A bound on thermal y-distortion of the cosmic neutrino background

TL;DR

This paper investigates the impact of a thermal y-distortion in the cosmic neutrino background on cosmological parameters, constraining its magnitude and exploring how it affects the Hubble constant and spectral index.

Contribution

It provides the first constraints on the thermal y-distortion of the cosmic neutrino background using CMB and BAO data, linking it to cosmological parameter shifts.

Findings

Upper bound on y-parameter: y ≤ 0.043

Increased y-parameter raises H_0 to 71.12 km/s/Mpc

Allows higher spectral index n_S up to 0.9842

Abstract

We consider the possibility that the cosmic neutrino background might have a nonthermal spectrum, and investigate its effect on cosmological parameters relative to standard $\Lambda$-Cold Dark Matter ($\Lambda$CDM) cosmology. As a specific model, we consider a thermal $y$-distortion, which alters the distribution function of the neutrino background by depleting the population of low-energy neutrinos and enhancing the high-energy tail. We constrain the thermal $y$-parameter of the cosmic neutrino background using Cosmic Microwave Background (CMB) and Baryon Acoustic Oscillation (BAO) measurements, and place a $95\%$-confidence upper bound of $y \leq 0.043$. The $y$-parameter increases the number of effective relativistic degrees of freedom, reducing the sound horizon radius and increasing the best-fit value for the Hubble constant $H_0$. We obtain an upper bound on the Hubble constant of $H_0 = 71.12\ \mathrm{km/s/Mpc}$ at $95\%$ confidence, substantially reducing the tension between CMB/BAO constraints and direct measurement of the expansion rate from Type-Ia supernovae. Including a spectral distortion also allows for a higher value of the spectral index of scalar fluctuations, with a best-fit of $n_{\mathrm{S}} = 0.9720 \pm 0.0063$, and a $95\%$-confidence upper bound of $n_{\mathrm{S}} \leq 0.9842$.