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B Is dark matter cold or warm

  1. Nov 12, 2016 #1


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    According to this article dark matter is warm, and provides a mechanism for showing the lack of satellite around the milky way.

    September 8, 2014
    Royal Astronomical Society (RAS)
    Scientists believe they have found a way to explain why there are not as many galaxies orbiting the Milky Way as expected. Computer simulations of the formation of our galaxy suggest that there should be many more small galaxies around the Milky Way than are observed through telescopes. This has thrown doubt on the generally accepted theory of cold dark matter, an invisible and mysterious substance that scientists predict should allow for more galaxy formation around the Milky Way than is seen. Now cosmologists think they have found a potential solution to the problem.
  2. jcsd
  3. Nov 12, 2016 #2
    The more precise wording of the question is "what is the average velocity of DM particles?"
  4. Nov 12, 2016 #3


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    Actually, the paper is discussing an alternative to warm DM in the form of self interacting DM [SIDM]. A good discussion of the SIDM model is offered here; https://arxiv.org/abs/1512.05349, ETHOS - An Effective Theory of Structure Formation: Dark matter physics as a possible explanation of the small-scale CDM problems. The interesting possibility that DM may consist of multiple particle species is not so subtly hinted at on page 3 of this paper:

    " We would like to stress that all these different modifications
    to the CDM paradigm are also motivated by the fact that the
    most simple CDM candidates (Weakly Interacting Massive Particles,
    WIMPs) have not been discovered despite decades long efforts
    (e.g., Bertone et al. 2005; Bertone 2010). Furthermore, there
    has also been no sign Supersymmetry at the LHC so far, which has
    provided a strong theoretical support for several excellent CDM-WIMP
    candidates like the neutralino (Jungman et al. 1996). Based
    on these null results, the time is ripe to think beyond simple CDM
    models. The fact that some of these alternative models can actually
    also solve astrophysical problems with CDM should be seen as a
    motivation to go beyond the purely particle physics based considerations.
    In the end there is also no compelling reason for the fact that
    the dark sector should be dominated by a single featureless WIMP
    particle only. After all, the visible sector is very rich, and this might
    also be true for the dark sector. From the perspective of structure
    formation, the CDM model is only an effective description that assumes
    that the only DM interaction that matters is gravity. Since
    such an assumption remains unverified, it is crucial that we develop
    a more generic effective framework that includes a broad range of
    allowed DM interactions."
    Last edited: Nov 12, 2016
  5. Nov 13, 2016 #4
    An even more precise wording could be: What is the variance of the velocity distribution of DM particles close to the centre of a "Dark Matter Object" ?
    An alternative way to ask this kind of questions could be:
    What kind of DM particle velocity distributions would allow stable "Dark Matter Objects", which survive gravitational interaction with "Normal Matter Objects" or with other Dark Matter Objects.
  6. Nov 13, 2016 #5
    A fundamental question beyond properties of dark matter is: Do large normal matter objects (suns, planets) contain a fraction of dark matter?
  7. Nov 13, 2016 #6
    No. You are changing the subject, whereas I was just making the question more quantitative. ("hot" and "cold" qualifiers are way too coarse, and they give incorrect impression that there are only two alternatives).

    Velocity alone is not enough to determine whether DM particles (if we assume DM is indeed particulate) can form bound objects. For example, neutrinos, even very slow ones, will not form any structure.

    We are almost certain DM particles do not form bound structures with normal matter.

    We don't know whether DM particles can form bound structures with themselves. Say, "mirror matter" subclass of possible DM theories has new, EM-like charges and interactions, and DM may in fact be "atoms" or "molecules" with several "mirror matter" particles bound into a tiny structure (which still does not interact with normal matter).
  8. Jan 9, 2017 #7
    Hi @nikkkom:
    I am quite curious about the above quote. By "We" I assume you refer to the population of physicists who study dark matter. Would you please cite any authoritative articles which explain this “almost certain” conclusion?

    I am mostly ignorant about the details of studies which discuss the likely properties of dark matter (DM). It seems to me logical that with some assumptions about the properties of DM, it would be expected that DM would sometimes form gravitationally bound structures with normal matter. Therefore I am guessing that there should be some studies that explain reasons why such assumptions are not consistent with observations and experiments.

    One example of such conceivable DM properties would be that they had some kind of force analogous with electro-magnetism (EM) with particles analogous to photons. The DM particles would not affect ordinary matter, but would enable a collection of DM to loose some their gravitational potential energy and form compact bodies in the same manner that EM has that effect on ordinary matter.

    Last edited: Jan 9, 2017
  9. Jan 9, 2017 #8
    Nikkkon was probably referring to small scale structures. It's believed that the largest scale structures were formed mostly by dark matter simply because it dominates the gravity.

    I think he's saying that we're certain there is no hidden dark matter particle inside of protons or invisibly hanging onto water molecules. The reason we don't is simply because the equations work now without it.

    I find the idea of complex dark matter so intriguing. I'm also hopeful that it's the case and we have a strange new chemistry to explore. Chemistry requires pretty low energy though doesn't it?

    A related tangent question: I know we've mapped the distribution of dark matter in the galaxy. If we used the exact same procedure to estimate the distribution of regular matter, what resolution would we see? Surely not stars, maybe clusters?
  10. Jan 9, 2017 #9
    Is that an assumption that is being made? I'd honestly never thought about it before but does it have to be? Energy gravitates, not just particles. The Higgs field has energy in it but isn't made of particles, I assume a model with another field that's not at it's ground state has been eliminated or just doesn't work for some reason that I'm missing?
  11. Jan 9, 2017 #10
    Hi @newjerseyrunner:
    I recently read a paper from 1999 by Robert Foot: "Have mirror planets been observed?"
    From https://en.wikipedia.org/wiki/Mirror_matter
    If mirror matter does exist in large abundances in the universe and if it interacts with ordinary matter via photon-mirror photon mixing, then this could be detected in dark matter direct detection experiments such as DAMA/NaI and its successor DAMA/LIBRA.​
    I gather from these (and other sources) that there are all kinds of ideas about hypothetical particles, some of which are proposed as the constituents of dark matter (DM) with a variety of properties. Do you know any observational or experimental results that makes it impossible for DM to form in the Milky Way collapsed structures such as a sun sized or planet sized object, although perhaps on a much slower time scale than ordinary matter suns and planet?

    If such objects could possibly occur in the Milky way, would it be possible to find them astronomically?

    Suppose there were two objects gravitationally bound as a binary system: (1) a sun like ours, and (2) a compressed DM object, say ten times the mass of our sun, but not so compressed as to form a black hole. The sun (1) in such a system should be as observable as any star similar to our sun might be seen in the Milky Way by a suitable telescope. The motion of (1) might well appear to be like a star gravitationally bound to a ordinary matter black hole (2). The mass and location of (2) in the sky could be calculated from the movements of (1) in the sky relative to more distant stars. Could (2) as a DM object (not a black hole) be distinguished from (2) as a black hole? I would guess that (2) as a black hole might create detectable gravitational lens effects, while a DM (2) would not.

    What do you think about this logical possibility? is it physically possible within the context of what is known about DM, as well as what is known to be unknown about DM?

  12. Jan 9, 2017 #11


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    That basically ignores all the evidence we have about DM, which pushes it into the realm of speculation. Speculation is good, sometimes even productive, but the odds are not all that great. The odds say science is fantastically good at modeling physical reality. Original models usually meet heavy resistance for good reasons. New ideas compete against many thousands of well educated and brilliant people who have dedicated their lives to studying the subject. A few few well thought out ideas survive this test, but, only a few. The people who have really studied this stuff are not idiots nor do they concede without posing serious questions.
  13. Jan 10, 2017 #12
    Hi @Chronos:
    I think you misunderstood the purpose of my questions. I am not trying to put forth ideas at odds with what is known about DM. I am trying to present examples of my ignorance in order to seek information about WHY these ideas are no good. I would much appreciate seeing some citations of articles which would explain WHY these ideas are no good, that is show the observational or experimental evidence that support models that demonstrate that these ideas no good. I do not have the skills to find such articles without help.

  14. Jan 10, 2017 #13


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    Infrared observations by the WISE telescope have ruled out the existence of a body in excess of 95 Earth masses out to about 10,000 AU, and a body exceeding 300 earth masses object out to about 26,000 AU. The orbits of known Kuiper belt objects further support these constraints. For discussion of the WISE mission and results, see; http://iopscience.iop.org/article/1...C5151774B037A401B90.c2.iopscience.cld.iop.org, A SEARCH FOR A DISTANT COMPANION TO THE SUN WITH THE WIDE-FIELD INFRARED SURVEY EXPLORER
  15. Jan 11, 2017 #14
    Hi @Chronos:
    Your post confuses me. I cannot understand what it has to do with the posts I have been making asking about the possibility of detecting a large (e.g., 10 solar masses) DM (not a black hole) object somewhere in the Milky Way. Your post seems to be about a failed search seeking to find a planet size ordinary matter object near our sun.

  16. Jan 11, 2017 #15
    That's extremely unlikely. Planets and stars form not just because of gravity, but because of friction. Huge amounts of gas and dust need to slow down into similar paths in order to become part of a protoplanet or star. Dark matter doesn't appear to have any friction. In the bullet cluster we can see where two galaxies have passed through each other. The gas, as we'd expect from particles that hit each other, all ended up in between them, but the dark matter hugged the galaxies, indicating that dark matter passes through other dark matter as though it weren't even there.
  17. Jan 11, 2017 #16
    Hi @newjerseyrunner:
    Thank you for your post. I would much appreciate citations for any sources that discusses the above quote. I have made a hypothetical guess that there might be some, although much less friction than that which ordinary matter has. I would very much like to see HOW that guess is known with confidence to be unsound.

  18. Jan 11, 2017 #17
  19. Jan 11, 2017 #18
    Since there is actually more DM than normal matter, If DM could form planet- and star-sized bound structures, then there would be many stars in the Galaxy which are paired with an invisible massive DM object instead of a normal, visible star. Something like 50% of double stars would be like that. This would be very easy to detect with todays telescopes, by making a multiple star system survey and doing basic statistics.

    This is definitely not observed.
  20. Jan 11, 2017 #19
    Hi @nikkkom:

    Thank you for your response. That is what I guessed would be the case, and I much appreciate your confirming this. I interpret this to mean that this result would enable a suitable educated DM researcher to calculate an estimate of the maximum amount of self "friction" DM has as a ratio to the "friction" which ordinary matter has via photon radiation. Have you ever seen such a calculation? If not, can you guess a rough estimate of what this ratio might be?

  21. Jan 11, 2017 #20


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    It appears your original question was in terms of " It seems to me logical that with some assumptions about the properties of DM, it would be expected that DM would sometimes form gravitationally bound structures with normal matter." Obviously, any such structures would be indistinguishable from normal matter in terms of gravitational influence on the orbits of other normal matter structures. While it is more difficult to gauge the effects of diffuse, non-clumping dark matter on orbital dynamics, it is certainly possible as discussed here; https://arxiv.org/abs/1306.5534, Constraints on Dark Matter in the Solar System. Taking the general case to extra solar structures. We would expect to find many multiple star systems where a visible star is being tugged around by an unseen companion. While it might be tempting to claim this as evidence for a black hole companion, it would be equally tempting to look for other symptom, such as xrays due to accretion of ordinary matter. This is the method used to affirm Cygnus X-1 as a probable black hole. I am unaware of any multiple star systems known to include an unseen massive companion that defies detection at all EM frequencies. Does that mean EM quiet massive bodies do not exist or merely such objects are difficult to detect? Absence of evidence is not evidence of absence, as the saying goes. It makes perfect sense to me to search your own backyard [solar system] for such objects before looking for clues on distant properties.
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