Thermal pressure on ultrarelativistic bubbles from a semiclassical formalism

We study a planar bubble wall that is traveling at an ultrarelativistic speed through a thermal plasma. This situation may arise during a first-order electroweak phase transition in the early universe. As particles cross the wall, it is assumed that their mass grows from ma to mb, and they are decelerated causing them to emit massless radiation (mc = 0). We are interested in the momentum transfer to the wall, the thermal pressure felt by the wall, and the resultant terminal velocity of the wall. We employ the semiclassical current radiation (SCR) formalism to perform these calculations. An incident-charged particle is treated as a point-like classical electromagnetic current, and the spectrum of quantum electromagnetic radiation (photons) is derived by calculating appropriate matrix elements. To understand how the spectrum depends on the thickness of the wall, we explore simplified models for the current corresponding to an abrupt and a gradual deceleration. For the model of abrupt deceleration, we find that the SCR formalism can reproduce the Ptherm ∝ γ0w scaling found in earlier work by assuming that the emission is soft, but if the emission is not soft the SCR formalism can be used to obtain Ptherm ∝ γ2w instead. For the model of gradual deceleration, we find that the wall thickness Lw enters to cutoff the otherwise log-flat radiation spectrum above a momentum of ∼ γ2w / Lw, and we discuss the connections with classical electromagnetic bremsstrahlung.