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