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 model selection analysis. We outline each component of this algorithm,
describing its response to simulated CMB skies with and without bubble
collisions. We rule out bubble collisions over a range of parameter space, and
find four features in the WMAP 7-year data that are consistent with being
bubble collisions. Data from the Planck satellite can be used to test if these
features are in fact signatures of other bubble universes.
\\ (http://arxiv.org/abs/1012.3667,  8531kb)