Unified dark energy models: A phenomenological approach

A phenomenological approach is proposed to the problem of the accelerated expansion of the Universe and of the nature of dark energy. A general class of models is introduced whose energy density depends on the redshift z in such a way that a smooth transition among the three main phases of the evolution of the Universe (radiation era, matter domination, asymptotical de Sitter state) is naturally achieved. We use the estimated age of the Universe, the Hubble diagram of type-Ia supernovae, and the angular size--redshift relation for compact and ultracompact radio structures, to test whether the model is in agreement with astrophysical observation and to constrain its main parameters. Although phenomenologically motivated, the model may be straightforwardly interpreted as a two-fluid scenario in which the quintessence is generated by a suitably chosen scalar field potential. On the other hand, the same model may also be read in the context of unified dark energy models or in the framework of modified Friedmann equation theories.