A signature of anisotropic bubble collisions

TL;DR

This paper explores how bubble collisions in an anisotropic universe with a compact dimension produce a unique observational signature, where collision disks align along a great circle, indicating anisotropic bubble nucleation.

Contribution

It introduces a novel signature of anisotropic bubble collisions, linking the collision geometry to the reduced symmetry of tunneling instantons in a universe with a compact dimension.

Findings

Collision regions appear as disks on the sky

Centers of disks align on a great circle

Signature indicates anisotropic bubble nucleation

Abstract

Our universe may have formed via bubble nucleation in an eternally-inflating background. Furthermore, the background may have a compact dimension---the modulus of which tunnels out of a metastable minimum during bubble nucleation---which subsequently grows to become one of our three large spatial dimensions. When in this scenario our bubble universe collides with other ones like it, the collision geometry is constrained by the reduced symmetry of the tunneling instanton. While the regions affected by such bubble collisions still appear (to leading order) as disks in an observer's sky, the centers of these disks all lie on a single great circle, providing a distinct signature of anisotropic bubble nucleation.