Dark Matter Candidates 2170- 1
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log(a) a^0DM baryonslog(δp/p)aeq = 3 10–5adec = 10 –3Figure 9.3Time-dependence of the DM and baryon density fluctuations,훿휌∕휌. The pertur-
bations in DM start to grow near the epoch of matter–radiation equality,푡eq. However, the
perturbations in the baryons cannot begin to grow until just after decoupling, when baryons
fall into the DM potential wells. Within a few expansion times the baryon perturbations catch
up with the DM perturbations.
determined by the baryonic fluctuations just before recombination. This is illustrated
in Figure 9.3.
The lightest DM particles are slow enough at time푡eqto be bound in perturbations
on galactic scales. They should then be found today in galaxy haloes together with
possible baryonic MACHOs. If the nonbaryonic DM in our galactic halo were to be
constituted by DM at a sufficient density, they should also be found inside the Sun,
where they lose energy in elastic collisions with the protons, and ultimately get cap-
tured. They could then contribute to the energy transport in the Sun, modifying solar
dynamics noticeably. So far this possible effect has not been observed.
On the other hand, if the DM overdensity only constituted early potential wells for
the baryons, but did not cluster so strongly, most WIMPs would have leaked out now
into the intergalactic space. In that case the DM distribution in clusters would be more
uniform than the galaxy (or light) distribution, so that galaxies would not trace mass.
Hot and Warm Dark Matter. Although the neutrinos decoupled from the thermal
plasma long before matter domination, they remained relativistic for a long time
because of their small mass. For this reason they would possibly constitutehot dark
matter(HDM), freely streaming at푡eq. The accretion of neutrinos to form haloes
around the baryon clumps would be a much later process. The CMB is then very little
perturbed by the clumps, because most of the energy is in neutrinos and in radiation.
However, we already know that the neutrino fraction is much too small to make it a
DM candidate, so HDM is no longer a viable alternative.