Cosmological Dynamics of D-BIonic and DBI Scalar Field and Coincidence
  Problem of Dark Energy
  We study the cosmological dynamics of D-BIonic and DBI scalar field, which is
coupled to matter fluid. For the exponential potential and the exponential
couplings, we find a new analytic scaling solution yielding the accelerated
expansion of the Universe. Since it is shown to be an attractor for some range
of the coupling parameters, the density parameter of matter fluid can be the
observed value, as in the coupled quintessence with a canonical scalar field.
Contrary to the usual coupled quintessence, where the value of matter couple
giving observed density parameter is too large to satisfy observational
constraint from CMB, we show that the D-BIonic theory can give similar solution
with much smaller value of matter coupling. As a result, together with the fact
that the D-BIonic theory has a screening mechanism, the D-BIonic theory can
solve the so-called coincidence problem as well as the dark energy problem.
