Phase Transitions in the Early Universe: Impacts on BBN and CMB Observables

We explore the effects of low-scale cosmological first-order phase transitions on Big Bang Nucleosynthesis (BBN) and Cosmic Microwave Background (CMB) anisotropy and distortion. We examine two scenarios: the distribution of the phase-transition energy in the cosmic background and the phase transition as an additional source of energy injection. Our analysis reveals that the CMB spectrum from the Planck 2018 dataset imposes stringent constraints on sub-MeV-scale phase transitions, while keV-scale phase transitions can induce significant CMB distortions. Consequently, BBN and CMB observables serve as complementary tools to constrain phase-transition parameters, alongside gravitational wave detections.