First Observational Tests of Eternal Inflation: Analysis Methods and WMAP 7-Year Results

TL;DR

This paper presents the first observational search for bubble collision signatures in the cosmic microwave background data from WMAP 7-year observations, using a novel Bayesian analysis method to test eternal inflation theories.

Contribution

It introduces a modular, unbiased algorithm for detecting bubble collision signatures in CMB data and applies it to WMAP data to constrain eternal inflation models.

Findings

No evidence of bubble collisions in WMAP data

Rules out certain parameter ranges for bubble collisions

Planck data could provide more definitive tests

Abstract

In the picture of eternal inflation, our observable universe resides inside a single bubble nucleated from an inflating false vacuum. Many of the theories giving rise to eternal inflation predict that we have causal access to collisions with other bubble universes, providing an opportunity to confront these theories with observation. We present the results from the first observational search for the effects of bubble collisions, using cosmic microwave background data from the WMAP satellite. Our search targets a generic set of properties associated with a bubble collision spacetime, which we describe in detail. We use a modular algorithm that is designed to avoid a posteriori selection effects, automatically picking out the most promising signals, performing a search for causal boundaries, and conducting a full Bayesian parameter estimation and model selection analysis. We outline each component of this algorithm, describing its response to simulated CMB skies with and without bubble collisions. Comparing the results for simulated bubble collisions to the results from an analysis of the WMAP 7-year data, we rule out bubble collisions over a range of parameter space. Our model selection results based on WMAP 7-year data do not warrant augmenting LCDM with bubble collisions. Data from the Planck satellite can be used to more definitively test the bubble collision hypothesis.