Generalized nonextensive thermodynamics applied to the cosmic background radiation in a Robertson-Walker universe

Statistical mechanics is useful to introduce generalizations of standard thermodynamics through the generalization of the entropy and other state functions. Along these lines the Tsallis nonextensive and the Bergmann group symmetric generalizations have proven to be very useful. We combine both formalisms to describe the nonextensive thermostatistics in a relativistic setting. We obtain the generalized forms of the first and second laws of thermodynamics for reversible processes, and apply the resulting theory to the cosmic blackbody radiation in a Robertson-Walker model of the Universe. We show that the temperature of the cosmic blackbody radiation varies as the inverse of the scale factor of the Universe, and is independent of the degree of nonextensivity.