Making predictions in the multiverse

TL;DR

This paper discusses the challenges of making predictions in an eternally inflating multiverse, focusing on the development and evaluation of different measures to regulate infinities and their implications for observable physics.

Contribution

It reviews existing measures for regulating infinities in eternal inflation, compares their equivalences, and discusses future tests to distinguish among them.

Findings

Some measures are ruled out by observational conflicts.

Several measures are shown to be equivalent.

Future theoretical and observational tests are proposed.

Abstract

I describe reasons to think we are living in an eternally inflating multiverse where the observable "constants" of nature vary from place to place. The major obstacle to making predictions in this context is that we must regulate the infinities of eternal inflation. I review a number of proposed regulators, or measures. Recent work has ruled out a number of measures by showing that they conflict with observation, and focused attention on a few proposals. Further, several different measures have been shown to be equivalent. I describe some of the many nontrivial tests these measures will face as we learn more from theory, experiment, and observation.