Dark D-brane Cosmology
  Disformally coupled cosmologies arise from Dirac-Born-Infeld actions in Type
II string theories, when matter resides on a moving hidden sector D-brane.
Since such matter interacts only very weakly with the standard model particles,
this scenario can provide a natural origin for the dark sector of the universe
with a clear geometrical interpretation: dark energy is identified with the
scalar field associated to the D-brane's position as it moves in the internal
space, acting as quintessence, while dark matter is identified with the matter
living on the D-brane, which can be modelled by a perfect fluid. The coupling
functions are determined by the (warped) extra-dimensional geometry, and are
thus constrained by the theory. The resulting cosmologies are studied using
both dynamical system analysis and numerics. From the dynamical system point of
view, one free parameter controls the cosmological dynamics, given by the ratio
of the warp factor and the potential energy scales. The disformal coupling
allows for new scaling solutions that can describe accelerating cosmologies
alleviating the coincidence problem of dark energy. In addition, this scenario
may ameliorate the fine-tuning problem of dark energy, whose small value may be
attained dynamically, without requiring the mass of the dark energy field to be
unnaturally low.
