Increasing Neff with particles in thermal equilibrium with neutrinos

TL;DR

This paper proposes that a light particle in thermal equilibrium with neutrinos can increase Neff, affecting early universe cosmology, and explores observational constraints and future detection prospects.

Contribution

It introduces a novel mechanism for increasing Neff via particles in thermal equilibrium with neutrinos, distinct from dark radiation models.

Findings

Neff can be greater than three due to such particles.

Current BBN and CMB data are relatively unconstraining.

Planck can significantly improve constraints and distinguish particle properties.

Abstract

Recent work on increasing the effective number of neutrino species (Neff) in the early universe has focussed on introducing extra relativistic species (`dark radiation'). We draw attention to another possibility: a new particle of mass less than 10 MeV that remains in thermal equilibrium with neutrinos until it becomes non-relativistic increases the neutrino temperature relative to the photons. We demonstrate that this leads to a value of Neff that is greater than three and that Neff at CMB formation is larger than at BBN. We investigate the constraints on such particles from the primordial abundance of helium and deuterium created during BBN and from the CMB power spectrum measured by ACT and SPT and find that they are presently relatively unconstrained. We forecast the sensitivity of the Planck satellite to this scenario: in addition to dramatically improving constraints on the particle mass, in some regions of parameter space it can discriminate between the new particle being a real or complex scalar.