Paper detail

The Hubble diagram for a system within dark energy: influence of some relevant quantities

We study the influence of relevant quantities, including the density of dark energy (DE), to the predicted Hubble outflow around a system of galaxies. In particular, we are interested in the difference between two models: 1) The standard $Λ$CDM model, with the everywhere constant DE density, and 2) the "Swiss cheese model", where the universe is as old as the standard model, but the DE density is zero on short scales, including the environment of the system. We calculate the current predicted outflow patterns of dwarf galaxies around the Local Group-like system, using different values for the mass of the group, the local dark energy density, and the time of ejection of the dwarf galaxies, treated as test particles. These results are compared with the observed Hubble flow around the Local Group. The predicted distance-velocity relations around galaxy groups are not alone very sensitive indicators of the dark energy density, due to the obsevational scatter and the uncertainties caused by the used mass of the group and a range in the ejection times. In general, the Local Group outflow data are in agreement with the local DE density being equal to the global one, if the mass is about $4 \times 10^{12} M_{\odot}$; a lower mass of about $2 \times 10^{12} M_{\odot}$ or less could suggest a zero local DE density. The dependence of the inferred DE density on the mass is a handicap in this and other common dynamical methods. This emphasizes the need to use different approaches together, for constraining the local dark energy density.