Stasis in an Expanding Universe: A Recipe for Stable Mixed-Component
  Cosmological Eras
  One signature of an expanding universe is the time-variation of the
cosmological abundances of its different components. For example, a
radiation-dominated universe inevitably gives way to a matter-dominated
universe, and critical moments such as matter-radiation equality are fleeting.
In this paper, we point out that this lore is not always correct, and that it
is possible to obtain a form of "stasis" in which the relative cosmological
abundances $\Omega_i$ of the different components remain unchanged over
extended cosmological epochs, even as the universe expands. Moreover, we
demonstrate that such situations are not fine-tuned, but are actually global
attractors within certain cosmological frameworks, with the universe naturally
evolving towards such long-lasting periods of stasis for a wide variety of
initial conditions. The existence of this kind of stasis therefore gives rise
to a host of new theoretical possibilities across the entire cosmological
timeline, ranging from potential implications for primordial density
perturbations, dark-matter production, and structure formation all the way to
early reheating, early matter-dominated eras, and even the age of the universe.
