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

Tuorla Observatory, Department of Physics and Astronomy, University of Turku, 21500 Piikkiö, Finland
e-mail: pekkatee@utu.fi

Received: 17 February 2014
Accepted: 19 June 2014

Abstract

Aims. 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 ΛCDM model, with the everywhere constant DE density, and 2) the “Swiss cheese model”, where the Universe is as old as the standard model and the DE density is zero on short scales, including the environment of the system.

Methods. We calculated 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 DE density, and the time of ejection of the dwarf galaxies, which are treated as test particles. These results are compared with the observed Hubble flow around the Local Group.

Results. The predicted distance–velocity relations around galaxy groups are not very sensitive indicators of the DE density, owing to the observational scatter and the uncertainties caused by the mass used for the group and a range in the ejection times. In general, the Local Group outflow data agree with the local DE density being equal to the global one, if the Local Group mass is about 4 × 10¹² M_⊙; a lower mass ≲ 2 × 10¹² M_⊙ 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 DE density.