A question relating to expansion rate of the Universe

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The discussion explores the factors influencing the expansion rate of the universe, questioning whether the conversion of mass into energy by stars significantly affects gravitational attraction and expansion. It concludes that the impact of star formation on expansion is negligible, as gravity acts on total energy density, and local gravitational forces can overpower expansion in bound regions. Observations suggest that voids between galaxy clusters are indeed expanding, consistent with a constant cosmological constant, although precise measurements of expansion rates in different regions remain challenging. The conversation also addresses dark matter's role, confirming that its gravitational influence is crucial for understanding the universe's expansion, and clarifies that the percentages of dark matter and ordinary matter change due to the dynamics of the early universe and the effects of cosmic expansion. Overall, the interplay of dark matter, energy density, and gravitational forces is essential for comprehending cosmic expansion.
  • #31
Answer is still the same as when posted in #25.

edit; Maybe I should say that's about as much as we think we know...
there seems to be a lot more we don't know than we know..."Do you have a good link for this description of DM?"

not so much...search these forums, and Wikipedia has a good outline under DARK MATTER...

and I think also discusses MACHOS and WIMPS...

and I see Drakkith has posted links to hot,warm, cold dark matter...

lots of candidates, lots of questions, not so many detailed answers yet.
 
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  • #32
Tanelorn said:
What stops DM from accumulating inside the rims and possibly near the centers of galaxies or is this unknown?

The issue is that DM can't interact through anything other than gravity. What happens is that DM follows an orbit around our galaxy just like everything else. In order for it to fall in towards the center of our galaxy and stay it must give up energy, otherwise it just swings back around all the way out to the halo region again. (or doesn't fall in at all)

When you have a dust cloud that is collapsing under its own gravity, the particles get rid of energy by colliding with each other and giving off EM radiation. This allows it to collapse into dense regions and eventually form stars, planets, etc.

But dark matter can't do this. It simply passes through itself without absorbing or giving off any energy. So it can't collapse into dense regions very easily, if at all.
 

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