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I How certain is dark matter?

  1. Aug 6, 2018 #1
    How certain are scientists that the solution to the dark matter problem is a new, unknown particle (or more)? Theories that eliminate the need for new particle(s) and suggest modifications to the current understanding of gravity seem to get less attention. Why is that?
     
  2. jcsd
  3. Aug 6, 2018 #2
    Probably because there doesn't seem to be much "give" to Einstein's General of Relativity. It just seems to be spot on every time it is tested.

    On the other hand, there has yet to be a good example of a galaxy that has significantly too much or too little Dark Matter - given its size.
    If one could be found, it would strongly argue for the particle theory.
    Here's an article that discusses a claim made earlier this year, a claim that is being hotly refuted:
    https://www.scientificamerican.com/article/oddball-galaxy-find-puts-dark-matter-theory-to-the-test/
     
  4. Aug 6, 2018 #3

    phinds

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    MOND theories have not done well against experimental evidence. In particular, for example, Google the "Bullet Cluster"
     
  5. Aug 6, 2018 #4
  6. Aug 6, 2018 #5
    So it's more about confidence than actual arguments against theoretical modifications?

    Why would that be against the new matter hypothesis? It cannot be that dark matter density is not constant?
     
  7. Aug 6, 2018 #6

    PeterDonis

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    It's not just that. Modified theories of gravity don't just affect cosmological observations. They affect all observations where gravity is involved. It's very, very difficult to find any modified theory of gravity that makes different predictions about cosmological observations (galaxy rotation curves, expansion of the universe) but doesn't make predictions different enough to be already falsified about other domains that involve gravity (such as the solar system).
     
  8. Aug 6, 2018 #7

    kimbyd

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    Modified theories of gravity have gotten a lot of attention, and continue to do so. But most cosmologists think they're highly unlikely at this point because the evidence just doesn't point that way. Still, these kinds of theories have a certain kind of allure to many theorists, so there continues to be work in modified gravity theories to attempt to explain dark matter (and dark energy).
     
  9. Aug 6, 2018 #8

    fresh_42

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    Here's another hit for DM:
    preprint (11/13/17) https://arxiv.org/abs/1711.04778
    publication (08/06/18), paywall https://www.nature.com/articles/s41550-018-0531-z
     
  10. Aug 6, 2018 #9

    kimbyd

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    I'm a bit confused as to what you mean by "hit for DM". It sounds like this is a paper which suggests that the reported signal which some people thought might have been from dark matter was more likely to be associated with stars instead. To me, that doesn't say anything one way or the other about DM. It's just a bit disappointing that the nature of dark matter is proving so difficult to discern.
     
  11. Aug 6, 2018 #10

    fresh_42

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    Yes. It's just another observation which can be explained by classical results, although some have hoped to be DM radiation. Just a chance less to resolve the mystery. I admit that the authors' reference to DM is a bit artificial and feeds the suspicion that it was due to PR, as was my formulation "hit". But how ever one might look at it, it's one chance less.
     
  12. Aug 6, 2018 #11

    mathman

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    There is another point in favor of something. ##\Omega=1## with ordinary matter ~ .04, the rest being dark matter ~ .25, and the bulk being dark energy.
     
  13. Aug 7, 2018 #12

    Vanadium 50

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    Bad example, despite the popular press. Abell 520 is a "reverse bullet cluster" in which the dark matter stays behind the gas, "impossible" in LCDM.

    The better argument is to admit that MOND works marginally better than LCDM on galactic scales, but nowhere else.
     
  14. Aug 7, 2018 #13

    phinds

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    Last edited: Aug 7, 2018
  15. Aug 7, 2018 #14

    Vanadium 50

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    Because Abell 520 and the Bullet Cluster are opposite each other in terms of the relative locations of luminous and dark matter. If your question is "why do some collisions look one way and others the other?" I don't think anyone knows. I just think one cannot say of the Bullet Cluster "This is a smoking gun - absolute irrefutable proof!" and of Abell 520, "Well you can't tell anything from one example, this is a complicated business...."
     
  16. Aug 7, 2018 #15

    ohwilleke

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    I think it is always useful to focus on the consensus before leaping into the controversy. So, here are four big points.

    FIRST

    There is no reasonable doubt that the phenomena attributed to dark matter exist and cannot be explained with general relativity as currently formulated and the particles and forces in the Standard Model of Particle Physics alone.

    This is an area of rock solid scientific consensus.

    Beyond "Core Theory" (i.e. Standard Model + General Relativity) physics are necessary to explain almost all astronomy phenomena at the dwarf galaxy scale or larger. General relativity alone can explain the solar system, most of the important details about black holes, most binary star systems, and gravitational lensing and red shift, without resort to dark matter or modifications to gravity, but beyond that you need the new physics that everyone is looking for. It is a great unsolved problem.

    There are other "unsolved problems" in physics like the hierarchy problem or the strong force CP problem or questions about why the initial conditions of the universe where what we have observed them to be as far back as we can measure with out available resources, that basically ask, "why are the laws of physics and its physical constants the way that they are."

    But, this problem isn't in that category. Everyone agrees that we need to add new physical laws/particles/constants to existing Core Theory applied using existing methods to explain what we see.

    (I say "to existing Core Theory applied using existing methods" to recognize a third possibility that could exist instead of, or in addition to the dark matter particles and modified gravity, which is that some or all of dark matter phenomena could arise from the possibility that using a Newtonian approximation of GR in lieu of complete GR to do galaxy plus scale estimates of behavior under GR could be less negligible than previously realized due to previously unrecognized flaws in how the discrepancy between a Newtonian approximation and a GR calculation is estimated. One or two people are argued this in published papers that were largely refuted, but neither side's analysis was really rigorous and extraordinarily careful so this isn't an impossibility.)

    SECOND

    There is no single widely accepted theory that explains all dark matter phenomena.

    Dark Matter Particle Theories

    Within the dark matter particle paradigm, the evidence has ruled out a lot of the potential dark matter particle parameter space. For example, both MACHOs like red dwarfs and stellar sized or larger black holes, and the hundreds of GeV WIMPs that interact via the weak force as anticipated in electroweak scale supersymmetry theories, have been all but ruled out experimentally. We know that dark matter can't be ordinary neutrinos. We know that dark matter can't be ordinary interstellar gas or planets or free neutrons.

    But, we don't have positive proof for any dark matter particle candidate to the exclusion of the alternatives. And, we have multiple competing hypotheses about potential kinds of dark matter particles. Some of the hottest models are "warm dark matter", "fuzzy dark matter", "self-interacting dark matter", "sterile neutrinos", "axion-like dark matter", "mirror matter", "bosonic dark matter", etc. But, in an effort to emphasize the consensus rather than the controversy, I'm not going to carefully parse which of these models still is or is not in the running in detail in this reply.

    (Note that the line between bosonic dark matter particles and a gravity modification mediated by a new fifth force carrier boson can start to overlap.)

    Modified Gravity Theories

    There is also no widely accepted modified gravity theory that explains all dark matter phenomena.

    MOND and TeVeS

    Milgrom's 1983 MOND theory is the most famous, and deservedly so. It explains the dynamics of everything from dwarf galaxies to elliptical galaxies and wide binary star systems with a single tweak to Newton's GMm/r2 law and one additional universal constant, with an accuracy as great as experimental error in the measurements that are being tested permits from ordinary matter distributions alone. It has made multiple accurate predictions in its domain of applicability that have been tested with subsequently acquired data, which no other theory has done. Also, while it underestimates dark matter phenomena in galactic clusters, it explains a significant share of the dynamics that GR does not even in those systems, and it does so without screwing up sub-galaxy scale systems like the solar system any more badly than Newtonian gravity does.

    (Dirty little secret: almost all galaxy scale gravitational models that are done with general relativity are actually done with plain old Newtonian gravity anyway, because the differences between Newtonian gravity and general relativity as currently formulated and as calculated appears to be negligible in those weak fields).

    But, no serious proponent of MOND has ever claimed that MOND itself is a correct theory of gravity. It is a phenomenological toy model that, at a minimum, must be correctly generalized to a relativistic version that can accurately predict things like black holes and gravitational lensing and gravitational red shift and the perihelion of Mercury and frame dragging and the like that general relativity does and Newtonian gravity does not.

    MOND was generalized into relativistic form by Bekenstein into a theory known as TeVeS (for tensor, vector, scalar, in contrast to GR which is a purely tensor theory), and that solved the really glaring and obvious problems that come from generalizing Newtonian gravity rather than general relativity. But, it has not fared very well in other tests designed to discriminate between competing theories designed to explain dark matter phenomena.

    Other Modified Gravity Theories

    There are other modified gravity theories, such as Moffat's MOG theories (almost everyone in the field has multiple version of their theory and I use the most well known names for each) which is not only relativistic, but solves the cluster problem that MOND has, explains the Bullet Cluster, and has been explored at a cosmology level and performed reasonably well (few other modified gravity theories have been explored at this level of detail). (For what it is worth, MOG, like TeVeS is a scalar, vector, tensor theory.)

    There are quite a few others, including some I am partial to, but I'm not going to bang the drum of particular little known theories in this reply.

    There are another class of theories such as f(R) theories and f(T) theories and tensor-scalar theories that make tweaks to Einstein's field equations, usually adding something extra where the stress-energy tensor would ordinarily go. These have the virtue of being mathematically consistent and fully relativistic in ordinary circumstances from the get go. Initially and still predominantly now, these theories are primarily targeted at finding an alternative to the cosmological constant to explain phenomena known as "dark energy" and to probe possibilities like cosmological inflation in a more bounded manner than just coming up with ad hoc inflation theories. But, some of them can reproduce, at least, basic dark matter phenomena too.

    Most of these theories are classical theories of gravity, like GR and Newtonian gravity and MOND, but a few are inspired by quantum gravity concepts. These are notable because for entirely independent and mostly theoretical reasons, we know that something in our Core Theory is wrong because the Standard Model of Particle Physics and GR are not fully compatible and we need at a minimum a quantum gravity theory to solve that disconnect, whether or not that quantum gravity theory solves any dark matter or dark energy phenomena issues. So, we really have two gravity related areas where we known to almost a certainty that we need new physics. Unfortunately, formulating a viable theory of quantum gravity is much harder than you would naively expect it to be.

    Quantum gravity is mostly hard because a self-interacting spin-2 massless graviton in four dimensions does not have a rapidly converging propagator in the way that, for example, the spin-1 massless photon which is not self-interacting does. The mathematical problems with doing calculations regarding the behavior of a self-interacting spin-2 massless graviton in four dimensions are similar in kind to the mathematical problems involved in quantum chromodynamics a.k.a. QCD a.k.a. the part of the Standard Model of Particle Physics that gives rise to the strong force, but worse. QCD is slow converging but can be calculated using perturbative QCD methods similar to those of QED (the quantum version of electromagnetism) and the weak force, except at low energies, but naive quantum gravity is not renormalizable. Theorists are looking for shortcuts and unexpected symmetries that simplify these calculations.

    Conventional wisdom in the quantum gravity field is that distinctions between quantum gravity and classical GR should matter most in strong field situations like near black holes and the Big Bang and at small distance scales, however, while both dark matter and dark energy phenomena, to the extent that they are gravity modifications, are instead tweaks to very weak gravitational fields (in terms of accelerations or forces) in very large systems.

    One of the problems on the side of the research is that few specific theories have received enough investigation and development to rigorously test them once they clear the first hurdle of explaining the galactic rotation curves that MOND does.

    THIRD

    Lots of new data is coming in on all sorts of fronts on a daily basis that is useful to solving the problem of dark matter phenomena. We have direct dark matter detection experiments, experiments trying to look for dark matter particle annihilation signatures, telescopes looking as the dynamics of matter in large systems at all sorts of frequencies and with new means like neutrinos, other cosmic rays and gravitational waves, telescopes looking at the very early universe by means other than the cosmic microwave background radiation, LHC searches, etc. Some examples can be helpful to illustrate this enterprise.

    Direct dark matter detection experiments and LHC searches can narrow the parameter space of any kind of dark matter particle that couples to ordinary matter or Standard Model force carrier bosons (or the Higgs boson). The null results to date of those searches imply that dark matter is either outside the range of roughly 1 GeV to 1000 GeV that has been throughly tested, or that it has very weak interactions (much weaker than neutrinos) with ordinary matter indeed. Progress is being made towards extending this exclusion range towards heavier and towards light dark matter particle mass ranges.

    One important kind of observation involve the RAVE apparatus, which, among other things, makes it possible to carefully observe the dynamics of the small number of stars outside the galactic plane in the Milky Way galaxy, which in turn, allows investigators to test dark matter particle halo shapes and modified gravity models that have indistinguishable effects on the main rotation dynamics of stars in the main galactic plane of our spiral galaxy, but give rise to different gravitational fields that can be distinguished outside the plane of that disk.

    CMB observations like those of the Planck collaboration and 21cm radio wave observations, that are just becoming available, probe the very early universe, in its first half-billion years or so.

    Gravitational wave telescopes combined with visible light and neutrinos observations of black hole-neutron star collisions (only one so far, but soon to be many more) provide independent constraints on Hubble's Constant (which is pertinent to dark energy) and severely constrains any deviation from a massless graviton in the theory's carrier bosons (some modified gravity theories have more than one carrier boson, one or more of which may be massive).

    Observations of the dynamics of very low surface brightness galaxies both near other galaxies and clusters, and in isolation, make it possible to probe whether a probe of a property of MOND all the back in 1983 that directly contradicts general relativity, called the "external field effect" does or does not exist, in a model dependent way.

    This is very unlike the story in high energy particle physics, were there is basically one mega-experiment in town that can't grow in power nearly as fast as we wish it could without a big new investment of $$$ that is a bottleneck on progressive on the experimental front.

    In dark matter physics, in contrast, we have lots of data, we are getting more every day, we are getting more kinds of data every few years, and we are finding better and better ways to collate and analyze these various streams of data. More input is coming in faster than we can analyze it and lots of that data is evidence that will be very useful in discriminating between dark matter hypotheses.

    FOURTH

    On the dark matter particle paradigm side of the enterprise of explaining dark matter phenomena, it isn't good enough to come up with an answer that simply give you one or several new particles with particular properties and possibly a force or two to govern their interactions.

    A dark matter particle theory needs to not only tell you what new particles and forces it needs, it needs to explain how those particles ended up in the precise places in the universe where we see them, and it needs to be capable of making precise and accurate predictions at all scales. A dark matter particle theory without an accompanying theory about the galaxy and mass assembly progress isn't much of a theory.

    Most glaringly, any dark matter particle theory that doesn't modify gravity has to explain why MOND manages to explain so much, so accurately, over such a broad range of applicability, with one little tweak to Newton's theory of gravity and one universal constant. Until your dark matter particle theory can do that, it is either wrong (if it fails to reproduce this phenomenological relationship) or incomplete.

    BEYOND CONSENSUS

    At this juncture, the task ahead is to carefully develop the existing theories of both types that haven't been ruled out so that we know what they predict, and to continue to hypothesize and explore new alternatives to existing theories, while continually tightening the noose of the acceptable parameter spaces and necessary experimental tests that must be met by any winner.

    Dark matter particle theorists and modified gravity theorists are marshaling their evidence and making their cases.

    On the dark matter particle side, probably the biggest barrier to any definitive conclusion is developing better N-body simulation or analytical simulation models that show what a particular kind of dark matter particle should imply. Current simulations are tuned to post-dict what we observe with a dozen plus adjustable parameters that give the theory as tested in the simulation far more degrees of freedom than modified gravity theories have, with parameters that don't alway have great justifications from fundamental physics. Until then, it is very hard to figure out which proposals can really reproduce the galaxy scale structure and inferred dark matter halo shapes that we infer from dynamics. When there isn't a match, we don't know if the simulation is broken or the hypothesis is wrong. When there is a match, we don't know if this is really an artifact of excessive tuning in our simulation, or if the theory is actually correct.

    On the modified gravity side, one of the biggest barriers is to find the resources to examine questions like what theories predict about the state of the early universe. Dark matter particle theorists point to the LamdaCDM "Standard Model of Cosmology's great success at explaining the cosmic background radiation and other much larger than galactic cluster scale features of the universe. But, often in the case of modified gravity theories, the problem isn't that the theories contradict the Standard Model of Cosmology, but that it is much harder to tell how the non-linearities in these modified gravity theories change the predictions for the early universe. Simply put, nobody has really gotten around to that rigorously for many of the most promising modified gravity theories.

    Still, this is a problem that honestly could have an evidence based solution in decades and not centuries. You have a good shot of living to hear the final consensus.
     
    Last edited: Aug 21, 2018
  17. Aug 7, 2018 #16

    kimbyd

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    This kind of thing doesn't really overturn the bullet cluster result at all. Nor does it argue against dark matter in favor of modified gravity. In a universe where dark matter exists alongside baryonic matter like our own, you expect there to be really complicated stuff that happens. The Bullet Cluster is a nice test because it is a relatively clean system: it is a system where a comparatively smaller galaxy cluster rammed through a larger galaxy cluster at high velocity. The simplicity of the Bullet Cluster makes it incredibly difficult to explain in any modified gravity model (the one example I'm aware of that comes close introduces its own, novel form of dark matter, a heavy neutrino).

    Abell 520, sometimes called the Train Wreck Cluster, is a vastly more complicated system which apparently formed at the crossing of three filaments of large-scale structure. This far more complex structure will naturally be more challenging to understand in detail. It's trivially easy to understand how such a structure might have a dark core (such dark cores will naturally form at the intersections of filaments). What's more confusing is why that dark core has so few galaxies in it: you'd generally expect that large concentrations of dark matter should coincide with very massive galaxies.

    Claiming that it argues against dark matter makes no sense whatsoever. It's rather like the argument that global warming is false because some places still get unseasonably cold in winter. A universe where the "missing mass" is explained by modified gravity has no mechanism whatsoever for there to be different amounts of dark matter vs. normal matter in different systems. A universe with dark matter has lots of potential mechanisms for this to happen.
     
  18. Aug 7, 2018 #17
    The short answer is it depends who you talk to. Some astronomers are very convinced about dark matter others not so. In the absence of a definite dark matter candidate the debate will continue. Possibly, the dark matter group just edge this with popularity and not forgetting many have spent a lot of money and long careers looking for dark matter!
     
  19. Aug 7, 2018 #18

    ohwilleke

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    I would disagree with your assessment about which way the evidence points, although in fairness, some of the strongest reasons to disagree involve quite recent evidence.

    One of the biggest involves dark matter particle parameter space. LambdaCDM isn't very demanding when it comes to dark matter candidates. They need to have 10eV/c2 or more of mass, it needs a density the declines proportionately to the expanding volume of the universe, and it needs to be "nearly collissionless" an approximation that isn't very exacting when you get down to brass tacks.

    Truly collisionless dark matter is easy to model. It forms halos that fit what is called an NFW profile (after the initials of the people who came up with it). But, recent observations pretty conclusively establish that inferred dark matter halos do not fit the NFW profile. And, equally important, it doesn't tightly, tightly align itself with the distribution of ordinary matter. Gravity causes the two to mutually influence the halo and ordinary matter distributions, but it shouldn't do so that strongly in a wide variety of galaxy systems. For example, if dark matter is truly collisionless, galaxies with bulges should have quite different halos from galaxies without them, even though they have quite similar rotational dynamics. You can tweak the NFW profile a bit by reducing the mass of the dark matter particles to the keV mass range, because some quantum effects start to kick in at that point, but it really isn't enough to reproduce observation (the lower mass of the dark matter particles also increases their anticipated average velocity which helps solve the fact that the amount of satellite galaxy and subhalo structure predicted for heavier dark matter particles that move more slowly is not observed). Truly collisionless dark matter is also a very bad fit to the 21cm radio wave observations of the early universe, that need dark matter to cool radiation era energy.

    But, if you have strong enough cross-sections of interaction to solve the problems with a truly collisionless model, by creating some sort of weak feedback between dark matter and ordinary matter, you start to screw up the CMB predictions that are the crowning achievement of LambdaCDM, although this could explain why galaxy formation observations seem to point to this happening earlier than predicted in a plain vanilla LambdaCDM scenario (something that also happens in the couple of modified gravity theories whose cosmologies have been explored more rigorously). The stronger the cross-section of interaction you have, the more of a problem the null results from the direct detection experiments and the LHC become as well.

    Apparent observations of the external field effect in the Crater II galaxy, and apparent MOND-like behavior in wide binary stars, are two more very convincing pieces of evidence in support of modified gravity theories that are both completely inconsistent with dark matter halo theories. The former contradicts GR, which no dark matter particle theory can do without a fifth forth of a kind that hasn't been well formulated at this point in the dark matter particle context. It is extremely hard to model that later effect through a dark matter halo.

    [Note to mods - I'll try to sprinkle in some citations soon, there are papers for everything that I have indexed links to, but I'm a work and have already spent way too much time on physics today, so I've got to go. I'll also try to go back for some copy editing.]
     
  20. Aug 7, 2018 #19

    kimbyd

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    Missing from this are two important facts:
    1. Modified gravity models have so far utterly failed to explain the CMB data (here's one formal argument along these lines from 2011, which I'm pretty sure remains valid to this day: https://arxiv.org/pdf/1112.1320.pdf).
    2. The above argument asks you to disregard observations of comparatively simple systems (e.g. the CMB) and instead focus on comparatively complex systems (e.g. galaxies). Systematic uncertainties are far, far more likely to muddle our understanding of complex systems.
     
  21. Aug 7, 2018 #20

    Vanadium 50

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    I'm not saying it does. I am saying that a single observation (in this case the Bullet Cluster) is not nearly as simple and as unambiguous as the popular press (and some astronomers) make it out to be.
     
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