How to relax the cosmological neutrino mass bound
  We study the impact of non-standard momentum distributions of cosmic
neutrinos on the anisotropy spectrum of the cosmic microwave background and the
matter power spectrum of the large scale structure. We show that the neutrino
distribution has almost no unique observable imprint, as it is almost entirely
degenerate with the effective number of neutrino flavours, $N_{\mathrm{eff}}$,
and the neutrino mass, $m_{\nu}$. Performing a Markov chain Monte Carlo
analysis with current cosmological data, we demonstrate that the neutrino mass
bound heavily depends on the assumed momentum distribution of relic neutrinos.
The message of this work is simple and has to our knowledge not been pointed
out clearly before: Cosmology allows that neutrinos have larger masses if their
average momentum is larger than that of a perfectly thermal distribution. Here
we provide an example in which the mass limits are relaxed by a factor of two.
