A study based on the thesis of Chiara Coviello (pictured right), who graduated in Physics from the University of Pisa in 2023 and is now at King's College London for a PhD, was recently published in the journal AVS Quantum Science. At the center of the study - titled “Gravitational waves and Black Hole perturbations in acoustic analogues” – are black holes, whose dark allure makes them some of the most fascinating yet challenging objects in the cosmos to analyze.
To better understand them, the interdisciplinary research team, of which Coviello is a part, examined acoustic black holes - analogue systems that trap sound waves and can be created in a tabletop experiment. The authors of the study include Professor Marilù Chiofalo of the University of Pisa, Professors Dario Grasso of the INFN in Pisa, Stefano Liberati of SISSA in Trieste and Massimo Mannarelli of the Gran Sasso National Laboratories, with Silvia Trabucco, a PhD student at the Gran Sasso Institute Science after graduating in Physics in Pisa.
Coviello and the other authors investigated whether acoustic black holes could be used to understand the interactions between gravitational waves and astrophysical black holes. In a theoretical analysis, they explored how to generate gravitational wave-like perturbations in a Bose-Einstein condensate of ultracold atoms, a state of matter in which a few hundred thousand atoms behave collectively as if they were one large molecule. In Bose-Einstein condensates, the lowest energy excitations are density perturbations, described by quantum particles called phonons.
In the study, phonons behave like massless particles within a geometry engineered to replicate certain characteristics of an astrophysical black hole for light quanta, or photons. In acoustic black hole analogs, phonons become trapped and simultaneously contribute to what is known as Hawking radiation, a phenomenon predicted by the renowned astrophysicist Stephen Hawking for astrophysical black holes. By leveraging existing knowledge about gravitational waves, the researchers developed a correspondence between astrophysical black holes and acoustic black holes. This approach aims to better understand how gravitational wave-like perturbations affect the acoustic horizon of a laboratory black hole. The idea is to use matter physics experiments on optical tables, spanning just a few square meters, as highly accurate and controllable quantum simulators to study properties of astrophysical and cosmological interest.
"We are excited that this physics can be explored in currently feasible experiments, such as those using ultracold atoms, offering a new way to analyze these systems in a controlled environment," said Chiara Coviello.
The results could be used to study the dissipation and reflection effects of gravitational wave-like perturbations in acoustic black holes. The authors believe this will help shed light on universal behaviour and the role of quantum fluctuations in astrophysical black holes.
The research team, consisting of a collaboration between several universities and research centres, intends to continue the study by analyzing the viscosity properties of the acoustic horizon in relation to its entropy, known to have universal behaviour, i.e. not dependent on the specific physical system. The results could provide new insights into the basic physical theory and symmetries of astrophysical black holes.