Modelling the thermodynamics of ultracold atomic bubbles in space*

With the recent observation of ultracold atomic bubbles in microgravity using the NASA Cold Atom Lab (CAL) aboard the International Space Station, we discuss modelling the thermodynamic properties of shell-shaped quantum fluids and directly compare to the data obtained from the experiment. We calculate the critical temperature required to achieve Bose-Einstein condensation (BEC) in the novel hollowed-out bubble geometries generated on CAL and, in line with experiment, model how the temperature evolves as an initially condensed gas is inflated into a bubble adiabatically. Using a simplified isotropic `bubble-trap' potential, we show that standard semiclassical methods overestimate the BEC critical temperature for atoms confined in quasi-2D thin shells and with this insight carry out our analysis of the anisotropic CAL trap using a hybrid spectral and semiclassical approach. We conclude by discussing the near-future possibility of achieving large condensed bubbles on CAL.