Forecasting constraints from the cosmic microwave background on eternal inflation

TL;DR

This paper forecasts how well future cosmic microwave background (CMB) data can constrain theories of eternal inflation through bubble collision signatures, highlighting the limited improvements from temperature data and modest gains from polarization.

Contribution

It introduces a Fisher matrix-based forecast for detecting bubble collisions in the CMB, incorporating advanced simulations and analyzing the impact of polarization and fundamental parameters.

Findings

Polarization data improves detectability by ~30%.

Future temperature data offers no significant advantage over current datasets.

Cosmic-variance-limited polarization data marginally improves constraints.

Abstract

We forecast the ability of cosmic microwave background (CMB) temperature and polarization datasets to constrain theories of eternal inflation using cosmic bubble collisions. Using the Fisher matrix formalism, we determine both the overall detectability of bubble collisions and the constraints achievable on the fundamental parameters describing the underlying theory. The CMB signatures considered are based on state-of-the-art numerical relativistic simulations of the bubble collision spacetime, evolved using the full temperature and polarization transfer functions. Comparing a theoretical cosmic-variance-limited experiment to the WMAP and Planck satellites, we find that there is no improvement to be gained from future temperature data, that adding polarization improves detectability by approximately 30%, and that cosmic-variance-limited polarization data offer only marginal improvements over Planck. The fundamental parameter constraints achievable depend on the precise values of the tensor-to-scalar ratio and energy density in (negative) spatial curvature. For a tensor-to-scalar ratio of $0.1$ and spatial curvature at the level of $10^{-4}$, using cosmic-variance-limited data it is possible to measure the width of the potential barrier separating the inflating false vacuum from the true vacuum down to $M_{\rm Pl}/500$, and the initial proper distance between colliding bubbles to a factor $\pi/2$ of the false vacuum horizon size (at three sigma). We conclude that very near-future data will have the final word on bubble collisions in the CMB.