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  • Open Access

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

Seth Koren1, Yuhsin Tsai1, and Runqing Wang1,2

  • 1Department of Physics and Astronomy, University of Notre Dame, South Bend, Indiana 46556, USA
  • 2Department of Physics and Center for Field Theory and Particle Physics, Fudan University, 2005 Song Hu Road, Shanghai 200438, China

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Phys. Rev. Lett. 137, 051002 – Published 28 July, 2026

DOI: https://doi.org/10.1103/1vyz-xdkb

Abstract

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.

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Phys. Rev. Lett. 137, 051002– Published 28 July, 2026
  • Received 30 September 2025
  • Revised 29 March 2026
  • Accepted 30 June 2026
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