Torrents, JordiDunjko, VanjaGonchenko, MarinaAstrakharchik, GrigoriOlshanii, Maxim2023-03-012023-03-012022-03-142542-4653https://hdl.handle.net/2445/194406We address the origins of the quasi-periodic breathing observed in [Phys. Rev.\ X vol. 9, 021035 (2019)] in disk-shaped harmonically trapped two-dimensional Bose condensates, where the quasi-period $T_{\text{quasi-breathing}}\sim$~$2T/7$ and $T$ is the period of the harmonic trap. We show that, due to an unexplained coincidence, the first instance of the collapse of the hydrodynamic description, at $t^{*} = \arctan(\sqrt{2})/(2\pi) T \approx T/7$, emerges as a `skillful impostor' of the quasi-breathing half-period $T_{\text{quasi-breathing}}/2$. At the time $t^{*}$, the velocity field almost vanishes, supporting the requisite time-reversal invariance. We find that this phenomenon persists for scale-invariant gases in all spatial dimensions, being exact in one dimension and, likely, approximate in all others. In $\bm{d}$ dimensions, the quasi-breathing half-period assumes the form $T_{\text{quasi-breathing}}/2 \equiv t^{*} = \arctan(\sqrt{d})/(2\pi) T$. Remaining unresolved is the origin of the period-$2T$ breathing, reported in the same experiment.application/pdfengcc-by (c) Torrents, Jordi et al., 2022https://creativecommons.org/licenses/by/4.0/Pertorbació (Dinàmica quàntica)Mecànica estadística del no equilibriMecànica estadísticaFísica estadísticaPerturbation (Quantum dynamics)Nonequilibrium statistical mechanicsStatistical mechanicsStatistical physicsinfo:eu-repo/semantics/article7309592023-03-01info:eu-repo/semantics/openAccess