σCDM coupled to radiation: Dark energy and Universe acceleration

Recently, the Chiral Cosmological Model (CCM) coupled to cold dark matter (CDM) has been investigated as sigma CDM model to study the observed accelerated expansion of the Universe. Dark sector fields (as Dark Energy content) coupled to cosmic dust were considered as the source of Einstein gravity in Friedmann-Robertson-Walker (FRW) cosmology. Such model had a beginning at the matter-dominated era. The purposes of our present investigation are two-fold: To extend life of the sigma CDM for earlier times to radiation-dominated era and to take into account variation of the exponential potential V = V-0 exp (-root lambda phi/M-P) + V-0 exp (-root lambda chi/M-P) via variation of the interaction parameter lambda. We use Markov Chain Monte Carlo (MCMC) procedure to investigate possible values of initial conditions constrained by the measured amount of the dark matter, dark energy and radiation component today. Our analysis includes dark energy contribution to critical density, the ratio of the kinetic and potential energies, deceleration parameter, effective equation of state (EoS) and evolution of DE EoS with variation of coupling constant lambda. A comparison with the Lambda CDM model was performed. A new feature of the model is the existence of some values of potential coupling constant, leading to a sigma CDM solution without transition into accelerated expansion epoch.