Aseyhe t1_itb8a8s wrote
In the early universe, both the gas and the dark matter were cold. With little thermal motion, both components were able to participate in the gravitational clustering that formed galaxies. Consequently, there are comparable amounts of dark matter and ordinary matter in galaxies. That means dark matter contributes significantly to gravitational dynamics.
When material falls into galaxies, it gains kinetic energy, becoming too hot to participate in further gravitational clustering. Gas can cool, e.g. through inelastic collisions, but dark matter cannot. Star systems, which form inside galaxies, thus form in an environment where the dark matter is hot but the gas is cold. That means the gas can participate in the gravitational clustering but the dark matter cannot. Consequently, star systems have much more ordinary matter than dark matter, so the contribution of dark matter to gravitational dynamics is negligible.
manugutito t1_itb8pzd wrote
The simple fact that dark matter seems to have coalesced in blobs means it must feel some interaction besides gravity, right? Otherwise it would not be able to 'cool down' and collect in a single place? Or could normal matter do a sort of 'sympathetic cooling' for dark matter?
Aseyhe t1_itb94zc wrote
Not necessarily. Dark matter cools in the early universe due to cosmic expansion (it's the same phenomenon as cosmological redshift, but it's even more efficient for nonrelativistic particles). The fact that dark matter clusters in galaxies does give us information about what dark matter could be, though! For example, it can't be primarily composed of ordinary neutrinos (which are technically dark matter), because they would be too hot to reproduce the observed clustering.
Nieshtze t1_itocg1w wrote
So dark matter halos in galaxies are formed as a result of cosmological expansion?
Aseyhe t1_itp1490 wrote
Assuming dark matter wasn't created cold enough to cluster on those scales, then yeah, that's a fair interpretation. However it should be noted that on galactic scales, the temperature of the dark matter was not the limiting influence on when structures began to form. That was set by the initial amplitude of variations in the density of the universe and the details of how they grew over time (which are determined by certain aspects of the history of the universe).
Since the dark matter temperature was not the limiting influence, we actually have no clear evidence what its temperature initially was. However, by probing dark matter halos at smaller and smaller scales, we might be able to determine it. The impact of temperature becomes more important at smaller scales.
Nieshtze t1_itrthi4 wrote
I see, that's very interesting! So the only 'dissipative' force on dark matter is the cosmological expansion, and the resulting clustering is determined by the density fluctuations of the early universe?
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