$\nu\Lambda$CDM: Neutrinos help reconcile Planck with the Local Universe

TL;DR

Introducing neutrino physics into the standard cosmological model helps reconcile discrepancies between Planck data and local universe measurements, notably in matter perturbation normalization and the Hubble constant.

Contribution

This paper demonstrates that adding sterile or active neutrinos with mass can resolve key tensions in cosmological data within the $ u\Lambda$CDM framework.

Findings

Massive sterile neutrinos improve agreement between Planck and cluster data.

Neutrino mass models are statistically favored over the minimal model.

An eV-scale sterile neutrino is compatible with current constraints.

Abstract

Current measurements of the low and high redshift Universe are in tension if we restrict ourselves to the standard six parameter model of flat $\Lambda$CDM. This tension has two parts. First, the Planck satellite data suggest a higher normalization of matter perturbations than local measurements of galaxy clusters. Second, the expansion rate of the Universe today, $H_0$, derived from local distance-redshift measurements is significantly higher than that inferred using the acoustic scale in galaxy surveys and the Planck data as a standard ruler. The addition of a sterile neutrino species changes the acoustic scale and brings the two into agreement; meanwhile, adding mass to the active neutrinos or to a sterile neutrino can suppress the growth of structure, bringing the cluster data into better concordance as well. For our fiducial dataset combination, with statistical errors for clusters, a model with a massive sterile neutrino shows 3.5$\sigma$ evidence for a non-zero mass and an even stronger rejection of the minimal model. A model with massive active neutrinos and a massless sterile neutrino is similarly preferred. An eV-scale sterile neutrino mass -- of interest for short baseline and reactor anomalies -- is well within the allowed range. We caution that 1) unknown astrophysical systematic errors in any of the data sets could weaken this conclusion, but they would need to be several times the known errors to eliminate the tensions entirely; 2) the results we find are at some variance with analyses that do not include cluster measurements; and 3) some tension remains among the datasets even when new neutrino physics is included.