Primordial Neutrinos
Steen Hannestad
TL;DR
Cosmology provides stringent probes of neutrino properties, including the total energy density, chemical potential, and absolute mass scale, via the cosmic microwave background, large-scale structure, and Big Bang nucleosynthesis, with sensitivity to quantities such as $\sum m_i$, $N_\nu$, and $\xi_\nu$. The paper reviews the thermal history establishing neutrino decoupling near $T \sim 1$ MeV and the resulting $T_\nu/T_\gamma = (4/11)^{1/3}$ relation, along with BBN constraints on $N_\nu$ and $\xi_\nu$ that depend on the primordial helium abundance and baryon density. It discusses potential late-time recoupling via new interactions, such as coupling to a massless scalar, which could modify relic neutrino densities. The review compares current cosmological bounds with laboratory limits on $m_{\nu_e}$ and outlines how upcoming CMB/LSS measurements could tighten constraints on $\sum m_i$, sterile species, and neutrino chemical potentials, underscoring the key interplay between particle physics and cosmology.
Abstract
The connection between cosmological observations and neutrino physics is discussed in detail. Neutrinos decouple from thermal contact in the early Universe at a temperature of order 1 MeV which coincides with the temperature where light element synthesis occurs. Observation of light element abundances therefore provides important information on such properties as neutrino energy density and chemical potential. Precision observations of the cosmic microwave background and large scale structure of galaxies can be used to probe neutrino masses with greater precision than current laboratory experiments. In this review I discuss current cosmological bounds on neutrino properties, as well as possible bounds from upcoming measurements.