Signatures of Bulk Neutrinos in the Early Universe

TL;DR

This paper investigates how large extra dimensions and bulk neutrinos influence early universe cosmology, analyzing constraints from cosmological data and proposing scenarios with low reheating temperatures to relax these limits.

Contribution

It revisits cosmological constraints on sterile neutrino towers from extra dimensions and explores low-temperature reheating as a way to relax these bounds.

Findings

Micron-sized extra dimensions are generally ruled out by cosmological constraints.

Low reheating temperatures can weaken constraints on extra dimensions.

Potential observational signatures can distinguish these scenarios.

Abstract

Neutrino masses and quantum gravity are strong reasons to extend the standard model of particle physics. A large extra dimension can be motivated by quantum gravity and can explain the small neutrino masses with new singlet states that propagate in the bulk. In such a case, a Kaluza-Klein tower of sterile neutrinos emerges. We revisit constraints on towers of sterile neutrinos that come from cosmological observables such as the effective number of noninteracting relativistic species and the dark matter density. These limits generically rule out micron-sized extra dimensions. We explore the weakening of these constraints to accommodate an extra dimension close to the micron size by assuming that the universe reheated after inflation to a low temperature. We discuss how such a possibility can be distinguished in the event of a positive signal in a cosmological observable.