Robustness of cosmic neutrino background detection in the cosmic microwave background

TL;DR

This paper examines the robustness of detecting the cosmic neutrino background through CMB measurements, showing current bounds remain stable across various models and assumptions, supporting the background's indirect detection.

Contribution

It generalizes the effective sound and viscosity speeds to massive neutrinos and dark radiation, demonstrating the robustness of CMB-based neutrino background detection under diverse assumptions.

Findings

Current bounds on effective parameters are stable across models.

Detection of cosmic neutrino background is robust to assumptions.

Generalized parameters for massive neutrinos do not significantly alter conclusions.

Abstract

The existence of a cosmic neutrino background can be probed indirectly by CMB experiments, not only by measuring the background density of radiation in the universe, but also by searching for the typical signatures of the fluctuations of free-streaming species in the temperature and polarisation power spectrum. Previous studies have already proposed a rather generic parametrisation of these fluctuations, that could help to discriminate between the signature of ordinary free-streaming neutrinos, or of more exotic dark radiation models. Current data are compatible with standard values of these parameters, which seems to bring further evidence for the existence of a cosmic neutrino background. In this work, we investigate the robustness of this conclusion under various assumptions. We generalise the definition of an effective sound speed and viscosity speed to the case of massive neutrinos or other dark radiation components experiencing a non-relativistic transition. We show that current bounds on these effective parameters do not vary significantly when considering an arbitrary value of the particle mass, or extended cosmological models with a free effective neutrino number, dynamical dark energy or a running of the primordial spectrum tilt. We conclude that it is possible to make a robust statement about the detection of the cosmic neutrino background by CMB experiments.