WallGo investigates: Theoretical uncertainties in the bubble wall velocity

We examine theoretical uncertainties in state-of-the-art calculations of the bubble wall velocity dur- ing 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 ther- mal 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 quasipar- ticle 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.