Abstract: Primordial sources of gravitational waves (GWs) have traditionally been probed through their contribution to the stochastic GW background, detectable via pulsar timing arrays and ground-based laser interferometers. However, these same tensor perturbations can also leave an imprint on the cosmic microwave background (CMB) in the form of B-mode polarization. While a detection of primordial B-modes has long been regarded as a smoking gun for inflation, recent work has shown that non-inflationary sources in the early universe can also produce B-mode polarization on observable scales with amplitudes potentially within reach of upcoming CMB experiments. 

In this talk, we examine two such sources: 1) first-order cosmological phase transitions occurring in the late, but pre-recombination, universe, and 2) scalar-induced tensor perturbations, generated at second order in cosmological perturbation theory from enhanced small-scale curvature perturbations. For each, we compute the angular spectrum of B-mode polarization and compare to that from inflation assuming tensor-to-scalar ratios targeted by next-generation CMB experiments. We find that in both cases, there exists viable parameter space yielding signals rivaling those from inflation. The scenarios can in principle be distinguished by making observations at multiple angular scales, since the spectral shapes are distinct from the inflationary case, featuring more power on smaller scales.

These results motivate broader observational strategies for uncovering primordial sources of GWs beyond inflation.