Cosmological Stasis: Theoretical Overview, Model Realizations, and Phenomenological Implications - Fei Huang (Weizmann)

Many theories of physics beyond the Standard Model give rise to a surprising phenomenon known as cosmological stasis, in which the abundances of different energy components such as matter, radiation, and vacuum energy remain constant across extended cosmological epochs, even though the universe is expanding. Interestingly, we find that stasis is actually a global dynamical attractor toward which the universe necessarily evolves whenever certain basic conditions are satisfied. As a result, an epoch of cosmological stasis is a common and potentially unavoidable feature within the expansion histories associated with many scenarios for physics beyond the Standard-Model. In this talk, I will provide a theoretical overview of the stasis phenomenon and discuss some of its implications and applications. I will also examine two particular model realizations of stasis: one involves the evaporation of a population of primordial black holes with an extended mass spectrum; the other involves a collection of scalar fields, sequentially undergoing transitions from overdamped to underdamped motion. As we shall see, these realizations of stasis not only emerge naturally within top-down scenarios for new physics, but may also have a number of phenomenological implications, including effects on inflationary observables and modifications to the spectrum of the stochastic gravitational-wave background. They may also potentially serve as the basis for a novel framework for realizing cosmic inflation.