The universe may still be hiding traces of dramatic events long after its earliest moments. In this talk, I will explore how first-order phase transitions — processes where bubbles of a new vacuum spontaneously nucleate and expand — can imprint both scalar and tensor perturbations on the cosmic microwave background (CMB) and large-scale structure (LSS) of the universe. 

 

As I will show, precise CMB and LSS measurements already have placed strong limits on such phase transitions, even if they happen in a completely hidden dark sector that interacts only through gravity. Future CMB temperature and B-mode measurements, however, can still have a chance to observe these phase transition signals. Moreover, when vacuum energy starts to dominate the energy budget of the universe, it may naturally trigger a new phase transition as thermal effects fade away. In that regime, CMB anisotropies again offer a uniquely sensitive probe, providing constraints far stronger than those from Hubble expansion measurements.