Cosmological constraints for an Eddington-Born-Infeld field

We consider the Eddington-Born-Infeld (EBI) model here without assuming any cosmological constant. The EBI scalar field is supposed to play a role of both dark matter and dark energy. Different eras in cosmology are reconstructed for the model. A comparison is drawn with the $\Lambda \mathrm{CDM}$ model using Supernova Type Ia data, WMAP7 and baryon acoustic oscillations data. It seems that the EBI field in this form does not give a good fit to observational data in comparison to the $\Lambda \mathrm{CDM}$ model.

Figure 1

Effective equation of state for the scalar field (left panel), and plot of the ratio $X/H$ (right panel). At early times the EBI scalar field behaves as a dark matter component ($w_X\approx 0$); then, at late times, it drives the evolution of the universe. This plot shows the evolution for the parameters which minimize the ${\chi }^2$ given in (44). Notice that since $\alpha >1$, the final state is not de Sitter ($w_X<-1$); rather it tends to the solution characterized by $H^{\prime }/H\to \mathrm{\text{constant}}$ (Fig. 2) and $X/H\to \mathrm{\text{constant}}$ (right panel).

Figure 2

Evolution for $H^{\prime }/H=\dot{H}/H^2$ (left panel), and for the variable $X^{\prime }/X$ (right panel). The evolution, starting from radiation domination, passing through matter domination, at late times, tends to a super-accelerating final state on the minimum-${\chi }^2$ solution.