Dark Matter - Hot and Cold

So the fluctuations in baryons can't grow up on their own to be galaxies. We have to help them along with non-baryonic dark matter.

How does this help? Remember that:

So if we can start the fluctuations growing earlier, we can get bigger fluctuations at a later time.

But remember that before recombination, the baryons and photons were coupled, and the photons kept the baryons from collapsing under their own gravity. So we can't start earlier if all the mass is baryonic. But if we postulate some sort of matter that doesn't interact with radiation the way baryons do, it can start growing much earlier.

When we look at the CMB, the fluctuations in baryons that we see are sitting on top of much stronger fluctuations in the dark matter. Once recombination occurs, the baryons are free to collapse under gravity, and they quickly fall into the dark matter concentrations to form into galaxies. We think...

Once we get to delta ~ 1, we say that the region has decoupled from the normal Hubble flow, and has started to collapse under gravity. The timescale for gravitational collapse is then on the order of the free fall time:

And so low density lumps collapse more slowly. Look at the density of different objects:

(Rough!) Overdensities of Objects
Object	rho [M_sun/pc³]	delta
galaxy nucleus	150	10⁹
galaxy	0.2	10⁶
galaxy cluster	10^-5	50

More massive structures are lower in density, and take longer to collapse.

So dark matter can grow early. It is important to distinguish now between different flavors of dark matter:

baryonic dark matter: doesn't help. we want nonbaryonic dark matter!
nonbaryonic dark matter: we characterize dark matter by the random velocities of the particles

hot dark matter: relativistic velocity, like neutrinos
cold dark matter: moving more slowly, no known particles (hypothetic axions, WIMPS, etc)

Different types of dark matter result in different evolutionary histories for the Universe.

Hot Dark Matter

Because HDM particles are moving so fast, they can escape from small mass density fluctuations. Since it is their mass that makes the density fluctuation, these small fluctuations will essentially dissolve. Calculations for neutrinos suggest that any density fluctuation smaller than about 10¹⁵ M_sun will dissolve away before recombination, so the baryons won't collapse into small lumps.
Instead, only the big surviving lumps will collapse. The scale of these lumps is like that of big clusters of galaxies, which have relatively low overdensities, so the collapse occurs slowly. Then after the big things collapse, fragmentation can occur (like individual stars form out of a bigger collapsing gas cloud). So,

structure forms slowly

structure forms "top down"

galaxies form very late in the Universe's history

Not like what we see! Hot dark matter doesn't work!

Cold Dark Matter

CDM particles do not diffuse out of small lumps. So lumps exist on all scales -- small and large. The little things collapse first, and the big things collapse later, incorporating the little things in as they collapse.

structure begins to form early

structure forms "bottom up"

galaxies form before galaxy clusters.

This gives a much better description of what we see in the universe, and leads to a picture for structure formation called hierarchical structure formation.

Hot versus Warm vesus Cold dark matter universes (courtesy ITC @ University of Zurich):

Top row: Simulations of what structure in Hot (left), Warm (middle), and Cold (Right) dark matter universes would look like at high redshift (early times).
Bottom row: same as top row, except now as they would look at the present time (z=0).