  We experimentally and theoretically observe the expansion behaviors of a
spherical Bose-Einstein condensate. A rubidium condensate is produced in an
isotropic optical dipole trap with an asphericity of 0.037. We measure the
variation of the condensate size during the expansion process. The free
expansion of the condensate is isotropic, which is different from that of the
condensate usually produced in the anisotropic trap. The expansion in the short
time is speeding and then after a long time the expansion velocity
asymptotically approaches a constant value. We derive an analytic solution of
the expansion behavior based on the spherical symmetry, allowing a quantitative
comparison with the experimental measurement. The interaction energy of the
condensate is gradually converted into the kinetic energy at the beginning of
the expansion and the kinetic energy dominates after a long-time expansion. We
obtain the interaction energy of the condensate in the trap by probing the
expansion velocity, which is consistent with the theoretical prediction.
