Analogue Gravity

Long-wavelength sound waves on a flowing fluid obey, to leading order, a wave equation that has the same mathematical form as a relativistic field on a curved spacetime. The role of the metric is played by the local flow profile and the sound speed; the role of light, by the sound mode itself. This correspondence — analogue gravity — turns laboratory hydrodynamics into a controlled platform for studying horizons, Hawking-like radiation, and cosmological evolution.

Third sound on a thin superfluid ⁴He film is an exceptionally clean platform for analogue gravity. It propagates with low loss, its dispersion can be engineered through the film thickness, and the underlying superfluid flow can be controlled via geometry and external drive. We use it to construct laboratory analogues of black-hole horizons — regions where the flow exceeds the local sound speed and acoustic excitations can no longer escape — and of expanding cosmologies with time-varying scale factors.

The work draws on, and contributes back to, our broader expertise in cavity optomechanics with superfluids: the same microwave readout used to probe third-sound modes also gives access to the analogue-spacetime dynamics with quantum-limited sensitivity. The hope is that analogue measurements of Hawking-like emission and cosmological particle production can serve as a counterpart to direct astrophysical observations and help discriminate between candidate models of the early universe.