From Cosmic Tensions to Fundamental Physics: A New Phase Transition in the Early Universe

Our cosmological standard model provides a remarkably successful description of the Universe, yet it treats the dark sector, including dark matter and dark energy, as a set of fluid components rather than a fundamental theory. Therefore, it is perhaps not surprising that with more precise cosmological data, this description appears increasingly incomplete. In this talk, I will argue that these observational challenges point towards new dark-sector physics, built on the same fundamental principles that govern the visible sector. Specifically, I will present a scenario in which the dark sector undergoes a symmetry-breaking phase transition in the early Universe, leading to a rapid reheating of the dark sector after Big Bang nucleosynthesis. In this scenario, a gauge force between dark matter and dark radiation gives rise to dark acoustic oscillations close in scale to baryon acoustic oscillations. This distinct feature produces a falsifiable signature in the matter power spectrum and provides a new way of addressing a recent anomaly in large-scale structure data, different from evolving dark energy.