High-frequency gravitational waves from first-order phase transitions

Abstract

First-order phase transitions in the early Universe are a well-motivated source of gravitational waves (GWs). In this Letter, we identify a previously overlooked GW production mechanism: gravitational transition radiation, arising from graviton emission by particles whose mass changes as they pass through expanding bubble walls. Unlike conventional sources such as bubble collisions or sound waves, this mechanism operates at the microscopic scale set by the Lorentz-contracted wall thickness, leading to GW emission at significantly higher frequencies. The resulting spectrum features a distinctive shape with a peak frequency redshifting to $f_{\rm peak}\sim T_0\sim 10^{10}\,{\rm Hz}$ where $T_0$ is the current temperature of the Universe. This mechanism is generic and is expected to operate similarly for domain walls and other relativistic interfaces.

Figures (4)

• Figure 1: GTR: a particle hitting an expanding wall emits a graviton due to the change in its mass.
• Figure 2: The shape of the power spectrum for $g_{\star,s}(T)=100$. The peak frequency today is at $f_{\rm peak}\approx 0.176 T_0$.
• Figure 3: Comparison of the GWs studied in this work (brown) with CGMB (orange) and those from conventional FOPT sources (red), using the benchmark parameters specified in Eq. \ref{['eq:BP']}. In computing the GW power spectrum, we have multiplied Eq. \ref{['eq:OmegaGW']} by a factor of 10 to account for multiple degrees of freedom. The figure is generated with the high-frequency GW plotter HFGWPlotter.
• Figure 4: The function $I_1$ in Eq. \ref{['eq:drhodlnk0-I']} for a few values of $\{m_{\chi,b},\gamma_w\}$. The additional dependence on these two parameters, aside from the dependence on the combination $k^0/\gamma_w T$, is weak. The GW power spectrum has the same shape as $I_1$.