Backreaction in an Analogue Black Hole Experiment

Rotating curved spacetime signatures from a giant quantum vortex

Gravity simulators1 are laboratory systems in which small excitations such as sound2 or surface waves3,4 behave as fields propagating on a curved spacetime geometry. The analogy between gravity and fluids requires vanishing viscosity2-4, a feature naturally realized in superfluids such as liquid helium or cold atomic clouds5-8. Such systems have been successful in verifying key predictions of quantum field theory in curved spacetime7-11. In particular, quantum simulations of rotating curved spacetimes indicative of astrophysical black holes require the realization of an extensive vortex flow12 in superfluid systems. Here we demonstrate that, despite the inherent instability of multiply quantized vortices13,14, a stationary giant quantum vortex can be stabilized in superfluid 4He. Its compact core carries thousands of circulation quanta, prevailing over current limitations in other physical systems such as magnons5, atomic clouds6,7 and polaritons15,16. We introduce a minimally invasive way to characterize the vortex flow17,18 by exploiting the interaction of micrometre-scale waves on the superfluid interface with the background velocity field. Intricate wave-vortex interactions, including the detection of bound states and distinctive analogue black hole ringdown signatures, have been observed. These results open new avenues to explore quantum-to-classical vortex transitions and use superfluid helium as a finite-temperature quantum field theory simulator for rotating curved spacetimes19.

Numerical Studies of Back Reaction Effects in an Analog Model of Cosmological Preheating

We theoretically propose an atomic Bose-Einstein condensate as an analog model of backreaction effects during the preheating stage of the early Universe. In particular, we address the out-of-equilibrium dynamics where the initially excited inflaton field decays by parametrically exciting the matter fields. We consider a two-dimensional, ring-shaped BEC under a tight transverse confinement whose transverse breathing mode and the Goldstone and dipole excitation branches simulate the inflaton and quantum matter fields, respectively. A strong excitation of the breathing mode leads to an exponentially growing emission of dipole and Goldstone excitations via parametric pair creation: Our numerical simulations of the BEC dynamics show how the associated backreaction effect results not only in an effective friction of the breathing mode, but also in a quick loss of longitudinal spatial coherence of the initially in-phase excitations. Implications of this result on the validity of the usual semiclassical description of backreaction are finally discussed.

Quantum Vacuum Excitation of a Quasinormal Mode in an Analog Model of Black Hole Spacetime

Vacuum quantum fluctuations near horizons are known to yield correlated emission by the Hawking effect. We use a driven-dissipative quantum fluid of microcavity polaritons as an analog model of a quantum field theory on a black-hole spacetime and numerically calculate correlated emission. We show that, in addition to the Hawking effect at the sonic horizon, quantum fluctuations may result in a sizable stationary excitation of a quasinormal mode of the field theory. Observable signatures of the excitation of the quasinormal mode are found in the spatial density fluctuations as well as in the spectrum of Hawking emission. This suggests an intrinsic fluctuation-driven mechanism leading to the quantum excitation of quasinormal modes on black hole spacetimes.

Cosmological analogies for geophysical flows, Lagrangians, and new analogue gravity systems

Formal analogies between the ordinary differential equations describing geophysical flows and Friedmann cosmology are developed. As a result, one obtains Lagrangian and Hamiltonian formulations of these equations, while laboratory experiments aimed at testing geophysical flows are shown to constitute analogue gravity systems for cosmology.

Ramp-up of Hawking Radiation in Bose-Einstein-Condensate Analog Black Holes

Inspired by a recent experiment by Steinhauer and co-workers, we present a simple model which describes the formation of an acoustic black hole in a Bose-Einstein condensate, allowing an analytical computation of the evolution in time of the corresponding density-density correlator. We show the emergence of analog Hawking radiation out of a "quantum atmosphere" region significantly displaced from the horizon. This is quantitatively studied both at T=0 and even in the presence of an initial temperature T, as is always the case experimentally.