Odds of observing the multiverse

TL;DR

This paper derives an upper bound on the probability of observing multiverse bubble collisions in eternal inflation, showing it depends on bubble nucleation rates and Hubble scales, with implications for detecting the multiverse.

Contribution

It provides a new derivation of the upper bound on collision observability in the multiverse, extending previous results to collisions with identical bubbles and correcting earlier errors.

Findings

Upper bound depends on bubble nucleation rate and Hubble scales

Bound applies to collisions with identical bubbles

Previous errors in derivation have been corrected

Abstract

Eternal inflation predicts our observable universe lies within a bubble (or pocket universe) embedded in a volume of inflating space. The interior of the bubble undergoes inflation and standard cosmology, while the bubble walls expand outward and collide with other neighboring bubbles. The collisions provide either an opportunity to make a direct observation of the multiverse or, if they produce unacceptable anisotropy, a threat to inflationary theory. The probability of an observer in our bubble detecting the effects of collisions has an absolute upper bound set by the odds of being in the part of our bubble that lies in the forward light-cone of a collision; in the case of collisions with bubbles of identical vacua, this bound given by the bubble nucleation rate times ($H_{\rm{O}}/H_{\rm{I}})^2$, where $H_{\rm{O}}$ is the Hubble scale outside the bubbles and $H_{\rm{I}}$ is the scale of the second round of inflation that occurs inside our bubble. Similar results were obtained by Freigovel \emph{et al.} using a different method for the case of collisions with bubbles of much larger cosmological constant; here it is shown to hold in the case of collisions with identical bubbles as well. A significant error in a previous draft was corrected in order to arrive at this result.