Axion hot dark matter bounds after Planck

TL;DR

This paper derives cosmological bounds on thermally produced axion masses using Planck, WMAP, SDSS, and local H_0 data, establishing the first CMB-only upper limit and analyzing the impact of different datasets on the constraints.

Contribution

It provides the first upper limit on axion mass from CMB data alone and assesses the influence of local H_0 measurements on axion and neutrino mass bounds.

Findings

Upper limit on axion mass: 0.67 eV with combined data.

CMB data alone constrains axion mass to 1.01 eV.

Neutrino mass sum upper limit: 0.27 eV (combined) and 0.84 eV (CMB alone).

Abstract

We use cosmological observations in the post-Planck era to derive limits on thermally produced cosmological axions. In the early universe such axions contribute to the radiation density and later to the hot dark matter fraction. We find an upper limit m_a < 0.67 eV at 95% C.L. after marginalising over the unknown neutrino masses, using CMB temperature and polarisation data from Planck and WMAP respectively, the halo matter power spectrum extracted from SDSS-DR7, and the local Hubble expansion rate H_0 released by the Carnegie Hubble Program based on a recalibration of the Hubble Space Telescope Key Project sample. Leaving out the local H_0 measurement relaxes the limit somewhat to 0.86 eV, while Planck+WMAP alone constrain the axion mass to 1.01 eV, the first time an upper limit on m_a has been obtained from CMB data alone. Our axion limit is therefore not very sensitive to the tension between the Planck-inferred H_0 and the locally measured value. This is in contrast with the upper limit on the neutrino mass sum, which we find here to range from 0.27 eV at 95% C.L. combining all of the aforementioned observations, to 0.84 eV from CMB data alone.