Phase transitions in the early Universe: Impacts on BBN and CMB observables

We explore the effects of low-scale cosmological first-order phase transitions (PTs) on big bang nucleosynthesis (BBN) and cosmic microwave background (CMB) anisotropies and distortions. We examine two scenarios: the PT energy acts as additional background energy, modifying the expansion history; and serves as energy injection into the baryon-photon fluid, generating CMB spectral distortions while having a negligible effect on the expansion history. Our analysis reveals that the CMB spectrum from the Planck 2018 dataset imposes stringent constraints on sub-MeV-scale PTs, while sub-keV-scale PTs can induce significant CMB distortions. Consequently, BBN and CMB observables serve as complementary tools to constrain PT parameters, alongside gravitational wave detections.