2 - The expanding universe

Abstract

The smooth universe is described by the FLRW metric. All relevant distances in the expanding universe can be obtained from the comoving distance, which is directly related to the conformal time. The geodesic equation for this metric predicts that the physical momentum of particles decays, so that the universe cools. After introducing the equilibrium distributions for different particles, we perform an inventory of the constituents of the universe: photons (CMB), neutrinos, baryons, cold dark matter, and dark energy, a constituent with negative pressure which, so far, is found to be consistent with being a cosmological constant. The universe has been dominated by matter and dark energy for the bulk of its history, but it was radiation dominated early on, before the epoch of equality. Due to their nonzero mass, cosmic neutrinos became nonrelativistic at late times.