Cosmic bubble collisions provide an important observational window on the dynamics of eternal inflation. In eternal inflation, our observable universe is contained in one of many bubbles formed from an inflating metastable vacuum. The collision between bubbles can leave a detectable imprint on the cosmic microwave background (CMB) radiation. Although phenomenological models of the observational signature have been proposed, to make the theory fully predictive one must determine the bubble collision spacetime, and thus the cosmological observables, from a scalar field theory giving rise to eternal inflation. Numerical simulations of bubble collisions in full General Relativity (GR) are needed to make a direct connection between a scalar field model giving rise to eternal inflation and the signature of bubble collisions expected to be present in the CMB. Numerics are important because the collision between bubbles is a highly non-linear process.

We recently took the first steps towards this goal, in collaboration with Matt Johnson and Luis Lehner at the Perimeter Institute in Canada. We simulated collisions between two bubbles in full GR. These simulations allowed us to accurately determine the outcome of bubble collisions, and examine their effect on the cosmology inside a bubble universe. Studying both vacuum bubbles and bubbles containing a realistic inflationary cosmology, we confirmed the validity of a number of approximations used in previous analytic work, and identified qualitatively new features of bubble collision spacetimes, such as oscillons. We identified the constraints on the scalar field potential that must be satisfied in order to obtain collisions that are consistent with our observed cosmology, yet leave detectable signatures.

The figures below show a couple of examples of bubble collisions in our simulations.