ZKDR Distance, Angular Size and Phantom Cosmology

The influence of mass inhomogeneities on the angular size-redshift test is investigated for a large class of flat cosmological models driven by dark energy plus a cold dark matter component. The results are presented in two steps. First, the mass inhomogeneities are modeled by a generalized Zeldovich-Kantowski-DyerRoeder (ZKDR) distance which is characterized by a smoothness parameter α(z) and a power index γ, and, second, we provide a statistical analysis to angular size data for a large sample of milliarcsecond compact radio sources. By marginalizing over the characteristic angular size l, fixing ΩM = 0.26 and assuming a Gaussian prior on ω, i.e., ω = −1± 0.3, the best fit values are ω = −1.03 and α = 0.9. By assuming a Gaussian prior on the matter density parameter, i.e., ΩM = 0.3± 0.1, the best fit model for a phantom cosmology with ω = −1.2 occurs at ΩM = 0.29 and α = 0.9 when we marginalize over the characteristic size of the compact radio sources. The results discussed here suggest that the ZKDR distance can give important corrections to the so-called background tests of dark energy.