WallGo investigates: Theoretical uncertainties in the bubble wall velocity

TL;DR

This paper analyzes the theoretical uncertainties in calculating bubble wall velocity during cosmological phase transitions, identifying key sources of error and assessing the reliability of current methods for gravitational wave predictions.

Contribution

It provides a comprehensive uncertainty budget for wall velocity calculations using WallGo, highlighting the impact of various modeling assumptions and approximations.

Findings

Uncertainties of order one from multiple modeling choices.

Linearisation of Boltzmann equations is a reliable approximation.

Limitations of the quasiparticle approximation in certain regions.

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.