Megahertz Gravitational Waves from Neutron Star Mergers

Abstract

Neutron star mergers provide a unique laboratory for the study of strong-field gravity coupled to quantum chromodynamics in extreme conditions. The frequencies and amplitudes of the resulting gravitational waves encode invaluable information about the merger. Simulations to date have shown that these frequencies lie in the kilohertz range. They have also shown that, if quantum chromodynamics possesses a first-order phase transition at high baryon density, then this is likely to be accessed during the merger dynamics. Here we show that this would result in the nucleation of superheated and/or supercompressed bubbles whose subsequent dynamics would produce gravitational waves in the megahertz range. We estimate the amplitude of this signal and compare it to the sensitivity of planned future detectors.

Figures (4)

• Figure 1: A possible phase transition in QCD, indicated by the solid red curve. $T$ and $\mu$ are the temperature and the baryon chemical potential, respectively. The dotted black curve shows a possible evolution of a region of a NS merger as this region is heated and/or compressed. The points dubbed $A$, $A'$ and $C$ correspond to the states shown in Fig. \ref{['meta']}.
• Figure 2: Energy density as a function of the position along the dashed black curve in Fig. \ref{['FOPT']}. Both $T$ and $\mu$ increase from left to right. The dotted black vertical line indicates the location of the phase transition, determined by the condition that the states $A$ and $A'$ have the same free energy density. Stable, metastable and unstable states are shown as solid green, dashed blue and doted red respectively. As a region is heated/compressed it enters the lower metastable branch. At $B$ the nucleation probability has been sufficiently enhanced, and bubbles of the preferred state $C$ on the upper stable branch are quickly nucleated.
• Figure 3: FOPT dynamics inside a HoCS. Once a HoCS enters the metastable region, it takes a time of order $\tau \simeq 1$ ms for the first few bubbles to nucleate inside the metastable phase (left). Bubbles then grow to a macroscopic size $R$ and collide in a time $\beta^{-1} \simeq 6\, \mu$s (center). They leave behind long-lived sound waves (with lifetime $\tau \simeq 1$ ms) of characteristic size $R$ propagating on top of the stable phase (right).
• Figure 4: Sensitivity of the proposal in Domcke:2024mfu (broadband and resonant) compared with the expected signal from an NS–NS collision at various distances from Earth.