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  • richardmitnick 12:55 pm on September 14, 2017 Permalink | Reply
    Tags: , , , Cold Dark Matter, , , Dark matter may have strong self-interactions, Physicists Offer Explanation for Diverse Galaxy Rotations, ,   

    From UC Riverside: “Physicists Offer Explanation for Diverse Galaxy Rotations” 

    UC Riverside bloc

    UC Riverside

    September 14, 2017
    Iqbal Pittalwala

    Hai-Bo Yu is an assistant professor of theoretical particle physics and astrophysics at UC Riverside. Photo credit: I. Pittalwala, UC Riverside.

    Identical twins are similar to each other in many ways, but they have different experiences, friends, and lifestyles.

    This concept is played out on a cosmological scale by galaxies. Two galaxies that appear at first glance to be very similar and effectively identical can have inner regions rotating at very different rates – the galactic analog of twins with different lifestyles.

    A team of physicists, led by Hai-Bo Yu of the University of California, Riverside, has found a simple and viable explanation for this diversity.

    Every galaxy sits within a dark matter halo that forms the gravitational scaffolding holding it together.

    Dark matter halo Image credit: Virgo consortium / A. Amblard / ESA

    The distribution of dark matter in this halo can be inferred from the motion of stars and gas particles in the galaxy.

    Yu and colleagues report in Physical Review Letters that diverse galactic-rotation curves, a graph of rotation speeds at different distances from the center, can be naturally explained if dark matter particles are assumed to strongly collide with one another in the inner halo, close to the galaxy’s center – a process called dark matter self-interaction.

    “In the prevailing dark matter theory, called Cold Dark Matter or CDM, dark matter particles are assumed to be collisionless, aside from gravity,” said Yu, an assistant professor of theoretical particle physics and astrophysics, who led the research. “We invoke a different theory, the self-interacting dark matter model or SIDM, to show that dark matter self-interactions thermalize the inner halo, which ties ordinary matter and dark matter distributions together so that they behave like a collective unit. The self-interacting dark matter halo then becomes flexible enough to accommodate the observed diverse rotation curves.”

    Yu explained that the dark matter collisions take place in the dense inner halo, where the luminous galaxy is located. When the particles collide, they exchange energy and thermalize. For low-luminous galaxies, the thermalization process heats up the inner dark matter particles and pushes them out of the central region, reducing the density, analogous to a popcorn machine in which kernels hit each other as they pop, causing them to fly up from the bottom of the machine. For high-luminous galaxies such as the Milky Way, thermalization pulls the particles into the deep potential well of the luminous matter and increases the dark matter density. In addition, the cosmological assembly history of halos also plays a role in generating the observed diversity.

    “Our work demonstrates that dark matter may have strong self-interactions, a radical deviation from the prevailing theory,” Yu said. “It well explains the observed diversity of galactic rotating curves, while being consistent with other cosmological observations.”

    Dark matter makes up about 85 percent of matter in the universe, but its nature remains largely unknown despite its unmistakable gravitational imprint on astronomical and cosmological observations. The conventional way to study dark matter is to assume that it has some additional, nongravitational interaction with visible matter that can be studied in the lab. Physicists do not know, however, if such an interaction between dark and visible matter even exists.

    Over the last decade, Yu has pioneered a new line of research based on the following premise: Setting aside whether dark matter interacts with visible matter, what happens if dark matter interacts with itself through some new dark force?

    Yu posited the new dark force would affect the dark matter distribution in each galaxy’s halo. He realized that there is indeed a discrepancy between CDM and astronomical observations that could be solved if dark matter is self-interacting.

    “The compatibility of this hypothesis with observations is a major advance in the field,” said Flip Tanedo, an assistant professor of theoretical particle physics at UC Riverside, who was not involved in the research. “The SIDM paradigm is a bridge between fundamental particle physics and observational astronomy. The consistency with observations is a big hint that this proposal has a chance of being correct and lays the foundation for future observational, experimental, numerical, and theoretical work. In this way, it is paving the way to new interdisciplinary research.”

    SIDM was first proposed in 2000 by a pair of eminent astrophysicists. It experienced a revival in the particle physics community around 2009, aided in part by key work by Yu and collaborators.

    “This is a special time for this type of research because numerical simulations of galaxies are finally approaching a precision where they can make concrete predictions to compare the observational predictions of the self-interacting versus cold dark matter scenarios,” Tanedo said. “In this way, Hai-Bo is the architect of modern self-interacting dark matter and how it merges multiple different fields: theoretical high-energy physics, experimental high-energy physics, observational astronomy, numerical simulations of astrophysics, and early universe cosmology and galaxy formation.”

    The research paper is included by Physical Review Letters as a “Editor’s Suggestion” and featured also in APS Physics.

    Yu was joined in the research by Ayuki Kamada, a postdoctoral researcher at UCR; and UC Irvine’s Manoj Kaplinghat and Andrew B. Pace.

    Yu’s research was supported by grants from the U.S. Department of Energy and the Hellman Fellows Fund. The National Science Foundation provided the research team with additional funding.

    See the full article here .

    [This article would have been helped with examples of galaxies.

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    UC Riverside Campus

    The University of California, Riverside is one of 10 universities within the prestigious University of California system, and the only UC located in Inland Southern California.

    Widely recognized as one of the most ethnically diverse research universities in the nation, UCR’s current enrollment is more than 21,000 students, with a goal of 25,000 students by 2020. The campus is in the midst of a tremendous growth spurt with new and remodeled facilities coming on-line on a regular basis.

    We are located approximately 50 miles east of downtown Los Angeles. UCR is also within easy driving distance of dozens of major cultural and recreational sites, as well as desert, mountain and coastal destinations.

  • richardmitnick 10:22 am on July 23, 2017 Permalink | Reply
    Tags: , , , , Cold Dark Matter, , , , FDM-Fuzzy Dark Matter, Lyman-alpha forest   

    From Astro Watch: “Flashes of Light on the Dark Matter” 

    Astro Watch bloc

    Astro Watch

    July 23, 2017
    No writer credit found


    A web that passes through infinite intergalactic spaces, a dense cosmic forest illuminated by very distant lights and a huge enigma to solve. These are the picturesque ingredients of a scientific research – carried out by an international team composed of researchers from the International School for Adavnced Studies (SISSA) and the Abdus Salam International Center for Theoretical Physics (ICTP) in Trieste, the Institute of Astronomy of Cambridge and the University of Washington – that adds an important element for understanding one of the fundamental components of our Universe: the dark matter.

    In order to study its properties, scientists analyzed the interaction of the “cosmic web” – a network of filaments made up of gas and dark matter present in the whole Universe – with the light coming from very distant quasars and galaxies. Photons interacting with the hydrogen of the cosmic filaments create many absorption lines defined “Lyman-alpha forest”. This microscopic interaction succeeds in revealing several important properties of the dark matter at cosmological distances. The results further support the theory of Cold Dark Matter, which is composed of particles that move very slowly. Moreover, for the first time, they highlight the incompatibility with another model, i.e. the Fuzzy Dark Matter, for which dark matter particles have larger velocities. The research was carried out through simulations performed on international parallel supercomputers and has recently been published in Physical Review Letters.

    Although constituting an important part of our cosmos, the dark matter is not directly observable, it does not emit electromagnetic radiation and it is visible only through gravitational effects. Besides, its nature remains a deep mystery. The theories that try to explore this aspect are various. In this research, scientists investigated two of them: the so-called Cold Dark Matter, considered a paradigm of modern cosmology, and an alternative model called Fuzzy Dark Matter (FDM), in which the dark matter is deemed composed of ultralight bosons provided with a non-negligible pressure at small scales. To carry out their investigations, scientists examined the cosmic web by analyzing the so-called Lyman-alpha forest. The Lyman-alpha forest consists of a series of absorption lines produced by the light coming from very distant and extremely luminous sources, that passes through the intergalactic space along its way toward the earth’s telescopes. The atomic interaction of photons with the hydrogen present in the cosmic filaments is used to study the properties of the cosmos and of the dark matter at enormous distances.

    Through simulations carried out with supercomputers, researchers reproduced the interaction of the light with the cosmic web. Thus they were able to infer some of the characteristics of the particles that compose the dark matter. More in particular, evidence showed for the first time that the mass of the particles, which allegedly compose the dark matter according to the FDM model, is not consistent with the Lyman-alpha Forest observed by the Keck telescope (Hawaii, US) and the Very Large Telescope (European Southern Observatory, Chile).

    Keck Observatory, Maunakea, Hawaii, USA

    ESO/VLT at Cerro Paranal, with an elevation of 2,635 metres (8,645 ft) above sea level

    Basically, the study seems not to confirm the theory of the Fuzzy Dark Matter. The data, instead, support the scenario envisaged by the model of the Cold Dark Matter.

    The results obtained – scientists say – are important as they allow to build new theoretical models for describing the dark matter and new hypotheses on the characteristics of the cosmos. Moreover, these results can provide useful indications for the realization of experiments in laboratories and can guide observational efforts aimed at making progress on this fascinating scientific theme.

    See the full article here .

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  • richardmitnick 5:00 pm on June 13, 2017 Permalink | Reply
    Tags: A different kind of dark matter could help to resolve an old celestial conundrum, , , , Cold Dark Matter, , , Dark matter superfluid, Dark matter vortices, , Kent Ford, , ,   

    From Quanta: “Dark Matter Recipe Calls for One Part Superfluid” 

    Quanta Magazine
    Quanta Magazine

    June 13, 2017
    Jennifer Ouellette

    A different kind of dark matter could help to resolve an old celestial conundrum.

    Markos Kay for Quanta Magazine

    For years, dark matter has been behaving badly. The term was first invoked nearly 80 years ago by the astronomer Fritz Zwicky, who realized that some unseen gravitational force was needed to stop individual galaxies from escaping giant galaxy clusters. Later, Vera Rubin and Kent Ford used unseen dark matter to explain why galaxies themselves don’t fly apart.

    Yet even though we use the term “dark matter” to describe these two situations, it’s not clear that the same kind of stuff is at work. The simplest and most popular model holds that dark matter is made of weakly interacting particles that move about slowly under the force of gravity. This so-called “cold” dark matter accurately describes large-scale structures like galaxy clusters. However, it doesn’t do a great job at predicting the rotation curves of individual galaxies. Dark matter seems to act differently at this scale.

    In the latest effort to resolve this conundrum, two physicists have proposed that dark matter is capable of changing phases at different size scales. Justin Khoury, a physicist at the University of Pennsylvania, and his former postdoc Lasha Berezhiani, who is now at Princeton University, say that in the cold, dense environment of the galactic halo, dark matter condenses into a superfluid — an exotic quantum state of matter that has zero viscosity. If dark matter forms a superfluid at the galactic scale, it could give rise to a new force that would account for the observations that don’t fit the cold dark matter model. Yet at the scale of galaxy clusters, the special conditions required for a superfluid state to form don’t exist; here, dark matter behaves like conventional cold dark matter.

    “It’s a neat idea,” said Tim Tait, a particle physicist at the University of California, Irvine. “You get to have two different kinds of dark matter described by one thing.” And that neat idea may soon be testable. Although other physicists have toyed with similar ideas, Khoury and Berezhiani are nearing the point where they can extract testable predictions that would allow astronomers to explore whether our galaxy is swimming in a superfluid sea.

    Impossible Superfluids

    Here on Earth, superfluids aren’t exactly commonplace. But physicists have been cooking them up in their labs since 1938. Cool down particles to sufficiently low temperatures and their quantum nature will start to emerge. Their matter waves will spread out and overlap with one other, eventually coordinating themselves to behave as if they were one big “superatom.” They will become coherent, much like the light particles in a laser all have the same energy and vibrate as one. These days even undergraduates create so-called Bose-Einstein condensates (BECs) in the lab, many of which can be classified as superfluids.

    Superfluids don’t exist in the everyday world — it’s too warm for the necessary quantum effects to hold sway. Because of that, “probably ten years ago, people would have balked at this idea and just said ‘this is impossible,’” said Tait. But recently, more physicists have warmed to the possibility of superfluid phases forming naturally in the extreme conditions of space. Superfluids may exist inside neutron stars, and some researchers have speculated that space-time itself may be a superfluid. So why shouldn’t dark matter have a superfluid phase, too?

    To make a superfluid out of a collection of particles, you need to do two things: Pack the particles together at very high densities and cool them down to extremely low temperatures. In the lab, physicists (or undergraduates) confine the particles in an electromagnetic trap, then zap them with lasers to remove the kinetic energy and lower the temperature to just above absolute zero.

    Lucy Reading-Ikkanda/Quanta Magazine

    The dark matter particles that would make Khoury and Berezhiani’s idea work are emphatically not WIMP-like. WIMPs should be pretty massive as fundamental particles go — about as massive as 100 protons, give or take. For Khoury’s scenario to work, the dark matter particle would have to be a billion times less massive. Consequently, there should be billions of times as many of them zipping through the universe — enough to account for the observed effects of dark matter and to achieve the dense packing required for a superfluid to form. In addition, ordinary WIMPs don’t interact with one another. Dark matter superfluid particles would require strongly interacting particles.

    The closest candidate is the axion, a hypothetical ultralight particle with a mass that could be 10,000 trillion trillion times as small as the mass of the electron. According to Chanda Prescod-Weinstein, a theoretical physicist at the University of Washington, axions could theoretically condense into something like a Bose-Einstein condensate.

    But the standard axion doesn’t quite fit Khoury and Berezhiani’s needs. In their model, particles would need to experience a strong, repulsive interaction with one another. Typical axion models have interactions that are both weak and attractive. That said, “I think everyone thinks that dark matter probably does interact with itself at some level,” said Tait. It’s just a matter of determining whether that interaction is weak or strong.

    Cosmic Superfluid Searches

    The next step for Khoury and Berezhiani is to figure out how to test their model — to find a telltale signature that could distinguish this superfluid concept from ordinary cold dark matter. One possibility: dark matter vortices. In the lab, rotating superfluids give rise to swirling vortices that keep going without ever losing energy. Superfluid dark matter halos in a galaxy should rotate sufficiently fast to also produce arrays of vortices. If the vortices were massive enough, it would be possible to detect them directly.

    Inside galaxies, the role of the electromagnetic trap would be played by the galaxy’s gravitational pull, which could squeeze dark matter together enough to satisfy the density requirement. The temperature requirement is easier: Space, after all, is naturally cold.

    Outside of the “halos” found in the immediate vicinity of galaxies, the pull of gravity is weaker, and dark matter wouldn’t be packed together tightly enough to go into its superfluid state. It would act as dark matter ordinarily does, explaining what astronomers see at larger scales.

    But what’s so special about having dark matter be a superfluid? How can this special state change the way that dark matter appears to behave? A number of researchers over the years have played with similar ideas. But Khoury’s approach is unique because it shows how the superfluid could give rise to an extra force.

    In physics, if you disturb a field, you’ll often create a wave. Shake some electrons — for instance, in an antenna — and you’ll disturb an electric field and get radio waves. Wiggle the gravitational field with two colliding black holes and you’ll create gravitational waves. Likewise, if you poke a superfluid, you’ll produce phonons — sound waves in the superfluid itself. These phonons give rise to an extra force in addition to gravity, one that’s analogous to the electrostatic force between charged particles. “It’s nice because you have an additional force on top of gravity, but it really is intrinsically linked to dark matter,” said Khoury. “It’s a property of the dark matter medium that gives rise to this force.” The extra force would be enough to explain the puzzling behavior of dark matter inside galactic halos.

    A Different Dark Matter Particle

    Dark matter hunters have been at work for a long time. Their efforts have focused on so-called weakly interacting massive particles, or WIMPs. WIMPs have been popular because not only would the particles account for the majority of astrophysical observations, they pop out naturally from hypothesized extensions of the Standard Model of particle physics.

    Yet no one has ever seen a WIMP, and those hypothesized extensions of the Standard Model haven’t shown up in experiments either, much to physicists’ disappointment.

    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.

    With each new null result, the prospects dim even more, and physicists are increasingly considering other dark matter candidates. “At what point do we decide that we’ve been barking up the wrong tree?” said Stacy McGaugh, an astronomer at Case Western Reserve University.

    Unfortunately, this is unlikely to be the case: Khoury’s most recent computer simulations suggest that vortices in the dark matter superfluid would be “pretty flimsy,” he said, and unlikely to offer researchers clear-cut evidence that they exist. He speculates it might be possible to exploit the phenomenon of gravitational lensing to see if there are any scattering effects, similar to how a crystal will scatter X-ray light that passes through it.

    Gravitational Lensing NASA/ESA

    Astronomers could also search for indirect evidence that dark matter behaves like a superfluid. Here, they’d look to galactic mergers.

    The rate that galaxies collide with one another is influenced by something called dynamical friction. Imagine a massive body passing through a sea of particles. Many of the small particles will get pulled along by the massive body. And since the total momentum of the system can’t change, the massive body must slow down a bit to compensate.

    That’s what happens when two galaxies start to merge. If they get sufficiently close, their dark matter halos will start to pass through each other, and the rearrangement of the independently moving particles will give rise to dynamical friction, pulling the halos even closer. The effect helps galaxies to merge, and works to increase the rate of galactic mergers across the universe.

    But if the dark matter halo is in a superfluid phase, the particles move in sync. There would be no friction pulling the galaxies together, so it would be more difficult for them to merge. This should leave behind a telltale pattern: rippling interference patterns in how matter is distributed in the galaxies.

    Perfectly Reasonable Miracles

    While McGaugh is mostly positive about the notion of superfluid dark matter, he confesses to a niggling worry that in trying so hard to combine the best of both worlds, physicists might be creating what he terms a “Tycho Brahe solution.” The 16th-century Danish astronomer invented a hybrid cosmology in which the Earth was at the center of the universe but all the other planets orbited the sun. It attempted to split the difference between the ancient Ptolemaic system and the Copernican cosmology that would eventually replace it. “I worry a little that these kinds of efforts are in that vein, that maybe we’re missing something more fundamental,” said McGaugh. “But I still think we have to explore these ideas.”

    Tait admires this new superfluid model intellectually, but he would like to see the theory fleshed out more at the microscopic level, to a point where “we can really calculate everything and show why it all works out the way it’s supposed to. At some level, what we’re doing now is invoking a few miracles” in order to get everything to fit into place, he said. “Maybe they’re perfectly reasonable miracles, but I’m not fully convinced yet.”

    One potential sticking point is that Khoury and Berezhiani’s concept requires a very specific kind of particle that acts like a superfluid in just the right regime, because the kind of extra force produced in their model depends upon the specific properties of the superfluid. They are on the hunt for an existing superfluid — one created in the lab — with those desired properties. “If you could find such a system in nature, it would be amazing,” said Khoury, since this would essentially provide a useful analog for further exploration. “You could in principle simulate the properties of galaxies using cold atoms in the lab to mimic how superfluid dark matter behaves.”

    While researchers have been playing with superfluids for many decades, particle physicists are only just beginning to appreciate the usefulness of some of the ideas coming from subjects like condensed matter physics. Combining tools from those disciplines and applying it to gravitational physics might just resolve the longstanding dispute on dark matter — and who knows what other breakthroughs might await?

    “Do I need superfluid models? Physics isn’t really about what I need,” said Prescod-Weinstein. “It’s about what the universe may be doing. It may be naturally forming Bose-Einstein condensates, just like masers naturally form in the Orion nebula. Do I need lasers in space? No, but they’re pretty cool.”

    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 8:28 pm on May 1, 2017 Permalink | Reply
    Tags: , , Cold Dark Matter, , , ,   

    From CERN Courier: “How dark matter became a particle 

    CERN Courier

    Apr 13, 2017
    Gianfranco Bertone, University of Amsterdam
    Dan Hooper, Fermi National Accelerator Laboratory and the University of Chicago.

    It took decades for dark matter to enter the lexicon of particle physics. Today, explaining the nature and abundance of dark matter is one of the most pressing problems in the field.

    EAGLE-project simulation

    Astronomers have long contemplated the possibility that there may be forms of matter in the universe that are imperceptible, either because they are too far away, too dim or intrinsically invisible. Lord Kelvin was perhaps the first, in 1904, to attempt a dynamical estimate of the amount of dark matter in the universe. His argument was simple yet powerful: if stars in the Milky Way can be described as a gas of particles acting under the influence of gravity, one can establish a relationship between the size of the system and the velocity dispersion of the stars. Henri Poincaré was impressed by Kelvin’s results, and in 1906 he argued that since the velocity dispersion predicted in Kelvin’s estimate is of the same order of magnitude as that observed, “there is no dark matter, or at least not so much as there is of shining matter”.

    The Swiss–US astronomer Fritz Zwicky is arguably the most famous and widely cited pioneer in the field of dark matter. In 1933, he studied the redshifts of various galaxy clusters and noticed a large scatter in the apparent velocities of eight galaxies within the Coma Cluster. Zwicky applied the so-called virial theorem – which establishes a relationship between the kinetic and potential energies of a system of particles – to estimate the cluster’s mass. In contrast to what would be expected from a structure of this scale – a velocity dispersion of around 80 km/s – the observed average velocity dispersion along the line of sight was approximately 1000 km/s. From this comparison, Zwicky concluded: “If this would be confirmed, we would get the surprising result that dark matter is present in much greater amount than luminous matter.”

    In the 1950s and 1960s, most astronomers did not ask whether the universe had a significant abundance of invisible or missing mass. Although observations from this era would later be seen as evidence for dark matter, back then there was no consensus that the observations required much, or even any, such hidden material, and certainly there was not yet any sense of crisis in the field. It was in 1970 that the first explicit statements began to appear arguing that additional mass was needed in the outer parts of some galaxies, based on comparisons between predicted and measured rotation curves. The appendix of a seminal paper published by Ken Freeman in 1970, prompted by discussions with radio-astronomer Mort Roberts, concluded that: “If [the data] are correct, then there must be in these galaxies additional matter which is undetected, either optically or at 21 cm. Its mass must be at least as large as the mass of the detected galaxy, and its distribution must be quite different from the exponential distribution which holds for the optical galaxy.” (Figure 1 below.)

    Several other lines of evidence began to appear that supported the same conclusion. In 1974, two influential papers (by Jaan Einasto, Ants Kaasik and Enn Saar, and by Jerry Ostriker, Jim Peebles and Amos Yahil) argued that a common solution existed for the mass discrepancies observed in clusters and in galaxies, and made the strong claim that the mass of galaxies had been until then underestimated by a factor of about 10.

    Kelvin, Rubin, Bosma

    By the end of the decade, opinion among many cosmologists and astronomers had crystallised: dark matter was indeed abundant in the universe. Although the same conclusion was reached by many groups of scientists with different subcultures and disciples, many individuals found different lines of evidence to be compelling during this period. Some astronomers were largely persuaded by new and more reliable measurements of rotation curves, such as those by Albert Bosma, Vera Rubin and others. Others were swayed by observations of galaxy clusters, arguments pertaining to the stability of disc galaxies, or even cosmological considerations. Despite disagreements regarding the strengths and weaknesses of these various observations and arguments, a consensus nonetheless began to emerge by the end of the 1970s in favour of dark-matter’s existence.

    Enter the particle physicists

    From our contemporary perspective, it can be easy to imagine that scientists in the 1970s had in mind halos of weakly interacting particles when they thought about dark matter. In reality, they did not. Instead, most astronomers had much less exotic ideas in the form of comparatively low-luminosity versions of otherwise ordinary stars and gas. Over time, however, an increasing number of particle physicists became aware of and interested in the problem of dark matter. This transformation was not just driven by new scientific results, but also by sociological changes in science that had been taking place for some time.

    Half a century ago, cosmology was widely viewed as something of a fringe science, with little predictive power or testability. Particle physicists and astrophysicists did not often study or pursue research in each other’s fields, and it was not obvious what their respective communities might have to offer one another. More than any other problem in science, it was dark matter that brought particle physicists and astronomers together.

    As astrophysical alternatives were gradually ruled out one by one, the view that dark matter is likely to consist of one or more yet undiscovered species of subatomic particle came to be held almost universally among both particle physicists and astrophysicists alike.

    Perhaps unsurprisingly, the first widely studied particle dark-matter candidates were neutrinos. Unlike all other known particle species, neutrinos are stable and do not experience electromagnetic or strong interactions – which are essential characteristics for almost any viable dark-matter candidate. The earliest discussion of the role of neutrinos in cosmology appeared in a 1966 paper by Soviet physicists Gershtein and Zeldovich, and several years later the topic began to appear in the West, beginning in 1972 with a paper by Ram Cowsik and J McClelland. Despite the very interesting and important results of these and other papers, it is notable that most of them did not address or even acknowledge the possibility that neutrinos could account for the missing mass that had been observed by astronomers on galactic and cluster scales. An exception included the 1977 paper by Lee and [Steven]Weinberg, whose final sentence reads: “Of course, if a stable heavy neutral lepton were discovered with a mass of order 1–15 GeV, the gravitational field of these heavy neutrinos would provide a plausible mechanism for closing the universe.”

    While this is still a long way from acknowledging the dynamical evidence for dark matter, it was an indication that physicists were beginning to realise that weakly interacting particles could be very abundant in our universe, and may have had an observable impact on its evolution. In 1980, the possibility that neutrinos might make up the dark matter received a considerable boost when a group studying tritium beta decay reported that they had measured the mass of the electron antineutrino to be approximately 30 eV – similar to the value needed for neutrinos to account for the majority of dark matter. Although this “discovery” was eventually refuted, it motivated many particle physicists to consider the cosmological implications of their research.

    No image caption, no image credit.

    Although we know today that dark matter in the form of Standard Model neutrinos would be unable to account for the observed large-scale structure of the universe, neutrinos provided an important template for the class of hypothetical species that would later be known as weakly interacting massive particles (WIMPs). Astrophysicists and particle physicists alike began to experiment with a variety of other, more viable, dark-matter candidates.

    Cold dark-matter paradigm

    The idea of neutrino dark matter was killed off in the mid-1980s with the arrival of numerical simulations. These could predict how large numbers of dark-matter particles would evolve under the force of gravity in an expanding universe, and therefore allow astronomers to assess the impact of dark matter on the formation of large-scale structure. In fact, by comparing the results of these simulations with those of galaxy surveys, it was soon realised that no relativistic particle could account for dark matter. Instead, the paradigm of cold dark matter – i.e. made of particles that were non-relativistic at the epoch of structure formation – was well on its way to becoming firmly established.

    Meanwhile, in 1982, Jim Peebles pointed out that the observed characteristics of the cosmic microwave background (CMB) also seemed to require the existence of dark matter.

    CMB per ESA/Planck


    If just baryons existed, then one could only explain the observed degree of large-scale structure if the universe started in a fairly anisotropic or “clumpy” state. But by this time, the available data already set an upper limit on CMB anisotropies at a level of 10–4 – too meagre to account for the universe’s structure. Peebles argued that this problem would be relieved if the universe was instead dominated by massive weakly interacting particles whose density fluctuations begin to grow prior to the decoupling of matter and radiation during which the CMB was born. Among other papers, this received enormous attention within the scientific community and helped establish cold dark matter as the leading paradigm to describe the structure and evolution of the universe at all scales.

    Solutions beyond the Standard Model

    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.

    Neutrinos might be the only known particles that are stable, electrically neutral and not strongly interacting, but the imagination of particle physicists did not remain confined to the Standard Model for long. Instead, papers started to appear that openly contemplated many speculative and yet undiscovered particles that might account for dark matter. In particular, particle physicists began to find new candidates for dark matter within the framework of a newly proposed space–time symmetry called supersymmetry.

    Standard model of Supersymmetry DESY

    The cosmological implications of supersymmetry were discussed as early as the late 1970s. In Piet Hut’s 1977 paper on the cosmological constraints on the masses of neutrinos, he wrote that the dark-matter argument was not limited to neutrinos or even to weakly interacting particles. The abstract of his paper mentions another possibility made within the context of the supersymmetric partner of the graviton, the spin-3/2 gravitino: “Similar, but much more severe, restrictions follow for particles that interact only gravitationally. This seems of importance with respect to supersymmetric theories,” wrote Hut.

    In their 1982 paper, Heinz Pagels and Joel Primack also considered the cosmological implications of gravitinos. But unlike Hut’s paper, or the other preceding papers that had discussed neutrinos as a cosmological relic, Pagels and Primack were keenly aware of the dark-matter problem and explicitly proposed that gravitinos could provide the solution by making up the missing mass. In many ways, their paper reads like a modern manuscript on supersymmetric dark matter, motivating supersymmetry by its various attractive features and then discussing both the missing mass in galaxies and the role that dark matter could play in the formation of large-scale structure. Around the same time, supersymmetry was being further developed into its more modern form, leading to the introduction of R-parity and constructions such as the minimal supersymmetric standard model (MSSM). Such supersymmetric models included not only the gravitino as a dark-matter candidate, but also neutralinos – electrically neutral mixtures of the superpartners of the photon, Z and Higgs bosons.

    Over the past 35 years, neutralinos have remained the single most studied candidate for dark matter and have been the subject of many thousand scientific publications. Papers discussing the cosmological implications of stable neutralinos began to appear in 1983. In the first two of these, Weinberg and Haim Goldberg independently discussed the case of a photino (a neutralino whose composition is dominated by the superpartner of the photon) and derived a lower bound of 1.8 GeV on its mass by requiring that the density of such particles does not overclose the universe. A few months later, a longer paper by John Ellis and colleagues considered a wider range of neutralinos as cosmological relics. In Goldberg’s paper there is no mention of the phrase “dark matter” or of any missing mass problem, and Ellis et al. took a largely similar approach by simply requiring only that the cosmological abundance of neutralinos not be so large as to overly slow or reverse the universe’s expansion rate. Although most of the papers on stable cosmological relics written around this time did not yet fully embrace the need to solve the dark-matter problem, occasional sentences could be found that reflected the gradual emergence of a new perspective.

    The Bullet Cluster

    During the years that followed, an increasing number of particle physicists would further motivate proposals for physics beyond the Standard Model by showing that their theories could account for the universe’s dark matter. In 1983, for instance, John Preskill, Mark Wise and Frank Wilczek showed that the axion, originally proposed to solve the strong CP problem in quantum chromodynamics, could account for all of the dark matter in the universe. In 1993, Scott Dodelson and Lawrence Widrow proposed a scenario in which an additional, sterile neutrino species that did not experience electroweak interactions could be produced in the early universe and realistically make up the dark matter. Both the axion and the sterile neutrino are still considered as well-motivated dark-matter candidates, and are actively searched for with a variety of particle and astroparticle experiments.

    The triumph of particle dark matter

    In the early 1980s there was still nothing resembling a consensus about whether dark matter was made of particles at all, with other possibilities including planets, brown dwarfs, red dwarfs, white dwarfs, neutron stars and black holes. Kim Griest would later coin the term “MACHOs” – short for massive astrophysical compact halo objects – to denote this class of dark-matter candidates, in response to the alternative of WIMPs. There is a consensus today, based on searches using gravitational microlensing surveys and determinations of the cosmic baryon density based on measurements of the primordial light-element abundances and the CMB, that MACHOs do not constitute a large fraction of the dark matter.

    Gravitational microlensing, S. Liebes, Physical Review B, 133 (1964): 835

    An alternative explanation for particle dark matter is to assume that there is no dark matter in the first place, and that instead our theory of gravity needs to be modified. This simple idea, which was put forward in 1982 by Mordehai Milgrom, is known as modified Newtonian dynamics (MOND) and has far-reaching consequences. At the heart of MOND is the suggestion that the force due to gravity does not obey Newton’s second law, F = ma. If instead gravity scaled as F = ma2/a0 in the limit of very low accelerations (a << a0 ~ 1.2 × 10−10 m/s2), then it would be possible to account for the observed motions of stars and gas within galaxies without postulating the presence of any dark matter.

    In 2006, a group of astronomers including Douglas Clowe transformed the debate between dark matter and MOND with the publication of an article entitled: A direct empirical proof of the existence of dark matter. In this paper, the authors described the observations of a pair of merging clusters collectively known as the Bullet Cluster (image above left). As a result of the clusters’ recent collision, the distribution of stars and galaxies is spatially separated from the hot X-ray-emitting gas (which constitutes the majority of the baryonic mass in this system). A comparison of the weak lensing and X-ray maps of the bullet cluster clearly reveals that the mass in this system does not trace the distribution of baryons. Another source of gravitational potential, such as that provided by dark matter, must instead dominate the mass of this system.

    Following these observations of the bullet cluster and similar systems, many researchers expected that this would effectively bring the MOND hypothesis to an end. This did not happen, although the bullet cluster and other increasingly precise cosmological measurements on the scale of galaxy clusters, as well as the observed properties of the CMB, have been difficult to reconcile with all proposed versions of MOND. It is currently unclear whether other theories of modified gravity, in some yet-unknown form, might be compatible with these observations. Until we have a conclusive detection of dark-matter particles, however, the possibility that dark matter is a manifestation of a new theory of gravity remains open.

    Today, the idea that most of the mass in the universe is made up of cold and non-baryonic particles is not only the leading paradigm, but is largely accepted among astrophysicists and particle physicists alike. Although dark-matter’s particle nature continues to elude us, a rich and active experimental programme is striving to detect and characterise dark-matter’s non-gravitational interactions, ultimately allowing us to learn the identity of this mysterious substance. It has been more than a century since the first pioneering attempts to measure the amount of dark matter in the universe. Perhaps it will not be too many more years before we come to understand what that matter is.

    See the full article here .

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