Gravitational-wave signatures from reheating

We initiate a study of the gravitational-wave signatures of a phase transition that occurs as the Universe's temperature increases during reheating. The gravitational-wave signatures of such a heating phase transition are different from those of a cooling phase transition, and their detection could allow us to probe reheating. In the lucky case that the gravitational-wave signatures from both the heating and cooling phase transitions were to be observed, information about reheating could in principle be obtained utilizing the correlations between the two transitions. Frictional effects, leading to a constant bubble-wall speed in one case, will instead behave as an ``antifriction'' force in the other and accelerate the bubble wall. This antifriction will often take the bubble into a runaway regime, significantly enhancing the amplitude of the heating phase transition gravitational-wave signal. The efficiency, strength, and duration of the phase transitions will be similarly correlated in a reheating-dependent way.