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Re Dark Matter and the Horizon Problem

  1. Sep 30, 2015 #1
    As I understand it, the statement of the horizon problem assumes that the uniformity of the CBR measured at opposite directions in the sky needs a mechanism to create this uniformity. I also understand that many cosmologists do not share this assumption.

    The purpose of this thread is to seek informed responses to the following three questions:

    Q1. Have any completed astronomical surveys of dark matter provided any clear evidence showing whether or not dark matter is approximately as uniform in its distribution towards opposite directions of the sky as is the CBR?

    Q2. If not, is it possible in principle that dark matter astronomy could show that dark matter is or is not approximately similar in its distribution towards opposite directions of the sky as compared with the CBR?

    Q3. Is the following reasoning cosmologically OK?
    I have the idea that if dark matter astronomy did show that dark matter is approximately similar with the CBR with respect to a high uniformity of mass/energy distribution towards opposite directions of the sky, then this would imply that inflation was unnecessary to solve the horizon problem. My reasoning is that if the distribution of dark matter is approximately uniform, then since inflation could not have produced such an effect, there is no reason to suppose it is necessary to produce the uniformity of the CBR distribution.
  2. jcsd
  3. Sep 30, 2015 #2


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    Yes. The brightness of the CMB is correlated with the total matter density, so there's a pretty strong relationship between the CMB anisotropies and the dark matter density in different directions. Surveys of dark matter from the nearby universe are very much in line with this.

    Why would you think this? Dark matter has nothing to do with the horizon problem.

    The horizon problem arises because if you simply take a universe with matter and radiation in it, and extrapolate that universe backward in time, then points on the CMB further from one another than about one degree on the sky could never have communicated. Dark matter wouldn't help different parts of the universe communicate. Inflation does help because it changes the expansion history so that there was plenty of time for every point on the sky to communicate and come into equilibrium.

    Furthermore, inflation's impact on the uniformity of dark matter is more or less the same as its impact on the uniformity of normal matter. The issue is that in order for dark matter production models to work, the dark matter particles couldn't have been produced until after inflation. When the inflaton decays, there is this incredibly hot soup of particles, hot enough for the dark matter particles to interact strongly. As it cools, the dark matter particles "freeze out" as their interactions are no longer very strong, and the density that remains at the time of freeze-out mostly sticks around to today.
  4. Sep 30, 2015 #3
    Hi Chalnoth:

    I much appreciate you response to my questions. The points you make are quite fascinating. I have never seen them before in anything I have read about dark matter.

    This points seems to say that in addition to solving the horizon problem and the flatness problem, inflation also is responsible for the creation of dark matter. Is the the only explanation for the origin of dark matter currenly accepted by cosmologists and physisists? Or is this a controversial idea? Can you recommend some sources I might read that discusses this idea in more detail?

    I assume you mean that the these strong interactions must have been between dark matter particles and baronic particles. Is this also a non-controversial idea?

  5. Sep 30, 2015 #4


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    No, not really. Inflation is responsible for setting up a smooth, high-temperature environment, as well as producing small variations in temperature from place to place due to zero-point quantum fluctuations. Dark matter would come from the physics that result from that early high-temperature environment.

    Yes, essentially.

    Neutrinos act exactly like this: neutrinos interact with other matter through the W and Z bosons, which mediate the weak nuclear force interactions. At high temperatures, neutrinos interact readily with normal matter. But once the temperature drops significantly below the masses of the W and Z bosons, those interactions become far more rare. Dark matter particles might act similarly: they could interact with the weak force just like neutrinos. Or they might only interact through some other weak force that we don't yet have a model for.

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