When Worlds Collide

TL;DR

This paper investigates the observable cosmological signatures resulting from collisions between Coleman-de Luccia bubbles, revealing potential effects on the cosmic microwave background and implications for vacuum energy selection.

Contribution

It generalizes previous models to include arbitrary cosmological constants and analyzes the resulting spacetime signatures and their observational consequences.

Findings

Collision signatures can affect the CMB in detectable ways.

Smaller cosmological constant bubbles are less affected by domain wall collisions.

Signatures can persist through long periods of inflation.

Abstract

We analyze the cosmological signatures visible to an observer in a Coleman-de Luccia bubble when another such bubble collides with it. We use a gluing procedure to generalize the results of Freivogel, Horowitz, and Shenker to the case of a general cosmological constant in each bubble and study the resulting spacetimes. The collision breaks the isotropy and homogeneity of the bubble universe and provides a cosmological "axis of evil" which can affect the cosmic microwave background in several unique and potentially detectable ways. Unlike more conventional perturbations to the inflationary initial state, these signatures can survive even relatively long periods of inflation. In addition, we find that for a given collision the observers in the bubble with smaller cosmological constant are safest from collisions with domain walls, possibly providing another anthropic selection principle for small positive vacuum energy.