Stasis in an Expanding Universe: A Recipe for Stable Mixed-Component Cosmological Eras

TL;DR

This paper demonstrates that stable periods of constant relative abundances of universe components, called stasis, can naturally occur and last long, challenging the traditional view of inevitable transitions in cosmic eras.

Contribution

It introduces the concept of cosmological stasis as a global attractor, showing that stable mixed-component eras are possible without fine-tuning.

Findings

Stasis periods can be long-lasting and naturally attract the universe's evolution.

Stable mixed-component eras are not fine-tuned but are generic in certain models.

Implications for primordial perturbations, dark matter, and structure formation.

Abstract

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.