A status report on the observability of cosmic bubble collisions

TL;DR

This paper reviews the current understanding of cosmic bubble collisions in eternal inflation, discussing their potential observational signatures, the physics involved, and the likelihood of detection through cosmological data.

Contribution

It synthesizes recent progress on the physics, observational effects, and probability of detecting bubble collisions, and suggests future research directions.

Findings

Progress in modeling bubble collision physics

Potential signatures in cosmic microwave background

Assessment of collision observability likelihood

Abstract

In the picture of eternal inflation as driven by a scalar potential with multiple minima, our observable universe resides inside one of many bubbles formed from transitions out of a false vacuum. These bubbles necessarily collide, upsetting the homogeneity and isotropy of our bubble interior, and possibly leading to detectable signatures in the observable portion of our bubble, potentially in the Cosmic Microwave Background or other precision cosmological probes. This constitutes a direct experimental test of eternal inflation and the landscape of string theory vacua. Assessing this possibility roughly splits into answering three questions: What happens in a generic bubble collision? What observational effects might be expected? How likely are we to observe a collision? In this review we report the current progress on each of these questions, improve upon a few of the existing results, and attempt to lay out directions for future work.