© CAMBRIDGE UNIVERSITY PRESS 1999

1.5 Dark-Matter Particles

1.5.1 Hot, Warm, and Cold Dark Matter

The current limits on the total and baryonic cosmological density parameters have been summarized, and it was argued in particular that ₀ 0.3 while _b 0.1. ₀ > _b implies that the majority of the matter in the universe is not made of atoms. If the dark matter is not baryonic, what is it? Summarized here are the physical and astrophysical implications of three classes of elementary particle DM candidates, which are called hot, warm, and cold. ⁽¹⁾ Table 1.3 gives a list of dark matter candidates, classified into these categories.

Table 1.3. Dark-matter candidates

Hot DM (HDM) refers to low-mass neutral particles that were still in thermal equilibrium after the most recent phase transition in the hot early universe, the Quantum Chromo-Dynamics (QCD) confinement transition, which took place at T_QCD 10² MeV. Neutrinos are the standard example of hot dark matter, although other more exotic possibilities such as ``majorons'' have been discussed in the literature. Neutrinos have the virtue that _e, _µ, and are known to exist, and as summarized in Section 1.5.3 there is experimental evidence that at least some of these neutrino species have mass, though the evidence is not yet really convincing. Hot DM particles have a cosmological number density roughly comparable to that of the microwave background photons, which implies an upper bound to their mass of a few tens of eV: m() = ₀ / n = 92 h² eV. Having ~ 1 implies that free streaming destroys any adiabatic fluctuations smaller than supercluster size, ~ 10¹⁵ M (Bond, Efstathiou, & Silk 1980). With the COBE upper limit, HDM with adiabatic fluctuations would lead to hardly any structure formation at all, although Hot DM plus some sort of seeds, such as cosmic strings (see, e.g., Zanchin et al. 1996), might still be viable. Another promising possibility is Cold + Hot DM with ~ 0.2 (CHDM, discussed in some detail below).

Warm DM particles interact much more weakly than neutrinos. They decouple (i.e., their mean free path first exceeds the horizon size) at T >> T_QCD, and they are not heated by the subsequent annihilation of hadronic species. Consequently their number density is expected to be roughly an order of magnitude lower, and their mass an order of magnitude higher, than hot DM particles. Fluctuations as small as large galaxy halos, 10¹¹ M, could then survive free streaming. Pagels and Primack (1982) initially suggested that, in theories of local supersymmetry broken at ~ 10⁶ GeV, gravitinos could be DM of the warm variety. Other candidates have also been proposed, for example light right-handed neutrinos (Olive & Turner 1982). Warm dark matter does not lead to structure formation in agreement with observations, since the mass of the warm particle must be chosen rather small in order to have the power spectrum shape appropriate to fit observations such as the cluster autocorrelation function, but then it is too much like standard hot dark matter and there is far too little small scale structure (Colombi, Dodelson, & Widrow 1996). (This, and also the possibly promising combination of hot and more massive warm dark matter, will be discussed in more detail in Section 1.7 below.)

Cold DM consists of particles for which free streaming is of no cosmological importance. Two different sorts of cold DM consisting of elementary particles have been proposed, a cold Bose condensate such as axions, and heavy remnants of annihilation or decay such as supersymmetric weakly interacting massive particles (WIMPs). As has been summarized above, a universe dominated by cold DM looks very much like the one astronomers actually observe, at least on galaxy to cluster scales.