Neutrino mixing and gravitational production via inflation

TL;DR

This paper develops a formalism to study gravitational production of mixed neutrinos during inflation, revealing that large scalar field fluctuations can significantly enhance neutrino production in the early universe.

Contribution

It introduces a Bogolyubov coefficient formalism for neutrino production with time-dependent mixing, accounting for inflationary scalar field fluctuations.

Findings

Gravitational production of neutrinos can be substantially enhanced during inflation.

An upper bound on neutrino abundance produced by gravity is established as $Y \\lesssim 10^{-11}$.

The formalism applies to both active and sterile neutrinos in the early universe.

Abstract

We develop the Bogolyubov coefficient formalism for gravitational production of fermions with time-dependent mixing, which allows us to study a prototype neutrino system. The neutrino masses and mixings depend on the scalar field values, i.e. the Higgs or singlet scalar expectation values. These are time-dependent in the Early Universe and, due to de Sitter fluctuations, can reach very large values during inflation. As a result, gravitational production of all types of neutrinos can be much enhanced. We obtain an upper bound on the abundance of active and sterile neutrinos produced by classical gravity, $Y \lesssim 10^{-11}$.