Cosmological Neutrinos and Their Influence on the Evolution of the Universe

Abstract

Being one of the most mysterious particles of the Standard Model, neutrinos have opened up new opportunities for astrophysical research. The high penetrating power of neutrinos provides insight into stellar interior and makes it possible to study the mechanisms of the origin of ultra-high energy cosmic rays. When a star explodes, a neutrino flare informs us about this event several hours earlier than electromagnetic radiation. Neutrino also plays a crucial role in cosmology, being the second most abundant known particle in the Universe. In the radiation-dominated epoch, neutrinos together with photons, determine the dynamics of the expansion of the Universe. Later, becoming nonrelativistic, the neutrinos increase the contribution Ω_m of nonrelativistic matter, which previously consisted of cold dark matter and baryonic matter. Since neutrinos affect the course of the evolution of the Universe, this fact should be taken into account when determining cosmological parameters. Recently, in addition to relic neutrinos from the Big Bang, antineutrinos of primordial nucleosynthesis have been theoretically predicted. Their detection could be additional evidence for baryon asymmetry of the Universe.

Current results from a number of independent experiments indicate the possibility of the existence of a light sterile neutrino (m_ν ~ 1–3 eV). The presence of such a neutrino is in poor agreement with the predictions of the Standard Cosmological Model, but these contradictions can be removed by its extension, for example, by the existence of a nonzero lepton asymmetry ξ_ν ~ 10⁻² of the Universe. Nowadays, there are little doubts about the existence of cosmological neutrinos, but unfortunately it is not yet possible to detect them directly due to the extremely small cross section of their interaction at low energies. However, if this can be done in the future, we will obtain direct information about the first seconds, minutes, and hours of the evolution of the Universe after the Big Bang. A review of key aspects related to the influence of cosmological neutrinos on the evolution of the Universe at different stages from the early Universe (primordial nucleosynthesis and primordial recombination) to present days is given.