Analyzing Cosmic Bubble Collisions

TL;DR

This paper develops analytic methods to study the impact of cosmic bubble collisions on inflationary cosmology, enabling model-independent predictions for observable effects like CMB temperature spots.

Contribution

It introduces a set of controlled approximations and a power series expansion to analyze bubble collision effects on the inflaton field during inflation.

Findings

Only one expansion coefficient is relevant in certain models, simplifying predictions.

The approach can determine whether collisions produce hot or cold spots in the CMB.

Analysis extends to thick-wall bubble collisions beyond the thin-wall approximation.

Abstract

We develop a set of controlled, analytic approximations to study the effects of bubble collisions on cosmology. We expand the initial perturbation to the inflaton field caused by the collision in a general power series, and determine its time evolution during inflation in terms of the coefficients in the expansion. In models where the observer's bubble undergoes sufficient slow-roll inflation to solve the flatness problem, in the thin wall limit only one coefficient in the expansion is relevant to observational cosmology, allowing nearly model-independent predictions. We discuss two approaches to determining the initial perturbation to the inflaton and the implications for the sign of the effect (a hot or cold spot on the Cosmic Microwave Background temperature map). Lastly, we analyze the effects of collisions with thick-wall bubbles, i.e. away from the thin-wall limit.