Dark D-brane cosmology: From background evolution to cosmological perturbations

We study the cosmological predictions of the dark D-brane model, in which dark matter resides on a D-brane moving in a higher-dimensional space. By construction, dark matter interacts only gravitationally with the standard model sector in this framework. The dark energy scalar field is associated with the position of the D-brane, and its dynamics is encoded in a Dirac-Born-Infeld action. On the other hand, dark matter is identified with matter on the D-brane, that naturally couples to dark energy via a disformal coupling. We analyze the numerical evolution of the cosmological background, highlighting the fact that there are two regimes of interest: one in which the coupling is positive throughout and another in which the coupling is negative at the present. In the latter, there is the enticing possibility of having scenarios in which the coupling is positive for a significant part of the evolution, before decreasing toward negative values. In both cases, the coupling is very small at early times and starts to grow only during the late matter-dominated era. We also derive the equations for the linear cosmological perturbations and an expression for the effective time-dependent gravitational coupling between dark matter particles and present the numerical results for the cosmic microwave background anisotropy and matter power spectra. This allows for a direct comparison of the predictions for the growth of large-scale structure with other disformal quintessence models.