Observable CMB Tensor Modes from Cosmological Phase Transitions

A B-mode polarization signal in the cosmic microwave background is widely regarded as smoking gun evidence for gravitational waves produced during inflation. In this talk, I demonstrate that tensor perturbations from a cosmological phase transition can nearly mimic the characteristic shape and power of inflationary predictions across a range of observable angular scales. Although phase transitions arise from sub-horizon physics, they nevertheless exhibit a white noise power spectrum outside the horizon. Thus, while B-mode power is suppressed on these large scales, it is not necessarily negligible. For viable phase transition parameters, the maximal B-mode amplitude at multipole moments around the recombination peak (l~100) can be comparable to all single-field inflationary predictions that can be tested with current and future experiments. This approximate degeneracy can be broken if a signal is measured at different angular scales, since the inflationary power spectrum is nearly scale invariant while the phase transition predicts a distinct suppression of power on large scales.