Primordial black holes place the Universe in stasis

A variety of scenarios for early-universe cosmology give rise to a population of primordial black holes (PBHs) with a broad spectrum of masses. The evaporation of PBHs in such scenarios has the potential to place the Universe into an extended period of “stasis” during which the abundances of matter and radiation—i.e., their energy densities divided by the critical density—remain absolutely constant despite cosmological expansion. This surprising phenomenon can give rise to new possibilities for early-universe dynamics and lead to distinctive signatures of the evaporation of such PBHs. In this paper, we discuss how this stasis epoch arises and explore a number of its phenomenological consequences, including implications for inflationary observables, the stochastic gravitational-wave background, baryogenesis, and the production of dark matter and dark radiation.