Simulating the Universe(s) III: Observables for the full bubble collision spacetime

TL;DR

This paper introduces a new numerical relativity method to simulate full bubble collision spacetimes in eternal inflation, predicting observable signatures like curvature perturbations and CMB anisotropies.

Contribution

It extends previous models by enabling full spacetime simulations of bubble collisions, providing detailed predictions for cosmological observables in single scalar field models.

Findings

Non-zero spatial curvature and CMB quadrupole from bubble collisions

Wall modes excite additional temperature anisotropies

Constant field surfaces can become nearly homogeneous after collisions

Abstract

This is the third paper in a series establishing a quantitative relation between inflationary scalar field potential landscapes and the relic perturbations left by the collision between bubbles produced during eternal inflation. We introduce a new method for computing cosmological observables from numerical relativity simulations of bubble collisions in one space and one time dimension. This method tiles comoving hypersurfaces with locally-perturbed Friedmann-Robertson-Walker coordinate patches. The method extends previous work, which was limited to the spacetime region just inside the future light cone of the collision, and allows us to explore the full bubble-collision spacetime. We validate our new methods against previous work, and present a full set of predictions for the comoving curvature perturbation and local negative spatial curvature produced by identical and non-identical bubble collisions, in single scalar field models of eternal inflation. In both collision types, there is a non-zero contribution to the spatial curvature and cosmic microwave background quadrupole. Some collisions between non-identical bubbles excite wall modes, giving extra structure to the predicted temperature anisotropies. We comment on the implications of our results for future observational searches. For non-identical bubble collisions, we also find that the surfaces of constant field can readjust in the presence of a collision to produce spatially infinite sections that become nearly homogeneous deep into the region affected by the collision. Contrary to previous assumptions, this is true even in the bubble into which the domain wall is accelerating.