Acoustic gravitational waves beyond leading order in bubble over Hubble radius

Abstract: We calculate the gravitational wave power spectrum from sound waves in a cosmological first order phase transition in the unexplored regime of large bubbles, by which we mean that the mean bubble spacing $R_$ is a non-negligible fraction of the Hubble length $\mathcal{H}_^{-1}$, i.e. $R_\mathcal{H}_ \lesssim \mathcal{O}(1)$. Since the amplitude of the gravitational wave signal increases with $R_\mathcal{H}_$, this is also the loud signal regime. In this regime the effects of gravity, hitherto neglected, become relevant. We carry out the calculation in cosmological perturbation theory expanding in the parameter $R_\mathcal{H}_$, or bubble over Hubble radius. The leading order term is the standard result for acoustic production of gravitational waves. At next-to-leading order we find three novel contributions: two contributions arise from general relativistic corrections to the dynamics of both sound and gravitational waves. A third contribution comes from gravitational waves induced by curvature perturbations. These contributions suppress the gravitational wave peak amplitude. The suppression factor, with respect to the leading order contribution, scales as $(R_\mathcal{H}_)^2$, and also depends on other transition parameters, such as the sound speed $c_s$, the duration of the acoustic source, and the peak wavenumber of the velocity field $k_p$. In a simplified model of the velocity field, we find that the suppression factor lies between $2\%$ and $15\%$ when $R_\mathcal{H}_ \simeq 0.5$, but is independent of the root mean squared fluid velocity. We provide analytical approximations to the next-to-leading order corrections, and a recipe to join them smoothly across different frequency regimes. Our work improves the precision of the current estimations of the gravitational wave power spectrum in the relatively unexplored regime of phase transition with large bubbles.