The Standard Model of Cosmology

Abstract

Cosmologists are now converging towards a unified picture of the Universe, similar to when particle physicists built the standard model of particle physics. The standard model of cosmology depicts the Universe as a mixture of five known particle species (photons, neutrinos, electrons, protons and neutrons), a hypothetical one (cold dark matter) and a mysterious dark energy component. The structure that we observe in the Universe (galaxies, clusters, filaments, voids and temperature fluctuations) is thought to have originated from the gravitational enhancement of small initial density perturbations over an otherwise homogeneous and isotropic background. In this chapter we build the standard model of cosmology step by step, starting from its foundations: the cosmological principle (Sect. 2.2), whereby the Universe is homogeneous and isotropic; the expansion of the Universe, first measured in the 1910s (Sect. 2.3); and general relativity (Sect. 2.4), the theory that enables us to compute the evolution of the Universe via the Einstein equation. The standard model naturally predicts that the Universe is permeated by a relic radiation, the cosmic microwave background (Sect. 2.5), whose discovery in the 1960s provided a spectacular confirmation of the Big Bang theory. The CMB is an instantaneous picture of the Universe after the Big Bang, and is therefore rich of cosmological information. This information, extracted thanks to space missions such as COBE, WMAP and Planck, allows to constrain the many models of cosmic inflation, the most popular theoretical description of the primordial Universe (Sect. 2.6). In particular, we stress that the non-Gaussianity of the CMB, via its bispectrum, is one of the most promising observables that can shed light on the physics in the early Universe.