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The evolution of a Universe modelled as a mixture of a Chaplygin gas andradiation is determined by taking into account irreversible processes. Thismixture could interpolate periods of a radiation dominated, a matter dominatedand a cosmological constant dominated Universe. The results of a Universemodelled by this mixture are compared with the results of a mixture whoseconstituents are radiation and quintessence. Among other results it is shownthat: (a) for both models there exists a period of a past deceleration with apresent acceleration; (b) the slope of the acceleration of the Universemodelled as a mixture of a Chaplygin gas with radiation is more pronounced thanthat modelled as a mixture of quintessence and radiation; (c) the energydensity of the Chaplygin gas tends to a constant value at earlier times thanthe energy density of quintessence does; (d) the energy density of radiationfor both mixtures coincide and decay more rapidly than the energy densities ofthe Chaplygin gas and of quintessence.Comment: 8 pages, 1 figure, to be published in GR

Letter: Irreversible Processes in a Universe Modelled as a Mixture of a Chaplygin Gas and Radiation