Boiling After the Dust Settles: Constraining First-Order Phase Transitions During Dark Energy Domination

A first-order phase transition could occur in the late Universe when vacuum energy begins dominating the energy density (z≲0.3) and convert some latent heat into other forms such as invisible radiation. This generic possibility also has concrete motivation in particle physics models, which invoke a multitude of vacua to address theoretical puzzles. The naïve constraint on such an event comes from measurements of the Hubble expansion rate, but this can only probe transitions involving O(10)% of the dark energy. In this Letter, we show that significantly tighter constraints appear when accounting for phase transition fluctuations affecting CMB photon propagation anisotropically. For instance, if a completed phase transition has β/H_{⋆}≲25, current CMB data limit the associated vacuum energy released to less than 1% of the dark energy. A transition to negative vacuum energy (quasi-anti-de Sitter) is allowed only for β/H_{⋆}≳300. For β/H_{⋆}≲500, the Universe will not crunch for at least 14 Gyr.