Solving the riddle: Visible in the lab but invisible in cosmology

The expansion history and thermal physical process that happened in the early Universe before big bang nucleosynthesis (BBN) remains relatively unconstrained by observations. The reheating temperature of the Universe is unknown beyond the requirement that it is $Trh > 1.8,mathrm{MeV}$ and is therefore a free parameter in studies of the energy content arising from the hot big bang. Low reheating temperature universes with normalcy temperatures of $T_mathrm{RH}sim 5$ MeV 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-$T_mathrm{RH}$ 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, and MAGNETO-$ u$. Cosmological constraints remain stringent for stable keV-scale sterile neutrinos. However, we show that sterile neutrinos with a dark decay to radiation through a $Z^prime$ or a new scalar are largely unconstrained by cosmology. In addition, such a mechanism provides 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.