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Bubbles in the Primordial Soup: Calculating Bubble Wall Velocity in Cosmic Phase Transitions

First-order cosmological phase transitions in the early universe expand as seething vacuum bubbles whose speed determines gravitational wave signatures; first-principles hydrodynamic calculations establish exact bubble wall velocity benchmarks without phenomenological friction.

Author
Benoit Laurent et al.
Published
2022
Journal
Physical review. D/Physical review. D.
Last updated
September 2026
Bubbles in the Primordial Soup: Calculating Bubble Wall Velocity in Cosmic Phase Transitions

Microseconds after the Big Bang, the universe underwent intense phase transitions, shifting from a seething plasma into broken-symmetry vacuum states through the nucleation and violent collision of macroscopic bubbles.

The amplitude and frequency spectrum of gravitational waves emitted by these ancient cosmic collisions depend sensitively on the terminal velocity of the expanding bubble walls, but calculating wall friction from microscopic particle collisions long remained an intractable theoretical challenge.

This work establishes a rigorous, first-principles field-theoretic framework to determine bubble wall velocity in relativistic thermal plasma, self-consistently solving coupled out-of-equilibrium Boltzmann transport equations and Higgs field profiles without phenomenological fudge factors.

These precise terminal velocity benchmarks provide the vital theoretical roadmap required to interpret future gravitational wave discoveries from space-based interferometers like LISA, offering a window into cosmic electroweak symmetry breaking.

Reference

Laurent, B., & Cline, J. M. (2022). First principles determination of bubble wall velocity. Physical Review D, 106(2).

Title

First principles determination of bubble wall velocity

Abstract

The terminal wall velocity of a first-order phase transition bubble can be calculated from a set of fluid equations describing the scalar fields and the plasma's state. We rederive these equations from the energy-momentum tensor conservation and the Boltzmann equation, without linearizing in the background temperature and fluid velocity. The resulting equations have a finite solution for any wall velocity. We propose a spectral method to integrate the Boltzmann equation, which is simple, efficient and accurate. As an example, we apply this new methodology to the singlet scalar extension of the standard model. We find that all solutions are naturally categorized as deflagrations (vw∼csv_w\sim c_s) or ultrarelativistic detonations (γw≳10\gamma_w\gtrsim10). Furthermore, the contributions from out-of-equilibrium effects are, most of the time, subdominant. Finally, we use these results to propose several approximation schemes with increasing levels of complexity and accuracy. They can be used to considerably simplify the methodology while correctly describing the qualitative behavior of the bubble wall.

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