Dark energy from dark radiation in strongly coupled cosmologies with no
  fine tuning
  A dual component made of non-relativistic particles and a scalar field,
exchanging energy, naturally falls onto an attractor solution, making them a
(sub)dominant part of the cosmic energy during the radiation dominated era,
provided that the constant \beta, measuring the coupling, is strong enough. The
density parameters of both components are then constant, as they expand as
a^{-4}. If the field energy is then prevalently kinetic, as is expected, its
energy is exactly half of the pressureless component; the dual component as a
whole, then, has a density parameter \Omega_{cd} = 3/4\beta^2 (e.g., for
\beta~2.5, \Omega_{cd}~0.1, in accordance with Dark Radiation expectations).
The stationary evolution can only be broken by the rising of other
component(s), expanding as a^{-3}. In a realistic scenario, this happens when
z~3-5x10^3. When such extra component(s) become(s) dominant, the densities of
the dual components also rise above radiation. The scalar field behavior can be
easily tuned to fit Dark Energy data, while the coupled DM density parameter
becomes O(10^{-3}). This model however requires that, at present, two different
DM components exist. The one responsible for the break of the stationary regime
could be made, e.g., by thermally distributed particles with mass even >>1$-2
keV (or non-thermal particles with analogous average speed) so accounting for
the size of observed galactic cores; in fact, a fair amount of small scale
objects is however produced by fluctuation re-generated by the coupled DM
component, in spite of its small density parameter, after the warm component
has become non-relativistic.
