The “cold” in cold dark matter means that CDM moves slowly so that it is non-relativistic when it decouples from photons. This allows it to condense and begin to form structure, while the baryons are still electromagnetically coupled to the photon fluid. After recombination, when protons and electrons first combine to form neutral atoms so that the cross-section for interaction with the photon bath suddenly drops, the baryons can fall into the potential wells already established by the dark matter, leading to a hierarchical scenario of structure formation with the repeated merger of smaller CDM clumps to form ever larger clumps.
Particle candidates for the CDM must be massive, non-baryonic, and immune to electromagnetic
interactions. The currently preferred CDM candidates are Weakly Interacting Massive Particles
(WIMPs, [46, 47, 48]) that condensed from the thermal bath of the early Universe. These
should have masses on the order of about 100 GeV so that (i) the free-streaming length is small
enough to create small-scale structures as observed (e.g., dwarf galaxies), and (ii) that thermal
relics with cross-sections typical for weak nuclear reactions account for the right amount of
matter density
(see, e.g., Eq. 28 of [48]). This last point is known as the WIMP
miracle5.
For lighter particle candidates (e.g., ordinary neutrinos or light sterile neutrinos), the damping scale
becomes too large. For instance, a hot dark matter (HDM) particle candidate with mass of a few to 15 eV
would have a free-streaming length of about 100 Mpc, leading to too little power at the
small-scale end of the matter power spectrum. The existence of galaxies at redshift
implies
that the coherence length should have been smaller than 100 kpc or so, meaning that even
warm dark matter (WDM) particles with masses between 1 and 10 keV are close to being
ruled out as well (see, e.g., [348]). Thus,
CDM presently remains the state-of-the-art in
cosmology, although some of the challenges listed in Section 4 are leading to a slow drift of
the standard concordance model from CDM to WDM [252
], but this drift brings along its
own problems, and fails to address most of the current observational challenges summarized
in the following Section 4, which might perhaps point to a more radical alternative to the
model.
http://www.livingreviews.org/lrr-2012-10 |
Living Rev. Relativity 15, (2012), 10
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