Cosmological mass limits on neutrinos, axions, and other light particles

The small-scale power spectrum of the cosmological matter distribution, together with other cosmological data, provides a sensitive measure of the hot dark matter fraction, leading to restrictive neutrino mass limits. We extend this argument to generic cases of low-mass thermal relics. We vary the cosmic epoch of thermal decoupling, the radiation content of the universe, and the new particle's spin degrees of freedom. Our treatment covers various scenarios of active plus sterile neutrinos or axion-like particles. For three degenerate massive neutrinos, we reproduce the well-known limit of m(nu)<0.34 eV. In a 3+1 scenario of 3 massless and 1 fully thermalized sterile neutrinos we find m(nu)<1.0 eV. Thermally produced QCD axions must obey m(a)<3.0 eV, superseding limits from a direct telescope search, but leaving room for solar eV-mass axions to be discovered by the CAST experiment.