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  • richardmitnick 9:02 pm on October 5, 2014 Permalink | Reply
    Tags: ars technica, ,   

    From ars technica: “Exploring the monstrous creatures at the edges of the dark matter map” 

    Ars Technica
    ars technica

    Sept 30 2014
    Matthew Francis

    So far, we’ve focused on the simplest dark matter models, consisting of one type of object and minimal interactions among individual dark matter particles. However, that’s not how ordinary matter behaves: the interactions among different particle types enable the existence of atoms, molecules, and us. Maybe the same sort of thing is true for dark matter, which could be subject to new forces acting primarily between particles.

    Some theories describe a kind of “dark electromagnetism” where particles carry charges like electricity, but they’re governed by a force that doesn’t influence electrons and the like. Just as normal electromagnetism describes light, these models include “dark photons,” which sound like something from the last season of Star Trek: The Next Generation (after the writers ran out of ideas).

    elec
    Diagram of a solenoid and its magnetic field lines. The shape of all lines is correct according to the laws of electrodynamics.

    Like many WDM candidates, dark photons would be difficult—if not impossible—to detect directly, but if they exist, they would carry energy away from interacting dark matter systems. That would be detectable by its effect on things like the structure of neutron stars and other compact astronomical bodies. Observations of these objects would let researchers place some stringent limits on the strength of dark forces. Another consequence is that dark forces would tend to turn spherical galactic halos into flatter, more disk-like structures. Since we don’t see that in real galaxies, there are strong constraints on how much dark forces can affect dark matter motion.

    som
    The “Sombrero” galaxy shows that matter interacting with itself flattens into disks. Dark matter doesn’t seem to do that, limiting the strength of possible interactions between particles.
    NASA, ESA, and The Hubble Heritage Team (STScI/AURA)

    NASA Hubble Telescope
    NASA/ESA Hubble

    Another side effect of dark forces is that there should be dark antimatter and dark matter-antimatter annihilation. The results of such interactions could include ordinary photons, another intriguing hint in the wake of observations of excess gamma-rays, possibly due to dark matter annihilation in the Milky Way and other galaxies.

    What’s cooler than cold dark matter?

    While most low-mass particles are “hot,” a hypothetical particle known as the axion is an exception. Axions were first predicted as a solution to a thorny problem in the physics of the strong nuclear force, but certain properties make them appealing as dark matter candidates. Mainly, they are electrically neutral and don’t interact directly with ordinary matter except through gravity.

    Axions are also very low-mass (at least in one proposed version), but unlike hot dark matter, they “condensed” in the early Universe into a slow, thick soup. In other words, they behave much like cold dark matter, but without the large mass usually implied by the term.

    Axions aren’t part of the Standard Model, but in a sense they’re a minimally invasive addition. Unlike supersymmetry, which involves adding one particle for each type in the Standard Model, axions are just one particle type, albeit one with some unique properties. (To be fair, these aren’t mutually exclusive concepts: it’s possible both SUSY particles and axions are real, and some versions of SUSY even include a hypothetical partner for axions.)

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

    Supersymmetry standard model
    Standard Model of Supersymmetry

    Like WDM, axions don’t interact directly with ordinary matter. But according to theory, in a strong magnetic field, axions and photons can oscillate into each other, switching smoothly between particle types. That means axions could be created all the time near black holes, neutron stars, or other places with intense magnetic fields—possibly including superconducting circuits here on Earth. This is how experiments hunt for axions, most notably the Axion Dark Matter eXperiment (ADMX).

    So far, no experiment has turned up axions, at least of the type we’d expect to see. Particle physics has a lot of wiggle-room for possibilities, so it’s too soon to say no axions exist, but axion partisans are disappointed. A universe with axions makes more sense than one without, but it wouldn’t be the first time something that really seemed to be a good idea didn’t quite work out.

    A physicist’s fear

    Long as it is becoming, this list is far from complete. We’ve excluded exotic particles with sufficiently tiny electric charges to be nearly invisible, weird (but unlikely) interactions that change the character of known particles under special circumstances, plus a number of other possibilities. One interesting candidate is jokingly known a WIMPzilla, which consists of one or more particle type more than a trillion times the mass of a proton. These would have been born at a much earlier era than WIMPs, when the Universe was even hotter. Because they are so much heavier, WIMPzillas can be rarer and interact more readily with normal matter, but—as with other more exotic candidates—they aren’t really considered to be a strong possibility.

    godzilla
    If the leading ideas for dark matter don’t hold up to experimental scrutiny, then we’ve definitely sailed off the map into the unknown.
    Castle Gallery, College of New Rochelle

    And more non-WIMP dark matter candidates seem to crop up every year, though many are implausible enough they won’t garner much attention even from other theorists. However, each guess—even unlikely ones—can help us understand what dark matter can be, and what it can’t.

    We’ve also omitted a whole other can of worms known as “modified gravity”—a proposition that the matter we see is all there is, and the observational phenomena that don’t make sense can be explained by a different theory of gravity. So far, no modified gravity model has reproduced all the observed phenomena attributed to dark matter, though of course that doesn’t say it can never happen.

    To put it another way: most astronomers and cosmologists accept that dark matter exists because it’s the simplest explanation that accounts for all the observational data. If you want a more grumpy description, you could say that dark matter is the worst idea, except for all the other options.

    Of course, Nature is sly. Perhaps more than one of these dark matter candidates is out there. A world with both axions and WIMPs—motivated as they are by different problems arising from the Standard Model—would be confounding but not beyond reason. Given the unexpected zoo of normal particles discovered in the 20th century, maybe we’ll be pleasantly surprised; after all, wouldn’t it be nice if several of our hypotheses were simultaneously correct for once? (I’m a both/and kind of guy.) More than one type might also help explain why we have yet to see any dark matter in our detectors so far. If a substantial fraction of dark matter particles is made of axions, then the density of WIMPs or WDM must be correspondingly lower and vice versa.

    But a bigger worry lurks in the minds of many researchers. Maybe dark matter doesn’t interact with ordinary matter at all, and it doesn’t annihilate in a way we can detect easily. Then the “dark sector” is removed from anything we can probe experimentally, and that’s an upsetting thought. Researchers would have a hard time explaining how such particles came to be after the Big Bang, but worse: without a way to study their properties in the lab, we would be stuck with the kind of phenomenology we have now. Dark matter would be perpetually assigned to placeholder status.

    In old maps made by European cartographers, distant lands were sometimes shown populated by monstrous beings. Today of course, everyone knows that those lands are inhabited by other human beings and creatures that, while sometimes strange, aren’t the monsters of our imagination. Our hope is that the monstrous beings of our theoretical space imaginings will some day seem ordinary, too, and “dark matter” will be part of physics as we know it.

    See the full article here.

    Ars Technica was founded in 1998 when Founder & Editor-in-Chief Ken Fisher announced his plans for starting a publication devoted to technology that would cater to what he called “alpha geeks”: technologists and IT professionals. Ken’s vision was to build a publication with a simple editorial mission: be “technically savvy, up-to-date, and more fun” than what was currently popular in the space. In the ensuing years, with formidable contributions by a unique editorial staff, Ars Technica became a trusted source for technology news, tech policy analysis, breakdowns of the latest scientific advancements, gadget reviews, software, hardware, and nearly everything else found in between layers of silicon.

    Ars Technica innovates by listening to its core readership. Readers have come to demand devotedness to accuracy and integrity, flanked by a willingness to leave each day’s meaningless, click-bait fodder by the wayside. The result is something unique: the unparalleled marriage of breadth and depth in technology journalism. By 2001, Ars Technica was regularly producing news reports, op-eds, and the like, but the company stood out from the competition by regularly providing long thought-pieces and in-depth explainers.

    And thanks to its readership, Ars Technica also accomplished a number of industry leading moves. In 2001, Ars launched a digital subscription service when such things were non-existent for digital media. Ars was also the first IT publication to begin covering the resurgence of Apple, and the first to draw analytical and cultural ties between the world of high technology and gaming. Ars was also first to begin selling its long form content in digitally distributable forms, such as PDFs and eventually eBooks (again, starting in 2001).

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  • richardmitnick 12:06 pm on July 29, 2014 Permalink | Reply
    Tags: ars technica, , , , ,   

    From ARS Technica: “Dark matter makes up 80% of the Universe—but where is it all?” 

    Ars Technica
    ARS Technica

    July 27 2014
    Matthew Francis

    It’s in the room with you now. It’s more subtle than the surveillance state, more transparent than air, more pervasive than light. We may not be aware of the dark matter around us (at least without the ingestion of strong hallucinogens), but it’s there nevertheless.

    dm
    Composite image of X-ray (pink) and weak gravitational lensing (blue) of the famous Bullet Cluster of galaxies.
    X-ray: NASA/CXC/CfA/ M.Markevitch et al.; Lensing Map: NASA/STScI; ESO WFI; Magellan/U.Arizona/ D.Clowe et al. Optical: NASA/STScI; Magellan/U.Arizona/D.Clowe et al.

    Although we can’t see dark matter, we know a bit about how much there is and where it’s located. Measurement of the cosmic microwave background shows that 80 percent of the total mass of the Universe is made of dark matter, but this can’t tell us exactly where that matter is distributed. From theoretical considerations, we expect some regions—the cosmic voids—to have little or none of the stuff, while the central regions of galaxies have high density. As with so many things involving dark matter, though, it’s hard to pin down the details.

    Cosmic Background Radiation Planck
    CMB per ESA/Planck

    ESA Herschel
    ESA/Planck

    Unlike ordinary matter, we can’t see where dark matter is by using the light it emits or absorbs. Astronomers can only map dark matter’s distribution using its gravitational effects. That’s especially complicated in the denser parts of galaxies, where the chaotic stew of gas, stars, and other forms of ordinary matter can mask or mimic the presence of dark matter. Even in the galactic suburbs or intergalactic space, dark matter’s transparency to all forms of light makes it hard to locate with precision.

    Despite that difficulty, astronomers are making significant progress. While individual galaxies are messy, analyzing surveys of huge numbers of them can provide a gravitational map of the cosmos. Astronomers also hope to overcome the messiness of galaxies and estimate how much dark matter must be in the central regions using careful observation of the motion of stars and gas.

    There’s also been a tantalizing hint of dark matter particles themselves in the form of a signal that may come from their annihilation near the center of the Milky Way. If this is borne out by other observations, it could constrain dark matter’s properties while avoiding messy gravitational considerations. Adding it all up, it’s a promising time for mapping the location of dark matter, even as researchers still build particle detectors to identify what it is.

    A (very) brief history of dark matter

    In the 1930s, Fritz Zwicky measured the motion of galaxies within the Coma galaxy cluster. Based on simple gravitational calculations, he found that they shouldn’t move as they did unless the cluster contained a lot more mass than he could see. As it turned out, Zwicky’s estimates of how much matter there was were too large by a huge factor. Still, he was correct in the broader picture: more than 80 percent of a galaxy cluster’s mass isn’t in the form of atoms.

    Zwicky’s work didn’t get a lot of attention at the time, but Vera Rubin’s later observations of spiral galaxies were another matter. She found that the combined stars and gas had too little mass to explain the rotation rates she measured. Between Rubin’s work and subsequent measurements, astronomers established that every spiral galaxy is engulfed by a roughly spherical halo (as it is called) of matter—matter that’s transparent to every form of light.

    The Bullet Cluster

    That leads us to the “Bullet Cluster,” one of the most important systems in astronomy.

    bullt
    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 galaxies interact. When the galaxies collided, based on black-body temperature 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.

    First described in 2006, it’s actually a pair of galaxy clusters observed in the act of colliding. Researchers mapped it in visible and X-ray light, finding that it consists of two clumps of galaxies. But it’s the stuff they couldn’t image directly that ensured the Bullet Cluster is rightfully cited as one of the best pieces of evidence for dark matter’s existence (the title of the paper announcing the discovery even calls it “direct empirical proof”).

    Galaxy clusters are the biggest individual objects in the Universe. They can contain thousands of galaxies bound to each other by mutual gravity. However, the stuff within those galaxies—stars, gas, dust—is outweighed by an extremely hot, gaseous plasma between them, which shines brightly in X-rays. In the Bullet Cluster, the collision between the two clusters created a shock wave in the plasma (the shape of this shock wave gives the structure its name).

    More dramatically, though, the astronomers who described the cluster used gravitational lensing—the distortion of light from more distant galaxies by the mass within the cluster—to map the distribution of most of the material in the Bullet Cluster. That method is known as “weak gravitational lensing.” Unlike the sexier strong lensing, weak lensing doesn’t create multiple images of the more distant galaxies. Instead, it slightly warps the light from background objects in a small but measurable way, depending on the amount and concentration of mass in the “lens”—in this case, the cluster.

    Astronomers found the shocked plasma, which represents most of the mass of the Bullet Cluster, was almost entirely in the region between the two clusters, separated from the galaxies. However, the mass was largely concentrated around the galaxies themselves. This enabled a clear, independent measurement of the amount of dark matter, separate from the mass of the gas.

    The results also confirmed some predictions about the behavior of dark matter. Thanks to the shock of the collision, the plasma stayed in the region between the two clusters. Since the dark matter doesn’t interact much with either itself or normal matter, it passed right through the collision without any noticeable change.

    It’s a phenomenal discovery, but it’s only one galaxy cluster, and that ain’t enough. Science is inherently greedy for evidence (as it should be). A single example of anything tells us very little in a Universe full of possibilities. We want to know if dark matter always clusters around galaxies or if it can be more widely dispersed. We want to know where all the dark matter is, in all galaxy clusters and beyond, throughout the entire cosmos.

    A dark matter census

    Weak gravitational lensing provides a method to search for dark matter in other galaxy clusters, too, as well as even larger and smaller structures. Princeton University astronomers Neta Bahcall and Andrea Kulier took a weak lensing census of 132,473 galaxy groups and clusters, all within a well-defined patch of the sky but at a range of distances from the Milky Way. (“Groups” are smaller associations of galaxies; for example, the Milky Way is the second largest galaxy in the Local Group, after the Andromeda galaxy.) While individual galaxy clusters usually can’t tell us much, a large sample allowed the astronomers to treat the problem statistically—weak lensing effects that were too small to spot for a single cluster became obvious when looking at hundreds of thousands.

    For example, a typical quantity used in studying galaxies is the mass-to-light ratio. To measure this statistically, Bahcall and Kulier looked at the cumulative amount of light (mostly emitted by stars) and weak lensing (mostly from dark matter), starting from the centers of each cluster and working outward. They found something intriguing: the amount of mass and light increased in tandem and then leveled off together. That means neither the dark matter nor the light extends farther than the other: the stars inside these groups and clusters were a very good tracer for the dark matter. That’s surprising because stars are typically less than two percent of the mass in a cluster, with the balance of ordinary matter made up by gas and dust.

    As Kulier told Ars, “The total amount of dark matter in galaxy groups and clusters might be accounted for entirely by the amount of dark matter in the halos of their constituent galaxies.” That’s an average result, though; the details could look quite different. “This does not necessarily imply that the halos are still ‘attached’ to the galaxies,” Kulier said. In other words, when galaxies came together to form clusters, the stronger forces acting on galaxies and their stars could in principle separate them from their dark matter but leave everything inside the cluster thanks to mutual gravity.

    Kulier pointed out that these results provide strong support for the “hierarchical” model of structure formation: “smaller structures collapse earlier than larger ones, so that galaxies form first and then merge together to form larger structures like clusters.” The Bullet Cluster is an archetypical example of this, but things could be otherwise. For instance, dark matter could have ended up in the center of clusters, separate from the galaxies and their individual halos.

    But that’s not what astronomers see. In their analysis, Bahcall and Kulier also calculated that the total ratio of dark matter to ordinary matter in galaxy clusters matches that of the Universe as a whole. That’s another strong piece of evidence in favor of the standard model in cosmology: maybe most of the dark matter everywhere is in galactic halos.

    Every galaxy wears a halo

    halo
    Computer reconstruction of the location of mass in terms of how it affects the image of distant galaxies through weak lensing.
    S. Colombi (IAP), CFHT Team

    So what about the halos themselves and the galaxies that wear them? Historically, dark matter was first recognized for its role in spiral galaxies. However, it’s one thing to say that dark matter is present. It’s another to map out where it is—especially in the dense, star-choked inner parts of galaxies.

    Spiral galaxies consist of three basic parts: the disk, the bulge, and the halo. The disk is a thin region containing the spiral arms and most of the bright stars. The bulge is the central, densest part, with large populations of older stars and (at its very heart) a supermassive black hole. The halo is a more or less spherical region containing a smattering of stars; it envelops the other regions, extending several times beyond the limit of the disk. To provide an example, the Milky Way’s disk is about 100,000 light-years in diameter, but its halo is between 300,000 to 1 million light-years across.

    Because of the relative sizes of the different regions, most of a galaxy’s dark matter is in the halo. Relatively little is in the disk; Jo Bovy and Scott Tremaine showed that the disk and halo contain less than the equivalent mass of 100 Earths in a cube one light-year across. That may sound like a lot, but Earth isn’t that large, and a light-year defines a big volume. That amount isn’t enough to affect the Sun’s orbit around the galactic center strongly. (It’s still enough for a few particles to drift through detectors like LUX, though.)

    By contrast, the amount of dark matter increases toward the galaxy’s center, so the density should be much higher in the bulge than anywhere else. For that reason, a number of astronomers look to the central part of the Milky Way for indications of dark matter annihilation, which (under some models) would produce gamma rays. This would occur if dark matter particles are their own antimatter partners, so that their (very rare) collisions result in mutual destruction and some high-energy photons. This winter, a group of researchers announced a possible detection of excess gamma rays originating in the Milky Way’s core, based on data from the orbiting [NASA] Fermi gamma ray observatory.

    NASA Fermi Telescope
    NASA/Fermi

    However, the bulge also has the highest density of stars, making it a tangled mess. Many things in that region could produce an excess of gamma rays. As University of Melbourne cosmologist Katherine Mack told me, “The Galactic Center is a really messy place, and the analysis of the signal is complicated. It’ll take a lot to show that the signal has to be dark matter annihilation rather than some un-accounted-for astrophysical source.” We can’t rule out the possibility of dark matter annihilation, but it’s definitely too soon to break out the champagne.

    The difference between the ease of calculating an average density and detecting the presence of dark matter is illustrative of the general problem with mapping dark matter inside galaxies. It’s relatively simple to put limits on how much there is in the disk, since that’s a small fraction of the total volume of a galaxy. The tougher questions include how steeply the density falls off from the galactic center, how far the halo actually extends, and how lumpy the halo is.

    For instance, our galaxy’s halo is big enough to encompass its satellite galaxies, including the Magellanic Clouds and a host of smaller objects. But these galaxies also have their own halos in accordance with the hierarchical model. Because they’re denser dark matter lumps inside the Milky Way’s larger halo, the satellites’ halos create a substructure.

    Our dark matter models predict how much substructure should be present. However, dwarf galaxies are very faint, so astronomers have difficulty determining if there are enough of them to account for all the predicted substructure. This is known as the “missing satellite problem,” but many astronomers suspect the problem will evaporate as they get better at finding these faint objects.

    A hopeful conclusion

    So where is the dark matter? Based on both theory and observation, it looks like most of it is in galactic halos. Surveys using weak gravitational lensing are ongoing, with many more planned for the future. These surveys will show where most of the mass in the Universe is located in unprecedented detail.

    How dark matter is distributed within those halos is still a bit mysterious, but there are several hopeful approaches. By looking for “dark galaxies”—small satellites with few stars but high dark matter concentrations—astronomers can determine the substructure within larger halos. The [ESA]Gaia mission is working to produce a three-dimensional map of a billion stars and their motions, which will provide information about the structure of the Milky Way and its surrounding satellites. That in turn will allow researchers to work backward, determining the gravitational field dictating the motion of these stars. With that data in hand, we should have a good map of the dark matter in many regions that are currently difficult to study.

    Dark matter may be subtle and invisible, but we’re much closer than ever to knowing exactly where it hides.


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  • richardmitnick 12:32 pm on December 17, 2010 Permalink | Reply
    Tags: ars technica, , ,   

    From ars technica:”Theorists seek dark matter in hot neutron stars” 

    By Chris Lee

    [I am not going to tell Chris's story here, he is an excellent writer and knows his subject. I just want to get you intrigued.]

    “Dark matter is an enigma wrapped in a conundrum. We have lots of gravitational evidence for the presence of dark matter. In fact, the evidence is from so many different types of observations, and is all so consistent, that very few astronomers or cosmologists appear to doubt that some type of dark matter exists. That is the enigma: it is very likely to exist, but we know very little of the specific details about what exactly exists.

    Going further than that has been a problem. The very nature of dark matter, which makes cosmologists so certain of its existence, means it has the very properties that make it so damn hard to find by any means other than gravity—something of a conundrum, really. A recent paper that looks at how dark matter might be detectable in neutron stars inadvertently makes this problem very clear.

    Most cosmologists believe that dark matter consists of weakly interacting massive particles (WIMPs). As the name suggests, they are heavy and they only talk to normal matter very rarely. But rarely is not never and, if we can find a place where there is an awful lot of both normal matter and dark matter, we might be able to observe the consequences of the two colliding. There are a fair number of experiments going on that attempt to do this, and they have some tantalizing results. But tantalizing is all they are—nothing that would get you calling your Mom in excitement in the middle of the night.

    So, when I stumbled across a paper discussing the effects of dark matter on neutron stars, I was intrigued. The basic idea, it turned out, was that neutron stars have huge densities, so the likelihood of dark matter colliding with normal matter is greater there than in any other objects in the observable universe. If the neutron star happens to be near the galactic center or in a globular cluster, then there should be a lot of WIMPs around to play with….”

    i2
    Credit: NASA

    So, click on the link and read Chris’s story here.

     
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