Gravitational wave detectors like LIGO observe in the hertz-to-kilohertz range — the frequencies of merging compact objects. Above that, the universe has been silent. High-frequency gravitational wave detectors in the megahertz band exist conceptually but lack known astrophysical targets. Why build a detector with nothing to detect?
This paper provides the target. During a galactic supernova, the nascent neutron star's core may undergo a phase transition from hadronic matter to deconfined quark matter. This transition, occurring in the seconds after core bounce, produces gravitational wave emission in the megahertz band — orders of magnitude above LIGO's range.
The signal exists because the phase transition is first-order: the core doesn't smoothly cross into the quark matter phase but nucleates bubbles that grow, collide, and generate gravitational radiation. The frequency is set by the transition's physical scale — centimeters to meters in the core of a neutron star, corresponding to megahertz gravitational waves.
The detection would probe quantum chromodynamics in a regime inaccessible to any other experiment. Terrestrial accelerators cannot reach the densities and temperatures of a neutron star core. Observations of neutron star masses and radii constrain the equation of state but cannot directly observe the transition. Megahertz gravitational waves would be a direct imprint of QCD phase structure — the strong force, heard rather than seen.