The Influence of the Effective Number of Active and Sterile Neutrinos on the Determination of the Values of Cosmological Parameters

Abstract

Neutrino, as the second most abundant known particle in the Universe, has a significant impact on its expansion rate during the radiation- and matter-dominated eras. For this reason, a change in the number of neutrino species can lead to substantial changes in the estimates of cosmological parameters, the most accurate values of which, at the moment, are obtained by analysing the anisotropy of the CMB. In the presented work we consider the influence of a hypothetical sterile neutrino (with eV-scale mass) on the determination of cosmological parameters. The possible existence of such neutrino is suggested by the analysis of a series of different experiments. If it is detected, it will be necessary to include it into the $\Lambda\rm CDM$ model with the fixed values of its mass $m_{\rm s}$ and mixing angle $\theta_{s}$, which is the main method used through this paper. Apart from that, the seesaw mechanism requires there to be at least two sterile states, one of them being much heavier than the active ones. The heavier sterile state ($m_{s}\sim 1$ keV) would decay and increase the effective number of active neutrinos. Therefore, the influence of a change in the effective number of relativistic neutrino species $N_{\textrm{eff}}$ was studied as well, which could be caused by various reasons, for example, by the decay processes of dark matter particles or the above-mentioned sterile neutrinos, as well as processes leading to an increase in the temperature of relic neutrinos $T_{\textrm{C}\nu\textrm{B}}$. The effects studied in this work lead to a significant change in the estimates of the cosmological parameters, including the value of $H_{0}$. It has been discovered that the accounting of the sterile neutrino with masses $m=1$ and $2.7$ eV leads to a decrease in the estimate of the current Hubble parameter value $H_{0}$ and, therefore, exacerbates the ‘‘$H_{0}$-tension’’ problem. An increase in the value of the effective number of relativistic neutrino species leads, on the contrary, to an increase in the $H_{0}$ estimate, resolving the above-mentioned problem at $N_{\textrm{eff}}=3.0+0.9$, which is equivalent to an increase of the neutrino temperature up to $T^{0}_{\textrm{C}\nu\textrm{B}}=1.95+0.14$ K. At the same time, the rest of the cosmological parameters do not change significantly, leaving us within the framework of the standard $\Lambda$CDM model.