ABSTRACT
The evolution of the universe depends crucially on the total matter density of the universe. Combined t to all cosmological measurements now implies that the total matter density equals the critical density of the universe, so the universe is at. Out of this, a contribution of about 70% comes from the vacuum energy (also called the dark energy) or the cosmological constant
Λ∼ 0.75×10−56 cm−2. (17.1)
Baryonic and nonbaryonic matter in the universe amount to only about 30% of the critical density, out of which only about 5% is baryonic matter (see section 15.2). The amount of nonbaryonic dark matter is then about 1/4 of the critical density of the universe, i.e.,
ρDM = 1.7×10−30 g cm−3. (17.2) Neutrinos could be a hot (relativistic) dark matter candidate, but the present limits on the neutrino masses imply that they cannot contribute to more than 1% of the dark matter. Moreover, the large scale structure formation and the cosmic microwave background anisotropy together require that most of the dark matter is nonrelativistic (cold dark matter). Thus, the ΛCDM (nonvanishing cosmological constant and cold dark matter) model is the most favored cosmological model at present, and the matter content implies that the universe is at and accelerating.
