Varying alpha generalized Dirac-Born-Infeld models

We study the cosmological consequences of a class of Dirac-Born-Infeld models, and assess their viability as a candidate for the recent acceleration of the Universe. The model includes both the rolling tachyon field and the generalized Chaplygin gas models as particular limits, and phenomenologically each of these provides a possible mechanism for a deviation of the value of the dark energy equation of state from its canonical (cosmological constant) value. The field-dependent potential that is characteristic of the rolling tachyon also leads to variations of the fine-structure constant $\alpha$, implying that the model can be constrained both by standard cosmological probes and by astrophysical measurements of $\alpha$. Our analysis, using the latest available low-redshfit data and local constraints from atomic clock and weak equivalence principle experiments, shows that the two possible deviations of the dark energy equation of state are constrained to be ${\log }_{10}(1+w_0{)}_V<-7.85$ and ${\log }_{10}(1+w_0{)}_C<-0.85$, respectively for the rolling tachyon and Chaplygin components, both being at the 95.4% confidence level (although the latter depends on the choice of priors, in a way that we quantify). Alternatively, the 95.4% confidence level bound on the dimensionless slope of the potential is ${\log }_{10}\lambda <-5.36$. This confirms previous analyses indicating that in these models the potential needs to be extremely flat.

Figure 1

Constraints on generalized DBI models from our cosmological data, using the equation of state focused parametrization. The black contours denote the one, two, and three sigma confidence levels, and the color map depicts the reduced ${\chi }^2$ of the fit for each set of model parameters (the white color corresponds to a reduced ${\chi }^2$ of 1.5 or higher, and the black one to a reduced ${\chi }^2$ of 0.5 or lower).

Figure 2

One-dimensional (marginalized) posterior likelihoods for the model parameters corresponding to the analysis in Fig. 1.

Figure 3

Constraints on generalized DBI models, using the equation of state focused parametrization. The black contours denote the one, two, and three sigma confidence levels, and the color map depicts the reduced ${\chi }^2$ of the fit for each set of model parameters (the white color corresponds to a reduced ${\chi }^2$ of 1.5 or higher).

Figure 4

One-dimensional (marginalized) posterior likelihoods for the model parameters corresponding to the analysis in Fig. 3.

Figure 5

Constraints on generalized DBI models, using the potential slope parametrization. The black contours denote the one, two, and three sigma confidence levels, and the color map depicts the reduced ${\chi }^2$ of the fit for each set of model parameters (the white color corresponds to a reduced ${\chi }^2$ of 1.5 or higher).

Figure 6

One-dimensional (marginalized) posterior likelihoods for the model parameters corresponding to the analysis in Fig. 5.