The Next Phase Transition of the Universe: From Metastable Higgs Vacuum Decay to Broken Color and Charge

by Amartya Sengupta

Friday 20 Mar 2026, 12:30 → 14:30 America/Los_Angeles

48/2-224 - Madrone (SLAC)

Our universe has already gone through a sequence of dramatic phase transitions—possibly including grand unification, electroweak symmetry breaking, and the QCD transition. However, this sequence may not be over yet. The Standard Model Higgs field may still reside in a metastable vacuum, and additional sectors may also undergo late-time symmetry breaking. Such transitions could be triggered by rare high-energy events, such as primordial black holes, leading to the nucleation of a true-vacuum bubble that expands at nearly the speed of light. In this talk, we investigate particle production from expanding bubble walls in several scenarios of late?time symmetry breaking. Using the vacuum mismatch method, we calculate the spectrum of particles produced by the bubble until the terminal wall velocity is reached. We also show that frictional dissipation at terminal velocity can generate a large population of thermally produced particles, which continues even after the mismatch channel shuts off. If the terminal velocity satisfies vterm < c, these signals can in principle outrun the bubble wall and may be detected before the bubble itself reaches us. These processes give rise to characteristic decay products and observable photon and neutrino spectra. As concrete examples, we consider metastable Higgs vacuum decay, a model with a TeV-scale colored scalar that breaks SU(3)c, and a model with a charged scalar that breaks U(1)EM. Although the underlying symmetries differ, all three cases share the same basic mechanism of bubble-wall particle production during a first-order phase transition. We conclude by discussing the broader phenomenological implications of such late-time phase transitions and the prospects for probing them observationally.