motivating that field, not to mention the obscure jargon! For most people, it takes years for scientific papers to become meaningful.
    Our goal is to solve this problem, one paper at a time. In 5 minutes a day reading Astrobites, you should not only learn about one interesting piece of current work, but also get a peek at the broader picture of research in a new area of astronomy.

  • richardmitnick 7:05 pm on November 30, 2016 Permalink | Reply
    Tags: , , , , Erik Verlinde, , ,   

    From Quanta: “The Case Against Dark Matter” 

    Quanta Magazine
    Quanta Magazine

    November 29, 2016
    Natalie Wolchover

    Erik Verlinde
    Ilvy Njiokiktjien for Quanta Magazine

    For 80 years, scientists have puzzled over the way galaxies and other cosmic structures appear to gravitate toward something they cannot see. This hypothetical “dark matter” seems to outweigh all visible matter by a startling ratio of five to one, suggesting that we barely know our own universe. Thousands of physicists are doggedly searching for these invisible particles.

    But the dark matter hypothesis assumes scientists know how matter in the sky ought to move in the first place. This month, a series of developments has revived a long-disfavored argument that dark matter doesn’t exist after all. In this view, no missing matter is needed to explain the errant motions of the heavenly bodies; rather, on cosmic scales, gravity itself works in a different way than either Isaac Newton or Albert Einstein predicted.

    The latest attempt to explain away dark matter is a much-discussed proposal by Erik Verlinde, a theoretical physicist at the University of Amsterdam who is known for bold and prescient, if sometimes imperfect, ideas. In a dense 51-page paper posted online on Nov. 7, Verlinde casts gravity as a byproduct of quantum interactions and suggests that the extra gravity attributed to dark matter is an effect of “dark energy” — the background energy woven into the space-time fabric of the universe.

    Instead of hordes of invisible particles, “dark matter is an interplay between ordinary matter and dark energy,” Verlinde said.

    To make his case, Verlinde has adopted a radical perspective on the origin of gravity that is currently in vogue among leading theoretical physicists. Einstein defined gravity as the effect of curves in space-time created by the presence of matter. According to the new approach, gravity is an emergent phenomenon. Space-time and the matter within it are treated as a hologram that arises from an underlying network of quantum bits (called “qubits”), much as the three-dimensional environment of a computer game is encoded in classical bits on a silicon chip. Working within this framework, Verlinde traces dark energy to a property of these underlying qubits that supposedly encode the universe. On large scales in the hologram, he argues, dark energy interacts with matter in just the right way to create the illusion of dark matter.

    In his calculations, Verlinde rediscovered the equations of “modified Newtonian dynamics,” or MOND. This 30-year-old theory makes an ad hoc tweak to the famous “inverse-square” law of gravity in Newton’s and Einstein’s theories in order to explain some of the phenomena attributed to dark matter. That this ugly fix works at all has long puzzled physicists. “I have a way of understanding the MOND success from a more fundamental perspective,” Verlinde said.

    Many experts have called Verlinde’s paper compelling but hard to follow. While it remains to be seen whether his arguments will hold up to scrutiny, the timing is fortuitous. In a new analysis of galaxies published on Nov. 9 in Physical Review Letters, three astrophysicists led by Stacy McGaugh of Case Western Reserve University in Cleveland, Ohio, have strengthened MOND’s case against dark matter.

    The researchers analyzed a diverse set of 153 galaxies, and for each one they compared the rotation speed of visible matter at any given distance from the galaxy’s center with the amount of visible matter contained within that galactic radius. Remarkably, these two variables were tightly linked in all the galaxies by a universal law, dubbed the “radial acceleration relation.” This makes perfect sense in the MOND paradigm, since visible matter is the exclusive source of the gravity driving the galaxy’s rotation (even if that gravity does not take the form prescribed by Newton or Einstein). With such a tight relationship between gravity felt by visible matter and gravity given by visible matter, there would seem to be no room, or need, for dark matter.

    Even as dark matter proponents rise to its defense, a third challenge has materialized. In new research that has been presented at seminars and is under review by the Monthly Notices of the Royal Astronomical Society, a team of Dutch astronomers have conducted what they call the first test of Verlinde’s theory: In comparing his formulas to data from more than 30,000 galaxies, Margot Brouwer of Leiden University in the Netherlands and her colleagues found that Verlinde correctly predicts the gravitational distortion or “lensing” of light from the galaxies — another phenomenon that is normally attributed to dark matter. This is somewhat to be expected, as MOND’s original developer, the Israeli astrophysicist Mordehai Milgrom, showed years ago that MOND accounts for gravitational lensing data. Verlinde’s theory will need to succeed at reproducing dark matter phenomena in cases where the old MOND failed.

    Kathryn Zurek, a dark matter theorist at Lawrence Berkeley National Laboratory, said Verlinde’s proposal at least demonstrates how something like MOND might be right after all. “One of the challenges with modified gravity is that there was no sensible theory that gives rise to this behavior,” she said. “If [Verlinde’s] paper ends up giving that framework, then that by itself could be enough to breathe more life into looking at [MOND] more seriously.”

    The New MOND

    In Newton’s and Einstein’s theories, the gravitational attraction of a massive object drops in proportion to the square of the distance away from it. This means stars orbiting around a galaxy should feel less gravitational pull — and orbit more slowly — the farther they are from the galactic center. Stars’ velocities do drop as predicted by the inverse-square law in the inner galaxy, but instead of continuing to drop as they get farther away, their velocities level off beyond a certain point. The “flattening” of galaxy rotation speeds, discovered by the astronomer Vera Rubin in the 1970s, is widely considered to be Exhibit A in the case for dark matter — explained, in that paradigm, by dark matter clouds or “halos” that surround galaxies and give an extra gravitational acceleration to their outlying stars.

    Searches for dark matter particles have proliferated — with hypothetical “weakly interacting massive particles” (WIMPs) and lighter-weight “axions” serving as prime candidates — but so far, experiments have found nothing.

    Lucy Reading-Ikkanda for Quanta Magazine

    Meanwhile, in the 1970s and 1980s, some researchers, including Milgrom, took a different tack. Many early attempts at tweaking gravity were easy to rule out, but Milgrom found a winning formula: When the gravitational acceleration felt by a star drops below a certain level — precisely 0.00000000012 meters per second per second, or 100 billion times weaker than we feel on the surface of the Earth — he postulated that gravity somehow switches from an inverse-square law to something close to an inverse-distance law. “There’s this magic scale,” McGaugh said. “Above this scale, everything is normal and Newtonian. Below this scale is where things get strange. But the theory does not really specify how you get from one regime to the other.”

    Physicists do not like magic; when other cosmological observations seemed far easier to explain with dark matter than with MOND, they left the approach for dead. Verlinde’s theory revitalizes MOND by attempting to reveal the method behind the magic.

    Verlinde, ruddy and fluffy-haired at 54 and lauded for highly technical string theory calculations, first jotted down a back-of-the-envelope version of his idea in 2010. It built on a famous paper he had written months earlier, in which he boldly declared that gravity does not really exist. By weaving together numerous concepts and conjectures at the vanguard of physics, he had concluded that gravity is an emergent thermodynamic effect, related to increasing entropy (or disorder). Then, as now, experts were uncertain what to make of the paper, though it inspired fruitful discussions.

    The particular brand of emergent gravity in Verlinde’s paper turned out not to be quite right, but he was tapping into the same intuition that led other theorists to develop the modern holographic description of emergent gravity and space-time — an approach that Verlinde has now absorbed into his new work.

    In this framework, bendy, curvy space-time and everything in it is a geometric representation of pure quantum information — that is, data stored in qubits. Unlike classical bits, qubits can exist simultaneously in two states (0 and 1) with varying degrees of probability, and they become “entangled” with each other, such that the state of one qubit determines the state of the other, and vice versa, no matter how far apart they are. Physicists have begun to work out the rules by which the entanglement structure of qubits mathematically translates into an associated space-time geometry. An array of qubits entangled with their nearest neighbors might encode flat space, for instance, while more complicated patterns of entanglement give rise to matter particles such as quarks and electrons, whose mass causes the space-time to be curved, producing gravity. “The best way we understand quantum gravity currently is this holographic approach,” said Mark Van Raamsdonk, a physicist at the University of British Columbia in Vancouver who has done influential work on the subject.

    The mathematical translations are rapidly being worked out for holographic universes with an Escher-esque space-time geometry known as anti-de Sitter (AdS) space, but universes like ours, which have de Sitter geometries, have proved far more difficult. In his new paper, Verlinde speculates that it’s exactly the de Sitter property of our native space-time that leads to the dark matter illusion.

    De Sitter space-times like ours stretch as you look far into the distance. For this to happen, space-time must be infused with a tiny amount of background energy — often called dark energy — which drives space-time apart from itself. Verlinde models dark energy as a thermal energy, as if our universe has been heated to an excited state. (AdS space, by contrast, is like a system in its ground state.) Verlinde associates this thermal energy with long-range entanglement between the underlying qubits, as if they have been shaken up, driving entangled pairs far apart. He argues that this long-range entanglement is disrupted by the presence of matter, which essentially removes dark energy from the region of space-time that it occupied. The dark energy then tries to move back into this space, exerting a kind of elastic response on the matter that is equivalent to a gravitational attraction.

    Because of the long-range nature of the entanglement, the elastic response becomes increasingly important in larger volumes of space-time. Verlinde calculates that it will cause galaxy rotation curves to start deviating from Newton’s inverse-square law at exactly the magic acceleration scale pinpointed by Milgrom in his original MOND theory.

    Van Raamsdonk calls Verlinde’s idea “definitely an important direction.” But he says it’s too soon to tell whether everything in the paper — which draws from quantum information theory, thermodynamics, condensed matter physics, holography and astrophysics — hangs together. Either way, Van Raamsdonk said, “I do find the premise interesting, and feel like the effort to understand whether something like that could be right could be enlightening.”

    One problem, said Brian Swingle of Harvard and Brandeis universities, who also works in holography, is that Verlinde lacks a concrete model universe like the ones researchers can construct in AdS space, giving him more wiggle room for making unproven speculations. “To be fair, we’ve gotten further by working in a more limited context, one which is less relevant for our own gravitational universe,” Swingle said, referring to work in AdS space. “We do need to address universes more like our own, so I hold out some hope that his new paper will provide some additional clues or ideas going forward.”

    Access mp4 video here .

    The Case for Dark Matter

    Verlinde could be capturing the zeitgeist the way his 2010 entropic-gravity paper did. Or he could be flat-out wrong. The question is whether his new and improved MOND can reproduce phenomena that foiled the old MOND and bolstered belief in dark matter.

    One such phenomenon is the Bullet cluster, a galaxy cluster in the process of colliding with another.

    X-ray photo by Chandra X-ray Observatory of the Bullet Cluster (1E0657-56). Exposure time was 0.5 million seconds (~140 hours) and the scale is shown in megaparsecs. Redshift (z) = 0.3, meaning its light has wavelengths stretched by a factor of 1.3. Based on today’s theories this shows the cluster to be about 4 billion light years away.
    In this photograph, a rapidly moving galaxy cluster with a shock wave trailing behind it seems to have hit another cluster at high speed. The gases collide, and gravitational fields of the stars and galalxies interact. When the galaxies collided, based on black-body temperture readings, the temperature reached 160 million degrees and X-rays were emitted in great intensity, claiming title of the hottest known galactic cluster.
    Studies of the Bullet cluster, announced in August 2006, provide the best evidence to date for the existence of dark matter.

    Superimposed mass density contours, caused by gravitational lensing of dark matter. Photograph taken with Hubble Space Telescope.
    Date 22 August 2006

    The visible matter in the two clusters crashes together, but gravitational lensing suggests that a large amount of dark matter, which does not interact with visible matter, has passed right through the crash site. Some physicists consider this indisputable proof of dark matter. However, Verlinde thinks his theory will be able to handle the Bullet cluster observations just fine. He says dark energy’s gravitational effect is embedded in space-time and is less deformable than matter itself, which would have allowed the two to separate during the cluster collision.

    But the crowning achievement for Verlinde’s theory would be to account for the suspected imprints of dark matter in the cosmic microwave background (CMB), ancient light that offers a snapshot of the infant universe.

    CMB per ESA/Planck
    CMB per ESA/Planck

    The snapshot reveals the way matter at the time repeatedly contracted due to its gravitational attraction and then expanded due to self-collisions, producing a series of peaks and troughs in the CMB data. Because dark matter does not interact, it would only have contracted without ever expanding, and this would modulate the amplitudes of the CMB peaks in exactly the way that scientists observe. One of the biggest strikes against the old MOND was its failure to predict this modulation and match the peaks’ amplitudes. Verlinde expects that his version will work — once again, because matter and the gravitational effect of dark energy can separate from each other and exhibit different behaviors. “Having said this,” he said, “I have not calculated this all through.”

    While Verlinde confronts these and a handful of other challenges, proponents of the dark matter hypothesis have some explaining of their own to do when it comes to McGaugh and his colleagues’ recent findings about the universal relationship between galaxy rotation speeds and their visible matter content.

    In October, responding to a preprint of the paper by McGaugh and his colleagues, two teams of astrophysicists independently argued that the dark matter hypothesis can account for the observations. They say the amount of dark matter in a galaxy’s halo would have precisely determined the amount of visible matter the galaxy ended up with when it formed. In that case, galaxies’ rotation speeds, even though they’re set by dark matter and visible matter combined, will exactly correlate with either their dark matter content or their visible matter content (since the two are not independent). However, computer simulations of galaxy formation do not currently indicate that galaxies’ dark and visible matter contents will always track each other. Experts are busy tweaking the simulations, but Arthur Kosowsky of the University of Pittsburgh, one of the researchers working on them, says it’s too early to tell if the simulations will be able to match all 153 examples of the universal law in McGaugh and his colleagues’ galaxy data set. If not, then the standard dark matter paradigm is in big trouble. “Obviously this is something that the community needs to look at more carefully,” Zurek said.

    Even if the simulations can be made to match the data, McGaugh, for one, considers it an implausible coincidence that dark matter and visible matter would conspire to exactly mimic the predictions of MOND at every location in every galaxy. “If somebody were to come to you and say, ‘The solar system doesn’t work on an inverse-square law, really it’s an inverse-cube law, but there’s dark matter that’s arranged just so that it always looks inverse-square,’ you would say that person is insane,” he said. “But that’s basically what we’re asking to be the case with dark matter here.”

    Given the considerable indirect evidence and near consensus among physicists that dark matter exists, it still probably does, Zurek said. “That said, you should always check that you’re not on a bandwagon,” she added. “Even though this paradigm explains everything, you should always check that there isn’t something else going on.”

    See the full article here .

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

  • richardmitnick 10:43 am on November 12, 2016 Permalink | Reply
    Tags: , , Erik Verlinde, ,   

    From EarthSky: “No need for dark matter?” 



    November 10, 2016
    Deborah Byrd

    Erik Verlinde just released the latest installment of his new theory of gravity. He now says he doesn’t need dark matter to explain the motions of stars in galaxies.

    Theoretical physicist Erik Verlinde has a new theory of gravity, which describes gravity not a force but as an illusion. The theory says gravity is an emergent phenomenon, possible to be derived from the microscopic building blocks that make up our universe’s entire existence. This week, he published the latest installment of his theory showing that – if he’s correct – there’s no need for dark matter to describe the motions of stars in galaxies.

    Verlinde, who is at the University of Amsterdam, first released his new theory in 2010. According to a statement released this week (November 8, 2016):

    … gravity is not a fundamental force of nature, but an emergent phenomenon. In the same way that temperature arises from the movement of microscopic particles, gravity emerges from the changes of fundamental bits of information, stored in the very structure of spacetime.

    Dark matter – the invisible “something” that most modern physicists believe makes up a substantial fraction of our universe – came to be necessary when astronomers found in the mid-20th century they couldn’t explain why stars in galaxies moved as they did. The outer parts of galaxies, including our own Milky Way, rotate much faster around their centers than they should, according to the theories of gravity as explained by Isaac Newton and Albert Einstein. According to these very accepted theories, there must be more mass in galaxies than that we can see, and thus scientists began speaking of invisible matter, which they called dark matter.

    They’ve been speaking of it, and trying to understand it, ever since.

    Verlinde is now saying we don’t need dark matter to explain what’s happening in galaxies. He says his new theory of gravity accurately predicts star velocities in the Milky Way and other galaxies. In his statement, he said:

    “We have evidence that this new view of gravity actually agrees with the observations. At large scales, it seems, gravity just doesn’t behave the way Einstein’s theory predicts.

    If true, it’s a revolution in science, since essentially all of modern cosmology – including the Big Bang theory that describes how our universe began – is based on Einstein’s theory of gravity. In recent decades, dark matter and its cousin dark energy have been bugaboos to the accepted theories; despite searches, for example, no one has ever actually observed dark matter.

    If Verlinde’s theory of gravity is true, it doesn’t mean Einstein’s theory is wrong, just as Einstein’s description of gravity didn’t exactly nullify Isaac Newton’s theory of gravity from two centuries before. Newton’s theory is still taught in physics classes, but Einstein’s theory was a refinement – a major one – in our way of thinking about gravity. Likewise, Verlinde’s theory, if correct, would be a refinement of Einstein’s ideas and a chance to have a deeper understanding of the way our universe works. Verlinde commented in his statement:

    “Many theoretical physicists like me are working on a revision of the [accepted modern theories of gravity], and some major advancements have been made. We might be standing on the brink of a new scientific revolution that will radically change our views on the very nature of space, time and gravity.”

    See the full article here .

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