Surviving the crash: assessing the aftermath of cosmic bubble collisions

TL;DR

This study explores the internal structure and observational consequences of cosmic bubble collisions in an inflationary universe, revealing conditions under which observers might detect collision effects and implications for cosmological measures.

Contribution

It provides a detailed analysis of bubble collision spacetime, highlighting how domain wall acceleration influences observable regions and measure problems in inflationary cosmology.

Findings

Observers can have collisions to their past due to spacelike surfaces of homogeneity.

Observable effects are confined to a small region on the sky for most observers.

Bubble collisions may significantly impact measure definitions in inflationary models.

Abstract

This paper is the third in a series investigating the possibility that if we reside in an inflationary "bubble universe", we might observe the effects of collisions with other such bubbles. Here, we study the interior structure of a bubble collision spacetime, focusing on the issue of where observers can reside. Numerical simulations indicate that if the inter-bubble domain wall accelerates away, infinite spacelike surfaces of homogeneity develop to the future of the collision; this strongly suggests that observers can have collisions to their past, and previous results then imply that this is very likely. However, for observers at nearly all locations, the restoration of homogeneity relegates any observable effects to a vanishingly small region on the sky. We find that bubble collisions may also play an important role in defining measures in inflation: a potentially infinite relative volume factor arises between two bubble types depending on the sign of the acceleration of the domain wall between them; this may in turn correlate with observables such as the scale or type of inflation.