Hydrogen Mixing as a Novel Mechanism for Colder Baryons in 21 cm Cosmology

The anomalous 21 cm absorption feature reported by EDGES has galvanized the study of scenarios in which dark matter (DM) siphons off thermal energy from the Standard Model (SM) gas. In a departure from the much-discussed models that achieve cooling by DM scattering directly with SM particles, we show that the same end can be achieved through neutral atomic hydrogen $H$ mixing with a degenerate dark sector state $H$. An analysis of in-medium $H$-$H$ oscillations reveals viable parameter space for generic types of $H$-DM interactions to provide the requisite cooling. This strategy stands in stark contrast to other proposals in many respects, including its cosmological dynamics, model building implications, and complementary observational signatures.