Echoes of Inflationary First-Order Phase Transitions in the CMB
  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.
