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This study addresses a critical uncertainty in modeling first-order cosmological phase transitions by properly accounting for thermal masses of particles in bubble-wall friction calculations. The researchers found that including thermal masses in both the mathematical operators and collision integrals significantly suppresses the artificially enhanced contribution from infrared gauge bosons, shifting the dominant friction mechanism to top quarks at momentum scales comparable to the plasma temperature. Testing their approach on the singlet-extended Standard Model, they demonstrated that thermal mass corrections reduce sensitivity to poorly understood infrared plasma behavior while producing only minor changes to top-quark contributions.
Why it matters
This work improves the theoretical reliability of predictions for early universe phase transitions, which are crucial for understanding gravitational wave signatures that current and future detectors aim to observe. More accurate friction calculations enable better interpretation of cosmological observations and constrain beyond-Standard-Model physics scenarios.
Understand the Science
arXiv:2607.14867v1 Announce Type: cross
Abstract: Bubble-wall friction controls the dynamics of first-order cosmological phase transitions. In Boltzmann-equation approaches, a major uncertainty arises from infrared gauge bosons, whose contribution is artificially enhanced in the massless approximation. We study the impact of thermal masses by including them consistently in both the Liouville operator and the collision integrals. Thermal masses suppress the source term for out-of-equilibrium perturbations while also reducing interaction rates. These effects largely cancel for top quarks, giving only percent-level changes, but they strongly suppress the infrared gauge-boson contribution, shifting the dominant momenta to scales of order the temperature. As a result, gauge bosons become subleading and wall velocities are close to those obtained from top-quark friction alone. We illustrate this in the singlet-extended Standard Model. Our results show that thermal masses reduce the sensitivity of friction calculations to the poorly controlled infrared sector of the plasma.
Source: Thermal Masses and Bubble-Wall Friction in Cosmological Phase Transitions