Multiple Cosmic Collisions and the Microwave Background Power Spectrum

TL;DR

This paper investigates how multiple cosmic bubble collisions, as predicted by false vacuum eternal inflation, could affect the CMB power spectrum, especially increasing low multipole power, and discusses their potential to constrain inflation models.

Contribution

It introduces a perturbative approach to analyze multiple bubble collisions and their impact on the CMB, highlighting a largely model-independent spectrum feature.

Findings

Multiple collisions can be approximated as a superposition of individual events.

The resulting CMB spectrum shows increased power at low multipoles.

Predicted spectrum features are largely model-independent and testable against observations.

Abstract

Collisions between cosmic bubbles of different vacua are a generic feature of false vacuum eternal inflation scenarios. While previous studies have focused on the consequences of a single collision event in an observer's past, we begin here an investigation of the more general scenario allowing for many "mild" collisions intersecting our past light cone (and one another). We discuss the general features of multiple collision scenarios and consider their impact on the cosmic microwave background (CMB) temperature power spectrum, treating the collisions perturbatively. In a large class of models, one can approximate a multiple collision scenario as a superposition of individual collision events governed by nearly isotropic and scale-invariant distributions, most appearing to take up less than half of the sky. In this case, the shape of the expected CMB temperature spectrum maintains statistical isotropy and typically features a dramatic increase in power in the low multipoles relative to that of the best-fit $\Lambda$CDM model, which is in tension particularly with the observed quadrupole. We argue that this predicted spectrum is largely model-independent and can be used to outline features of the underlying statistical distributions of colliding bubbles consistent with CMB temperature measurements.