The dark degeneracy and interacting cosmic components
  We study some properties of the dark degeneracy, which is the fact that what
we measure in gravitational experiments is the energy momentum tensor of the
total dark sector, and any split into components (as in dark matter and dark
energy) is arbitrary. In fact, just one dark fluid is necessary to obtain
exactly the same cosmological and astrophysical phenomenology as the LCDM
model. We work explicitly the first order perturbation theory and show that
beyond the linear order the dark degeneracy is preserved under some general
assumptions. Then, we construct the dark fluid from a collection of interacting
fluids. Finally, we try to break the degeneracy with a general class of
couplings to baryonic matter. Nonetheless, we show that these interactions can
also be understood in the context of the LCDM model as between dark matter and
baryons. For this last investigation we choose two independent
parameterizations for the interactions, one inspired in electromagnetism and
the other in Chameleon theories. Then, we constrain them with a joint analysis
of CMB and Supernovae observational data.
