Cosmological Neutrinos and their Impact on the Evolution of Universe

Abstract

The thermal history of the Universe implies the existence of a diffuse neutrino component that has survived since the earliest cosmic epochs. While neutrinos are characterized by extremely small masses and very weak interactions, their collective presence becomes relevant on cosmological scales due to their high abundance. In this study, we focus on how this component emerges, which of its properties are most relevant for cosmological applications, and how it is incorporated into the contemporary concordance model of the Universe. When neutrino mass is taken into account, the description of matter in the Universe is slightly altered compared to the idealized massless case. This alteration does not appear as a dominant effect but instead manifests as a consistent shift in model outcomes. As a result, the distribution of matter inferred from such models becomes less pronounced, indicating a reduced contrast relative to scenarios that neglect neutrino mass. Neutrino effects are not treated here as a separate add-on, but as part of the working cosmological description adopted throughout the text. This choice is motivated by the fact that neutrinos influence several model outputs simultaneously, so their impact is best assessed within the full setup rather than through isolated statements. As new measurements extend both their precision and their scope, they can distinguish between small differences that are otherwise degenerate. In this sense, future data provide a practical way to check whether the adopted description remains adequate once neutrino effects are handled consistently.