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For reference:

Referring to a question I posted in Feb. 2015:

Could Hyper-Massive Black Holes be due to Dark Matter in the Early Universe?

User10851 answered, and I replied. He then replied to my comment with:

"at 1 Gyr old the average density of the universe was less than 1000 times what it is now"

Which does not make sense to me, and leads to my question:

How can the universe's density increase over time? To me, if the universe is expanding, the density is decreasing unless new particles are popping into existence at a greater rate than the expansion (which I don't believe).

The reason this is important to me is that the density fact seemed to be the stop-fact for that theory, but I don't believe it's accurate. At the same time, I don't like questioning experienced physicists.

I believe that if Dark Matter truly does not interact with electromagnetic fields, and is weakly-interacting, but remains affected by gravity, expansion, and other forces, then the quantity of dark matter would have been significantly greater (perhaps exponentially) after the big bang, and these early black holes that are larger than they are supposed to be could be explained by their gobbling up of massive quantities of dark matter in the early universe. Since Thompson Scattering and Eddington Luminosity do not affect dark matter as they do other forms of energy and matter, this could be an explanation for larger-than-expected black holes 900 million years after the universe's birth, which in the end leaves us with the quantity of dark matter we have today (about 5.75 times that of visible matter).

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  • $\begingroup$ Where am I going wrong here: if dark energy is accelerating the expansion, does that energy not increase the overall density over time? I am nothing but curious, not deliberately glib or any other motivation. At the same time, I don't like questioning experienced physicists I am definetly not in that category, or near it, but to me, that's the beauty of this site :) $\endgroup$
    – user167453
    Oct 10, 2017 at 19:58
  • $\begingroup$ @Countto10 If the dark energy is in the form of a cosmological constant then it wouldn't make sense that the energy density would be lower in the past. $\endgroup$
    – A. C. A. C.
    Oct 10, 2017 at 20:41
  • $\begingroup$ @OP The dark matter density around black holes in the early universe is lower because it hasn't had time to collect into clouds and are thus distributed pretty evenly. The evolution of galaxy super-clusters have the dark matter and normal matter forming filaments and voids tha have very little matter, the total matter density in the universe might be decreasing but over time where there is matter, there is more of it now and will continue to grow due to interactions of gravity. $\endgroup$
    – A. C. A. C.
    Oct 10, 2017 at 20:47

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It very much sounds like you misread the comment. "The average density of the universe was less than 1000 times what it is now" means that it was more than it is now, but less than 1000 times so. So, say 200 times so.

The density of the universe has definitely decreased over its lifetime. The dark energy density remains constant (in the simplest models, and nearly constant over the lifetime so far regardless), while the other components have decreased: Universe density

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  • $\begingroup$ I love clear crisp graphs like this. Where can one that goes back to fractions of a second from t = 0 be found? $\endgroup$
    – Richard Benish
    Dec 22, 2021 at 23:17
  • $\begingroup$ The red and blue density distribution of matter and radiation in that graph is wrong, since the volume expands exponentially in later times the density of matter and radiation shouldn't converge to a constant value, they should sink rapidly. The curves are bent the wrong way, they should look like yukterez.net/f/einstein.equations/files/t#9 (although the colors of radiation and matter are switched there, but nevertheless) $\endgroup$
    – Yukterez
    Feb 26, 2022 at 4:34

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