Late reheating in the era of precision cosmology

We study the minimum cosmic reheating temperature allowed by recent measurements of primordial nuclear abundances and of the cosmic microwave background. We find that reheating by decays of a light particle into electrons and neutrinos is compatible with both at temperatures around 0.4 MeV, an order of magnitude below the bound for purely electromagnetic channels. This scenario has implications for cosmology and laboratory searches. For example, nearly standard neutrino radiation can coexist with a smaller active neutrino mass density at unchanged physical masses, by up to a factor of five for normal ordering. The weaker suppression of clustering changes matter power at the percent level, which forthcoming galaxy surveys and lensing measurements may probe. Late reheating also suppresses sterile neutrino production at mixings within the projected reach of laboratory experiments.