The expansion history and thermal physical process that happened in the early
Universe before big bang nucleosynthesis (BBN) remains relatively unconstrained
by observations. Low reheating temperature universes with normalcy temperatures
of TRH∼2MeV remain consistent with all observations,
and accommodate several new physics scenarios that would normally be
constrained by high-temperature reheating models, including massive sterile
neutrinos. We explore such scenarios' production of keV scale sterile neutrinos
and their resulting constraints from cosmological observations. The parameter
space for massive sterile neutrinos is much less constrained than in
high-TRH thermal histories, though several cosmological constraints
remain. Such parameter space is the target of several current and upcoming
laboratory experiments such as TRISTAN (KATRIN), HUNTER, MAGNETO-ν, and
PTOLEMY. Cosmological constraints remain stringent for stable keV-scale sterile
neutrinos. However, we show that sterile neutrinos with a dark decay to
radiation through a Z′ or a new scalar are largely unconstrained by
cosmology. In addition, this mechanism of sterile neutrinos with large mixing
may provide a solution to the Hubble tension. We find that keV-scale sterile
neutrinos are therefore one of the best probes of the untested pre-BBN era in
the early Universe and could be seen in upcoming laboratory experiments.Comment: 9 pages, 3 figures, comments welcom