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. 
Alejandro Aviles and Jorge L. Cervantes-Cota
astro-ph.CO gr-qc