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I Qs: What is known and known to be unknown about DM

  1. Jan 11, 2017 #1
    This thread is seeking help in finding authoritative sources regarding questions about dark matter (DM). It is motivated by the discussion in two other threads:

    Here is what I think is well known. If this is wrong, I would appreciate being corrected.
    1. There is approximately four times as much DM in the universe as ordinary matter.
    2. This was learned from three sources.
    (a) Velocity profiles from the stars in galaxies
    (b) Estimates of ordinary matter density from visible sources.
    (c) finding the best fit solution to Friedmann equation (below) that fits astronomical red-shift and brightness data from distant supernovas.​
    3. Over a decade of investigations regarding the possibility that DM consists of WIMPs has so far been unsuccessful.
    4. Not much else is known with any high degree of certainty.

    Here are a few questions to get started.
    1. What is known about the distribution of DM surrounding a galaxy, if any?
    2. If there is such knowledge, is this knowledge obtained from only velocity profiles or also from other methods?
    3. Is the distribution known to be
    (a) close to being uniform,
    (b) non-uniform, but spherically symmetric or nearly so,
    (c) something else, or
    (d) unknown?​

    Any responses will be much appreciated.
  2. jcsd
  3. Jan 11, 2017 #2


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    This is incomplete. Better evidence comes from studies of galaxy clusters. The strongest evidence, in my mind, comes from CMB observations, which demonstrates that there was a form of matter which didn't interact with photons in the early universe, and is the most accurate measurement available of dark matter's density.

    Some evidence of the distribution comes from measuring satellites of galaxies, but this can only be done for our own galaxy and a handful of very nearby ones, as those satellites are so faint.

    For galaxy clusters far away, the evidence comes primarily from gravitational lensing.

    In general, I believe the distribution of the dark matter is similar to elliptical galaxies (that is, spheroidal, but not spherically symmetric).
  4. Jan 11, 2017 #3
    Hi @Chalnoth:

    Thank you much for your response, especially your adding the information about the CMB observations.

    About the quote above, two items.
    1. I would much appreciate a citation of a references that discusses the plausible distribution of DM and explains the reasoning.
    2. I am not sure what your description of the density means. My guess is that you mean the distribution is like a partially flattened sphere, e.g. like the earth's flattening at the poles. Also, I am guessing that this means the distribution would be radially symmetric using an adjusted coordinate system in which the z-axis is enlarge enough to remove the flattening. Are these guesses correct?

  5. Jan 11, 2017 #4


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    Wikipedia goes into it a bit, and has some references:

    I'm not sure that a dark matter halo would be radially symmetric.
  6. Jan 11, 2017 #5
    Hi @Chalnoth:
    Thanks for the Wikipedia reference. It looks familiar. I think I looked at this a while ago, and had forgotten it.

    Here are some quotes I would like to ask your opinion about.
    The formation of dark matter halos is believed to have played a major role in the early formation of galaxies. During the phase of time in the universe when galaxies were first formed, the temperature of the baryonic matter should have still been much too high for it to form gravitationally self-bound objects, thus requiring the prior formation of dark matter structure to add additional gravitational interactions. The current hypothesis for this is based on cold dark matter (CDM) and its formation into structure early in the universe.
    The hypothesis for CDM structure formation begins with density perturbations in the Universe that grow linearly until they reach a critical density, after which they would stop expanding and collapse to form gravitationally bound dark matter halos. These halos would continue to grow in mass (and size), either through accretion of material from their immediate neighborhood, or by merging with other halos.​

    This seems to me to be saying that the DM experiences some form of gravitational collapse. That is, it experiences some form of "friction" which enables gravitational potential energy to dissipate, in a similar manner like ordinary matter collapses due to EM interactions which removes some potential energy by radiating photons. Does this seem plausible to you, or do you know of any alternative explanation? In looking over the Wikipedia article, I do not find any explanation of the collapse mechanism.

  7. Jan 11, 2017 #6


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    Not quite. These models assume no dynamical friction at all for the dark matter.

    My understanding is that what happens early-on is that there were a number of regions that were slightly more dense, and others that were slightly less. As the expansion slowed down over time, the slightly more dense regions stopped expanding due to their self-gravity. These structures would then remain stable as the rest of the universe expanded. So it's not so much that the dark matter collapses, as it stops expanding once the expansion slows enough that the self-gravity of the dark matter can take over.
  8. Jan 11, 2017 #7
    Hi @Chalnoth:
    I need to think some more about this explanation, maybe try to do a little math.

    What seems a bit odd is that during the early universe, say just following inflation, the expansion was decelerating. (The accelerating period came much later, long after the formation of galaxies.) The following is a bit oversimplified, but I think as a first approximation this deceleration implies that once a more-or-less spherical body of matter, say of mass M, is within a certain radius R, AND that M was sufficiently large so that the Hubble expansion at a distance R from the center of mass was less than the escape velocity at radius R, THEN most of the matter inside this sphere would be forever remain bound to itself. I would guess this to be an enormous amount of matter, with a extremely large radius. What would make that large mass of DM get smaller, say to the size of a galaxy super-cluster? Wouldn't that shrinkage require friction?

  9. Jan 11, 2017 #8


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    I don't think it did get smaller. I think it stayed at the roughly the same size.

    This is all discussing a linearized approximation of the dynamics, though. I'm sure things get more complicated for smaller regions in particular.
  10. Jan 11, 2017 #9


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    No, it doesn't. Deceleration is not the same as being bound. For a simpler example, consider an object thrown upward from the surface of the Earth at just above escape velocity. It will continually decelerate as it rises, but it is not bound to the Earth; it will never come to rest and fall back down again.
  11. Jan 12, 2017 #10
    Hi @Chalnoth:
    I have been attempting to read the following article.
    GALAXY CLUSTER 1E 0657–56
    I confess that the technical language makes it very difficult for me to understand much of what I read. Here is a quote from the abstract.
    The weak-lensing mass map reveals a dark matter clump lying ahead of the collisional gas bullet, but coincident with the effectively collisionless galaxies. From these observations, one can directly estimate the cross-section of the dark matter self-interaction.​

    My admittedly uncertain interpretation of this is the following. The article reports detecting a clump of dark matter, and I think this must be very much smaller than those "clumps" which we discussed previously as being created soon after the inflation period. Do you agree with this interpretation of what the article says? If so, how would you reconcile this with the following?
  12. Jan 12, 2017 #11
    Hi @PeterDonis:

    I see now that the way I described the relationship between (a) the nature of the expansion and (b) the gravitationally bound condition of a clump of matter was rather sloppy.

    I have come to understand since a discussion in a thread over a year ago that comparing Hubble velocity with escape velocity is NOT a best way to determining whether a body of matter is sufficiently gravitationally bound to survive expansion.
    I then gave the following source for this heuristic approach:
    An Introduction to the Science of Cosmology by D J Raine & E G Thomas (2001).​
    On page 13 this approach is used discussing material not escaping from a galaxy.

    In the above cited thread I discussed another approach which compares (a) the rate of acceleration which the Hubble expansion causes at a distance R between co-moving coordinates with (b) the gravitational acceleration on a test particle due to a point mass M at a distance R. The point of that thread was a discussion of these two different methods of deciding how big sphere of matter of mass M could be and remain gravitationally bound with respect to Hubble expansion.

    For my post in this thread I used the heuristic velocity comparison method to avoid the complexity of describing the acceleration comparison method.

    The point I was trying to make about the deceleration of the expansion was that if a body of (non-shrinking) DM is gravitationally bound as stable with respect to expansion at some particular time, it will continue to be stable as long as the expansion continues to decelerate. However, if the expansion accelerates, then this would not be necessarily so.

  13. Jan 12, 2017 #12


    Staff: Mentor

    This I agree with. The initial condition that the system is bound at some particular time makes a big difference.
  14. Jan 13, 2017 #13
    Hi @Chalnoth:
    I have a question about the Wikipedia article you cited in your post 4. Here is a quote from the article:
    The collapse of overdensities in the cosmic density field is generally aspherical. So, there is no reason to expect the resulting halos to be spherical. Even the earliest simulations of structure formation in a CDM universe emphasized that the halos are substantially flattened.[17] Subsequent work has shown that halo equidensity surfaces can be described by ellipsoids characterized by the lengths of their axes.[18]

    Because of uncertainties in both the data and the model predictions, it is still unclear whether the halo shapes inferred from observations are consistent with the predictions of ΛCDM cosmology.​
    I have underlines the text I want to ask you about.

    I would lie to learn about the mechanism that caused the flattening of the halos. With ordinary matter the mechanism for flattening is "friction" due to interactions with electromagnetic fields and the loss of gravitational potential energy through photon radiation. Can you describe the DM mechanism for flattening?
    The reference [17] is
    Davis, M., Efstathiou, G., Frenk, C. S., White, S. D. M. (1985), ApJ. 292, 371​
    I tracked this to a abstract at
    I do not have access to this journal. Can you help me?

  15. Jan 13, 2017 #14


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    Have you tried this:
    http://articles.adsabs.harvard.edu/cgi-bin/nph-iarticle_query?1985ApJ...292..371D&data_type=PDF_HIGH&whole_paper=YES&type=PRINTER&filetype=.pdf [Broken]
    Last edited by a moderator: May 8, 2017
  16. Jan 13, 2017 #15
    Hi @fresh_42:
    Thank you very much. I downloaded from your link and it worked just fine.

    Last edited: Jan 13, 2017
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