3.2 Cold Dark Matter (CDM)

In ΛCDM, dark matter is assumed to be made of non-baryonic dissipationless massive particles [48Jump To The Next Citation Point], the “cold dark matter” (CDM). This dark matter outweighs the baryons that participate in BBN by about 5:1. The density of baryons from the CMB is Ω = 0.046 b, grossly consistent with BBN [229Jump To The Next Citation Point]. This is a small fraction of the critical density; with the non-baryonic dark matter the total matter density is Ωm = ΩCDM + Ωb = 0.27.

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, 48Jump To The Next Citation Point]) 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 Ωm (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 z ∼ 6 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, ΛCDM 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 [252Jump To The Next Citation Point], 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.

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