Observable CMB B-modes from Cosmological Phase Transitions

TL;DR

This paper shows that cosmological phase transitions can produce B-mode polarization signals in the CMB, which could mimic inflationary gravitational waves and affect interpretation of observational data.

Contribution

It demonstrates that tensor perturbations from first-order phase transitions in a dark sector can generate observable B-modes, challenging the inflationary paradigm.

Findings

Phase transitions can produce a white noise tensor spectrum on super-horizon scales.

B-mode signals from phase transitions can be comparable to inflationary predictions.

Different sources can be distinguished by their angular scale dependence.

Abstract

A B-mode polarization signal in the cosmic microwave background (CMB) is widely regarded as smoking gun evidence for gravitational waves produced during inflation. Here, we demonstrate that tensor perturbations sourced during non-inflationary epochs can yield non-negligible B-mode signals, which can in principle complicate the interpretation of future observational data. As a case study, we consider tensor perturbations sourced in the bubble collision stage of a first-order cosmological phase transition occurring in a secluded dark sector. Although phase transitions arise from causal sub-horizon physics, they nevertheless exhibit a white noise power spectrum on super-horizon scales. Power is suppressed on the large scales relevant for CMB B-mode polarization, but it is not necessarily negligible. We show that for appropriately chosen phase transition parameters, the maximal B-mode amplitude can compete with inflationary predictions that can be tested with current and future experiments. These scenarios can be differentiated by performing measurements on multiple angular scales, since the phase transition signal predicts peak power on smaller scales.