Speed of sound in cosmological phase transitions and effect on gravitational waves

Understand

The energy budget for gravitational waves of a cosmological first order phase transitions depends on the speed of sound in the thermal plasma in both phases around the bubble wall.

• Working in the real-singlet augmented Standard Model, which admits a strong two-step electroweak phase transition, we compute higher order corrections to the pressure and sound speed.
• We compare our result to lower-order approximations to the sound speed and the energy budget and investigate the impact on the gravitational wave signal.
• We find that deviations in the speed of sound from $c_s^2 = 1/3$ are enhanced up to $\mathcal O(5\%)$ in our higher-order computation.