Echoes of Inflationary First-Order Phase Transitions in the CMB

TL;DR

This paper explores how first-order cosmological phase transitions during inflation could produce distinctive gravitational wave signatures in the CMB, offering a new way to probe early universe physics and baryogenesis.

Contribution

It proposes using CMB observations of bubble-generated gravitational waves to detect first-order phase transitions during inflation, linking cosmology with particle physics.

Findings

Bubble-generated GWs have scale-dependent B-mode spectra.

CMB can test electroweak baryogenesis scenarios.

Potential to distinguish phase transition signatures from primordial GWs.

Abstract

Cosmological phase transitions (CPTs), such as the Grand Unified Theory (GUT) and the electroweak (EW) ones, play a significant role in both particle physics and cosmology. In this letter, we propose to probe the first-order CPTs, by detecting gravitational waves (GWs) which are generated during the phase transitions through the cosmic microwave background (CMB). If happened around the inflation era, the first-order CPTs may yield low-frequency GWs due to bubble dynamics, leaving imprints on the CMB. In contrast to the nearly scale-invariant primordial GWs caused by vacuum fluctuation, these bubble-generated GWs are scale dependent and have non-trivial B-mode spectra. If decoupled from inflaton, the EWPT during inflation may serve as a probe for the one after reheating where the baryon asymmetry could be generated via EW baryogenesis (EWBG). The CMB thus provides a potential way to test the feasibility of the EWBG, complementary to the collider measurements of Higgs potential and the direct detection of GWs generated during EWPT.