Cycles in the Multiverse

TL;DR

This paper explores cyclic bubble universes within eternal inflation, proposing that such models can mitigate measure problem issues and offer new insights into predicting outcomes in an infinite multiverse.

Contribution

It introduces cyclic bubble universe models in eternal inflation that alleviate measure problem issues and provide a new framework for cosmological predictions.

Findings

Cyclic models can reduce youngness and Boltzmann Brain problems.

Eternal inflation can naturally include cyclic universes with dark energy phases.

Implications for measure problem and cosmological predictions are discussed.

Abstract

Eternal inflation is a seemingly generic consequence of theories that give rise to accelerated expansion of the universe and possess multiple vacuum states. Making predictions in an eternally inflating universe is notoriously difficult because one must compare infinite quantities, and a wide variety of regulating procedures yield radically different results. This is the measure problem of eternal inflation. In this paper, we analyze models of eternal inflation which allow for the possibility of cyclic bubble universes: in each bubble, standard cosmological evolution is re-played over and over again. Eternal inflation can generically arise in cyclic models that include a dark energy dominated phase. In such models, several problematic consequences of standard regulating procedures, such as the youngness and Boltzmann Brain problems, are substantially alleviated. We discuss the implications for making predictions in cyclic models, as well as some general implications for understanding the measure problem in eternal inflation.