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We calculate the partition function for a relativistic Bose-Einstein gas. The excitation energies are identified and compared with known results in various limits. In particular, this treatment shows how the Goldstone bosons of a spontaneously broken symmetry and the quasiparticles of the nonrelativistic gas emerge as special cases of one general formula. New results are obtained for the spectrum of Goldstone modes in a medium of nonzero density, and the fully relativistic case is discussed along with a possible cosmological application.
Bernstein et al. (1991) studied this question.