Reconstruction of the interaction term between dark matter and dark   energy using SNe Ia
  We apply a parametric reconstruction method to a homogeneous, isotropic and spatially flat Friedmann-Robertson-Walker (FRW) cosmological model filled of a fluid of dark energy (DE) with constant equation of state (EOS) parameter interacting with dark matter (DM). The reconstruction method is based on expansions of the general interaction term and the relevant cosmological variables in terms of Chebyshev polynomials which form a complete set orthonormal functions. This interaction term describes an exchange of energy flow between the DE and DM within dark sector. To show how the method works we do the reconstruction of the interaction function expanding it in terms of only the first six Chebyshev polynomials and obtain the best estimation for the coefficients of the expansion assuming three models: (a) a DE equation of the state parameter $w =-1$ (an interacting cosmological $\Lambda$), (b) a DE equation of the state parameter $w =$ constant with a dark matter density parameter fixed, (c) a DE equation of the state parameter $w =$ constant with a free constant dark matter density parameter to be estimated, and using the Union2 SNe Ia data set from "The Supernova Cosmology Project" (SCP) composed by 557 type Ia supernovae. In both cases, the preliminary reconstruction shows that in the best scenario there exist the possibility of a crossing of the noninteracting line Q=0 in the recent past within the $1\sigma$ and $2\sigma$ errors from positive values at early times to negative values at late times. This means that, in this reconstruction, there is an energy transfer from DE to DM at early times and an energy transfer from DM to DE at late times. We conclude that this fact is an indication of the possible existence of a crossing behavior in a general interaction coupling between dark components. 
Freddy Cueva Solano and Ulises Nucamendi
astro-ph.CO gr-qc