Universal thermodynamics of strongly interacting Fermi gases

Strongly interacting Fermi gases are of great current interest. Not only are fermions the most common particles in the universe, but they are also thought to have a universal thermodynamic behaviour for strong interactions1,2,3. Recent experiments on ultracold Fermi gases provide an unprecedented opportunity to test universality in the laboratory4,5,6,7,8. In principle this allows—for example—the interior properties of hot, dense neutron stars to be investigated on earth. Here we carry out a detailed test of this prediction. We analyse results from three ultracold fermion experiments involving two completely distinct atomic species in different kinds of atomic-trap environment6,7,8. The data are compared with the predictions of a recent strong-interaction theory9,10. Excellent agreement is obtained, with no adjustable parameters. By extrapolating to zero temperature, we show that the experimental measurements yield a many-body parameter β≃−0.59±0.07, describing the universal energy of strongly interacting Fermi gases.