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  • richardmitnick 12:30 pm on August 2, 2020 Permalink | Reply
    Tags: "An Alternative to Dark Matter Passes Critical Test", , , , , , , MOND, ,   

    From Quanta Magazine via Nautilus: “An Alternative to Dark Matter Passes Critical Test” 

    From Quanta Magazine

    Via

    Nautilus

    July 2020
    Charlie Wood

    1
    A view of the center of the Milky Way galaxy. Theories of modified gravity have had a hard time describing the universe from relatively small scales like this all the way up to the scale of the universe as a whole. Credit: NASA/JPL-Caltech/ESA/CXC/STScI

    For decades, a band of rebel theorists has waged war with one of cosmology’s core concepts—the idea that an invisible, intangible form of matter forms the universe’s primary structure. This Dark Matter [see below, “Dark Matter Background” ], which seems to outweigh the stuff we’re made of 5-to-1, accounts for a host of observations: the tight cohesion of galaxies and packs of galaxies, the way light from faraway galaxies will bend on its way to terrestrial telescopes, and the mottled structure of the early universe, to name a few.

    The would-be revolutionaries seek an alternative cosmic recipe. In place of dark matter, they substitute a subtly modified force of gravity. But attempts to translate their rough idea into precise mathematical language have always run afoul of at least one key observation. Some formulations get galaxies right, some get the contortion of light rays right, but none have pierced dark matter’s most bulletproof piece of evidence: precise maps of ancient light, known as the cosmic microwave background (CMB).

    CMB per ESA/Planck

    ESA/Planck 2009 to 2013

    “A theory must do really well to agree with this data,” said Ruth Durrer, a cosmologist at the University of Geneva. “This is the bottleneck.”

    Now, two theorists say they’ve finally squeezed an alternative theory of gravity past that obstacle. Their work, which was posted online [“A new relativistic theory for Modified Newtonian Dynamics” ( https://arxiv.org/abs/2007.00082 )] in late June 2020 and has not yet passed peer review, uses a tweaked version of Einstein’s theory of gravity to reproduce an iconic map of the early universe, a feat that even some rebels feared to be impossible. “For 15 years we’ve just been dead in the water,” said Stacy McGaugh, an astronomer at Case Western Reserve University and a longtime advocate for modified-gravity theories who was not involved in the research. “It’s a huge leap forward.”

    Others agree that the model’s preliminary results appear promising. “It’s a bit baroque, but since nothing else has worked so far, I’m still impressed that it seems to work,” Durrer said.

    Most cosmologists still prefer dark matter as the simpler of the two paradigms, but they agree that the new theory could be intriguing—if it can truly match additional cosmological observations. “That would be a big barrier,” said Dan Hooper, an astrophysicist at the University of Chicago. “That would be pretty interesting.”

    Threading the Needle

    The challenges for alternative gravity theories, collectively known as modified Newtonian dynamics or MOND, were spelled out in a separate preprint [“What is the price of abandoning dark matter? Cosmological constraints on alternative gravity theories” ( https://arxiv.org/abs/2007.00555 ) ] coincidentally published the day after the latest model appeared.

    Mordehai Milgrom, MOND theorist, is an Israeli physicist and professor in the department of Condensed Matter Physics at the Weizmann Institute in Rehovot, Israel http://cosmos.nautil.us

    MOND Rotation Curves with MOND Tully-Fisher

    MOND Modified Newtonian Dynamics a Humble Introduction Marcus Nielbock

    MOND UMD

    Chief among them is recasting the leading role dark matter plays in drawing the universe together, as described by a well-established cosmological model known as Lambda cold dark matter (LCDM).

    Lamda Cold Dark Matter Accerated Expansion of The universe http scinotions.com the-cosmic-inflation-suggests-the-existence-of-parallel-universes
    Alex Mittelmann, Coldcreation

    Simply put, LCDM says that we wouldn’t be here without dark matter. The infant universe was so smooth that the gravitational attraction of ordinary matter alone wouldn’t have been enough to gather particles into galaxies, stars and planets. Enter dark matter particles. LCDM uses their collective bulk to sculpt normal matter into the modern cosmic structures studied by astronomers.

    LCDM became the standard model of cosmology in part because it so precisely agrees with the CMB. This map of the early universe shows almost imperceptibly thick and thin spots rippling through the cosmos. More recently, researchers have been able to measure the orientation or polarization of the CMB’s light more precisely. Any successful cosmology will need to establish a comprehensive history of the cosmos by reproducing these three observations: the CMB’s temperature, the CMB’s polarization, and the current distribution of galaxies and galaxy clusters.

    2
    Lucy Reading-Ikkanda/Quanta Magazine; source: doi: 1303.5076v3

    In the second preprint, Kris Pardo, an astrophysicist at NASA’s Jet Propulsion Laboratory, and David Spergel, director of the Center for Computational Astrophysics at the Flatiron Institute, quantified how difficult it would be for any alternative theory of gravity to compete with one particular feature of LCDM. (Quanta Magazine is an editorially independent publication sponsored by the Simons Foundation, which also funds the Flatiron Institute.) When denser zones of dark matter dragged matter toward them, eventually forming galaxies and stars, this would have largely—but not entirely—washed out the ripples initially moving through the matter. By comparing the CMB’s polarization with today’s patterns of matter, cosmologists can cleanly measure just such an effect: ripple remnants 100 times smaller than the undulations seen in the CMB persist today.

    Re-creating these and other features without LCDM’s titular ingredient, Spergel showed, requires the finest of theoretical needle threading. “We haven’t disproven the existence of all these [modified-gravity theories],” he said. “But any alternative theory has to jump through these hoops.”

    Dark Dust

    Tom Złosnik and Constantinos Skordis, theorists at the Central European Institute for Cosmology and Fundamental Physics, believe they’ve done just that—although in a way that might surprise MOND skeptics and fans alike. They managed to construct a theory of gravity that contains an ingredient that acts exactly like an invisible form of matter on cosmic scales, blurring the line between the dark matter and MOND paradigms.

    Their theory, dubbed RelMOND, adds to the equations of general relativity an omnipresent field that behaves differently in different arenas. On the grandest scales, where the universe noticeably stretches as it expands, the field acts like invisible matter. In this mode, which Złosnik refers to as “dark dust,” the field could have shaped the visible universe just as dark matter would. The model faithfully reproduces the temperature of the CMB—the result that the duo published in their preprint—and Złosnik says it can also match the polarization spectrum and the matter distribution, although they have not yet published these plots.

    “[RelMOND] cannot do worse than LCDM,” said Złosnik, because it very closely mimics that theory for the universe as a whole.

    But if we zoom in on a galaxy, where the fabric of space holds rather still, the field acts in a way that’s true to its MOND roots: It entwines itself with the standard gravitational field, beefing it up just enough to hold a galaxy together without extra matter. (The researchers aren’t yet sure how the field acts for larger clusters of galaxies, a perennial MOND sore spot, and they suggest that this intermediate scale might be a good place to look for observational clues that could set the theory apart.)

    Despite the pair’s mathematical achievement, dark matter remains the simpler theory. Constructing the new field takes four new moving mathematical parts, while LCDM handles dark matter with just one. Hooper likens the situation to a detective debating whether the person at a murder scene is the murderer, or if they were framed by the CIA. Even if the available evidence matches both theories, one requires less of a leap.

    All the same, he doesn’t begrudge others working on what he considers a cosmological conspiracy theory. “I’m glad smart people are thinking about MOND,” he said.

    Złosnik hopes dark matter will be detected soon, but in the meantime, he sees his work on MOND more as an exercise in stretching general relativity to its limits than as a full assault on the cosmological establishment. For now, he’s just pleased to have helped show that the mathematics of gravity may accommodate weirder phenomena than many thought.

    “There’s a danger of missing out on something useful just by assuming that it’s not possible,” Złosnik said. “It might point the way to something a bit more successful.”

    ______________________________________________

    Dark Matter Background
    Fritz Zwicky discovered Dark Matter in the 1930s when observing the movement of the Coma Cluster., Vera Rubin a Woman in STEM denied the Nobel, did most of the work on Dark Matter.

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

    Coma cluster via NASA/ESA Hubble

    In modern times, it was astronomer Fritz Zwicky, in the 1930s, who made the first observations of what we now call dark matter. His 1933 observations of the Coma Cluster of galaxies seemed to indicated it has a mass 500 times more than that previously calculated by Edwin Hubble. Furthermore, this extra mass seemed to be completely invisible. Although Zwicky’s observations were initially met with much skepticism, they were later confirmed by other groups of astronomers.

    Thirty years later, astronomer Vera Rubin provided a huge piece of evidence for the existence of dark matter. She discovered that the centers of galaxies rotate at the same speed as their extremities, whereas, of course, they should rotate faster. Think of a vinyl LP on a record deck: its center rotates faster than its edge. That’s what logic dictates we should see in galaxies too. But we do not. The only way to explain this is if the whole galaxy is only the center of some much larger structure, as if it is only the label on the LP so to speak, causing the galaxy to have a consistent rotation speed from center to edge.

    Vera Rubin, following Zwicky, postulated that the missing structure in galaxies is dark matter. Her ideas were met with much resistance from the astronomical community, but her observations have been confirmed and are seen today as pivotal proof of the existence of dark matter.

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


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


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

    The Vera C. Rubin Observatory currently under construction on the El Peñón peak at Cerro Pachón Chile, a 2,682-meter-high mountain in Coquimbo Region, in northern Chile, alongside the existing Gemini South and Southern Astrophysical Research Telescopes.

    LSST Data Journey, Illustration by Sandbox Studio, Chicago with Ana Kova

    See the full article here .


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

    Please help promote STEM in your local schools.

    Stem Education Coalition

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

    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 11:53 am on July 28, 2020 Permalink | Reply
    Tags: "What dark matter is (probably) not", , , , , , MOND, ,   

    From Symmetry: “What dark matter is (probably) not” 

    Symmetry Mag
    From Symmetry<

    07/28/20
    Meredith Fore

    No one knows for sure what dark matter is. But we know we need something to explain what we see in the universe, and we’ve crossed a few ideas off of our list.

    1
    Illustration by Sandbox Studio, Chicago with Corinne Mucha

    For a recent YouTube video, a theorist with a postdoctoral fellowship at CERN named Dorota Grabowska discussed dark matter with popular science figure Dianna Cowern (more commonly known as Physics Girl). Below the video, a viewer typed the comment, “Dark Matter is science speak for ‘we don’t have a clue.’”

    Grabowska says she was surprised at the number of commenters who seemed to believe physicists knew nothing about the subject.

    “They were saying, ‘Oh, it’s just a miscounting of space dust’ or, ‘We really just don’t understand magnetism,’” Grabowska says. “People weren’t realizing the steps that physicists go through to actually test a hypothesis and say, ‘This doesn’t work; let’s think differently.’”

    2

    The birth of a dark matter theory

    Science hypotheses often come from an abnormal observation—something that appears unexpectedly or acts in an unexpected way. Three commonly cited unusual astrophysical observations credited to dark matter are abnormal galaxy rotation speeds, patterns in the large-scale structure of the universe, and the Bullet Cluster.

    ________________________________________________

    Abnormal galaxy rotation speeds

    Based on the visible matter in galaxies, physicists can use the laws of gravity to predict how quickly stars revolve around galaxies. The prediction was that matter closest to the center of a galaxy would move the fastest, with matter farther from the center feeling less of the pull of gravity and moving more slowly.

    However, astronomers first noticed in the early 20th century that the opposite was happening: The matter in the outer regions of galaxies was moving much faster than expected [see below Dark Matter Background]. Finding these abnormal dynamics was one of the first hints that we might be missing something. If invisible matter makes up much of the mass of a galaxy, then the faster-than-expected regions can be explained.

    ________________________________________________

    Large-scale structure

    The hot early universe was in part made up of charged nuclei and electrons. Photons could drag on these particles, preventing growth. But dark matter was immune to this interaction and, with the help of gravity, began to form its own structures. When the universe cooled enough that atoms could form, these atoms (“baryons”) were pulled into the dark matter structures, falling into their gravitational potential wells. This has left a signature in the sky called baryon acoustic oscillations, reminiscent of the ripples of a stone splashing into water.

    Comparing the predictions of a dark matter theory to observational data from these oscillations, called BAOs, and the overall shape of our universe’s “cosmic web” are necessary tests of the theory’s strength.

    ________________________________________________

    The Bullet Cluster

    Bullet Cluster NASA Chandra NASA ESA Hubble, evidence of shock

    The Bullet Cluster is a cluster of galaxies created by a collision between two other galaxy clusters. The clusters interacted gravitationally, causing the matter in both to slow down as they passed through each other. The gas from each collided, heating up and creating a bright signal in X-rays.

    But while there are two bright X-ray clusters of galaxies near the point of collision, a significant amount of mass is located in two dark “lobes” even farther from the center. It’s like the galaxies collided, but some of their matter didn’t care and kept going unimpeded. Dark matter seems to be passing through regular matter like a ghost.

    The Bullet Cluster is considered a crucial piece of evidence for the existence of dark matter. It is famous for being the first galaxy merger to be deeply studied, but astronomers have since found many more galaxy mergers that show the same dark “lobe” structure.

    ________________________________________________

    To be viable, any theory of dark matter must adequately explain all three of these phenomena. “The baton gets passed from the astrophysical observers to the theorists, to the experimentalists,” Grabowska says.

    Observers announce an unexpected phenomenon to theorists, who think of a possible explanation for it; experimentalists then devise a way to test that explanation through detection methods such as looking for a gravitational signature or a particle interaction. “It’s this massively collaborative process where no one really has the one way to look at it. You need everyone to put the whole picture together.”

    Scientists may not yet know what the explanation for these unusual astronomical observations is, but they do have some idea what it is not.

    Disfavored: The theories that have fallen behind

    Grabowska is careful to note that it’s difficult to completely throw away a theory of dark matter. The range of possible masses for a dark matter particle spans many orders of magnitude, all the way from the axion (its mass a tiny fraction of that of a proton) to stellar black holes (thousands of times more massive than the sun).

    “It is much easier to say you know what dark matter could be than what dark matter can’t be,” she says. “You can’t say a candidate is disfavored until you actually look for it, and dark matter is very hard to look for.”

    One thing physicists are sure of? They haven’t miscounted any mass.

    Galaxy rotation curves could be explained if there were simply regular matter that observers didn’t measure or account for. Some theories posited that perhaps there was more regular mass out there, but it consisted of things like brown dwarfs, burnt-out stars, and small, ancient black holes that are too dim to see properly. Scientists posited that these massive compact halo objects (or “MACHOs”) could make up the missing matter.

    But not only do MACHOs not explain other phenomena that point to the existence of dark matter, widespread searches for these objects have not found enough of them to represent all of dark matter.

    Another set of dark matter theories that many scientists consider to have fallen short is modified Newtonian dynamics (or “MOND”).

    Mordehai Milgrom, MOND theorist, is an Israeli physicist and professor in the department of Condensed Matter Physics at the Weizmann Institute in Rehovot, Israel http://cosmos.nautil.us

    These theories propose a modification to Newton’s laws of motion that would apply only at very small accelerations, which would adequately explain the abnormal rotation speed of galaxies. But like MACHOs, these theories are inadequate to explain BAOs or the dynamics of larger-scale galaxy clusters. They also struggle to describe the Bullet Cluster.

    Some of the strongest candidates for dark matter—which would explain abnormal galaxy rotation speeds, the Bullet Cluster, BAOs and the large-scale structure of the universe—are weakly interacting particles (or “WIMPs”). But so far, thorough, decades-long searches have not yet turned up any evidence for the existence of WIMP dark matter candidates; instead, they have set stricter and stricter limits on their possible properties.

    The range of viable candidates for a dark matter particle is still vast. It includes axions, sterile neutrinos and WIMPs with rare interactions or extremely low masses.

    Expanding the field even further is the possibility that there isn’t just one answer to dark matter: It could be a combination of different types of particles or objects, or there could be an entire “dark sector” of dark matter particles, like a mirror-universe version of the Standard Model.

    Definitively disproving a dark matter theory or a specific dark matter candidate is challenging—when do you stop searching for a particle that could have nearly any mass? But the breadth of evidence for dark matter allows for very thorough vetting. A dark matter theory has so much to explain; and as experiments get more precise and telescopes get sharper, we may be able to cross a few more off our list.

    ________________________________________________

    Dark Matter Background
    Fritz Zwicky discovered Dark Matter in the 1930s when observing the movement of the Coma Cluster., Vera Rubin a Woman in STEM denied the Nobel, did most of the work on Dark Matter.

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

    Coma cluster via NASA/ESA Hubble

    In modern times, it was astronomer Fritz Zwicky, in the 1930s, who made the first observations of what we now call dark matter. His 1933 observations of the Coma Cluster of galaxies seemed to indicated it has a mass 500 times more than that previously calculated by Edwin Hubble. Furthermore, this extra mass seemed to be completely invisible. Although Zwicky’s observations were initially met with much skepticism, they were later confirmed by other groups of astronomers.

    Thirty years later, astronomer Vera Rubin provided a huge piece of evidence for the existence of dark matter. She discovered that the centers of galaxies rotate at the same speed as their extremities, whereas, of course, they should rotate faster. Think of a vinyl LP on a record deck: its center rotates faster than its edge. That’s what logic dictates we should see in galaxies too. But we do not. The only way to explain this is if the whole galaxy is only the center of some much larger structure, as if it is only the label on the LP so to speak, causing the galaxy to have a consistent rotation speed from center to edge.

    Vera Rubin, following Zwicky, postulated that the missing structure in galaxies is dark matter. Her ideas were met with much resistance from the astronomical community, but her observations have been confirmed and are seen today as pivotal proof of the existence of dark matter.

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


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


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

    The Vera C. Rubin Observatory currently under construction on the El Peñón peak at Cerro Pachón Chile, a 2,682-meter-high mountain in Coquimbo Region, in northern Chile, alongside the existing Gemini South and Southern Astrophysical Research Telescopes.

    LSST Data Journey, Illustration by Sandbox Studio, Chicago with Ana Kova

    See the full article here .


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

    Please help promote STEM in your local schools.


    Stem Education Coalition

    Symmetry is a joint Fermilab/SLAC publication.


     
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