PDF《18. DARK MATTER》

1 (1998) available on the PDG WWW pages (URL: http://pdg.lbl.gov/) June 24,

1998 14:34

2 18. Dark matter There are, also, several indirect arguments which argue for a substantial amount of nonbaryonic dark matter. First, nucleosynthesis gives the limits 0.010 ≤ ?bh2

0 ≤ 0.016 for the total mass of baryons;

h0 is de?ned in Eq. (15.6) (in Section

15 on Big-Bang Cosmology in this Review). The upper limit on ?b is substantially below the value ?dm &

0.3 given by large scale measurements, even if h0 is near the lower end of its optimistically allowed range, 0.4 ≤ h0 ≤ 1.0. A second, purely theoretical argument is that in?ationary models (widely regarded as providing explanations of a number of otherwise puzzling paradoxes) generically predict ?total = 1. Finally, it is di?cult to construct a model of galaxy formation without nonbaryonic dark matter that predicts su?ciently small ?uctuations in the cosmic microwave background radiation [7]. For purposes of galaxy formation models, nonbaryonic dark matter is classi?ed as hot or cold, depending on whether the dark matter particles were relativistic or nonrelativistic at the time when the horizon of the universe enclosed enough matter to form a galaxy. If the dark matter particles are in thermal equilibrium with the baryons and radiation, then only the mass of a dark matter particle is relevant to knowing whether the dark matter is hot or cold, with the dividing line being mdm ?

1 keV. In addition, specifying a model requires giving the power spectrum of initial density ?uctuations. In?ationary models generically predict a power spectrum which is nearly scale invariant. Given this, models with only cold dark matter are much more successful than models with only hot dark matter at reproducing the observed structure of our universe, but there are still serious discrepancies [8]. Some of the suggestions proposed to alleviate these include a nonzero value of the cosmological constant Λ [9], signi?cant deviations from scale invariance in the spectrum of initial ?uctuations [10], and a mixture of both hot and cold dark matter [11]. Another class of models uses mass ?uctuations due to topological defects [12]. The best candidate for hot dark matter is one of the three neutrinos, endowed with a Majorana mass mν. Such a neutrino would contribute ?ν = 0.56........