Simulating the universe(s) II: phenomenology of cosmic bubble collisions in full General Relativity

TL;DR

This paper models and predicts the observable signatures of cosmic bubble collisions in the multiverse using relativistic simulations and analytic methods, providing insights into inflationary physics and potential observational constraints.

Contribution

It offers a detailed relativistic numerical analysis and analytic predictions of bubble collision phenomenology, linking scalar field properties to observable signatures.

Findings

Analytic and numerical results show excellent agreement.

Signature depends on bubble profile and separation distance.

Power-law fit with index between 1 and 2 describes the collision signatures.

Abstract

Observing the relics of collisions between bubble universes would provide direct evidence for the existence of an eternally inflating Multiverse; the non-observation of such events can also provide important constraints on inflationary physics. Realizing these prospects requires quantitative predictions for observables from the properties of the possible scalar field Lagrangians underlying eternal inflation. Building on previous work, we establish this connection in detail. We perform a fully relativistic numerical study of the phenomenology of bubble collisions in models with a single scalar field, computing the comoving curvature perturbation produced in a wide variety of models. We also construct a set of analytic predictions, allowing us to identify the phenomenologically relevant properties of the scalar field Lagrangian. The agreement between the analytic predictions and numerics in the relevant regions is excellent, and allows us to generalize our results beyond the models we adopt for the numerical studies. Specifically, the signature is completely determined by the spatial profile of the colliding bubble just before the collision, and the de Sitter invariant distance between the bubble centers. The analytic and numerical results support a power-law fit with an index $1< \kappa \lesssim 2$. For collisions between identical bubbles, we establish a lower-bound on the observed amplitude of collisions that is set by the present energy density in curvature.