CERN Accelerating science

Article
Report number	arXiv:2510.27691 ; CERN-TH-2025-221
Title	WallGo investigates: Theoretical uncertainties in the bubble wall velocity
Author(s)	van de Vis, Jorinde (CERN) ; Schicho, Philipp (U. Geneva (main)) ; Niemi, Lauri (Espoo, Sci. Computing Ctr.) ; Laurent, Benoit (McGill U. ; Perimeter Inst. Theor. Phys.) ; Hirvonen, Joonas (U. Nottingham) ; Gould, Oliver (U. Nottingham)
Publication	2026-04-03
Imprint	2025-10-31
Number of pages	51
In:	JHEP 2604 (2026) 041
DOI	10.1007/JHEP04(2026)041
Subject category	Astrophysics and Astronomy ; Particle Physics - Phenomenology
Abstract	We examine theoretical uncertainties in state-of-the-art calculations of the bubble wall velocity during first-order cosmological phase transitions. By utilising the software WallGo for two extensions of the Standard Model, we find several $O(1)$ uncertainties arising from the number of particles taken out of equilibrium, the logarithmically and power enhanced collision integrals, the treatment of thermal masses, the nucleation temperature, the $\tanh$ ansatz, and the perturbative order of the effective potential. However, we show that the linearisation of the Boltzmann equations is generally a good approximation with much smaller associated errors. We further clarify the limitations of the quasiparticle approximation in regions with negative mass squared. This study provides a detailed uncertainty budget and highlights where future efforts should be directed to improve the reliability of wall velocity and hence gravitational wave predictions.
Copyright/License	publication: © 2026 The Author(s) (License: CC-BY-4.0), sponsored by SCOAP³ preprint: (License: CC BY 4.0)

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