Cosmological data can be used to search for—and characterize—light particles in the standard model, if these populate our Universe. In addition to the well-known effect of these light relics in the background cosmology, usually parametrized through a change in the effective number $N_{\mathrm{eff}}$ of neutrino species, these particles can become nonrelativistic at later times, affecting the growth of matter fluctuations due to their thermal velocities. An extensively studied example is that of massive neutrinos, which are known to produce a suppression in the matter power spectrum due to their free streaming. Galaxies, as biased traces of matter fluctuations, can therefore provide us with a wealth of information about both known and unknown degrees of freedom in the standard model. To harness this information, however, the galaxy bias has to be determined in the presence of massive relics, which is expected to vary with scale. Here, we present the code RelicFast, which efficiently computes the scale-dependent bias induced by relics of different masses, spins, and temperatures, through spherical collapse and the peak-background split. Using this code, we find that, in general, the bias induced by light relics partially compensates for the suppression of power and should be accounted for in any search for relics with galaxy data. In particular, for the case of neutrinos, we find that both the normal and inverted hierarchies present a percent-level step in the Lagrangian bias, with a size scaling linearly with the neutrino-mass sum, in agreement with recent simulations. This effect persists at the subpercent level even if one defines the Eulerian bias with respect to dark matter only, suggesting that it has to be properly included in cosmological searches for the neutrino mass. RelicFast can compute halo bias in under a second, allowing for this effect to be properly included for different cosmologies, and light relics, at little computational cost.

• Received 8 June 2018

DOI:https://doi.org/10.1103/PhysRevD.98.043503