The standard model has a gap in physics scales below approximately 1 MeV. Current and upcoming cosmological observations will probe the nature of the universe when it was at these temperatures. An important consideration is what sorts of new physics could appear in this range which would have observable consequences. We will discuss the possibility that neutrino oscillations might equilibrate a new sector and consider how naturally that might occur. Such scenarios may have implications for existing cosmological tensions, such as the Hubble tension and the S8 tension, which we will discuss.