  Neutrino masses and quantum gravity are strong reasons to extend the standard
model of particle physics. A large extra dimension can be motivated by quantum
gravity and can explain the small neutrino masses with new singlet states that
propagate in the bulk. In such a case, a Kaluza-Klein tower of sterile
neutrinos emerges. We revisit constraints on towers of sterile neutrinos that
come from cosmological observables such as the effective number of
noninteracting relativistic species and the dark matter density. These limits
generically rule out micron-sized extra dimensions. We explore the weakening of
these constraints to accommodate an extra dimension close to the micron size by
assuming that the universe reheated after inflation to a low temperature. We
discuss how such a possibility can be distinguished in the event of a positive
signal in a cosmological observable.
