The future of the local large scale structure: the roles of dark matter and dark energy

We study the distinct effects of dark matter and dark energy on the future evolution of nearby large scale structures using constrained N-body simulations. We contrast a model of cold dark matter and a cosmological constant (Lambda CDM) with an open CDM (OCDM) model with the same matter density Omega m = 0.3 and the same Hubble constant h = 0.7. Already by the time the scale factor has increased by a factor of 6 ( 29 Gyr from now in Lambda CDM; 78 Gyr from now in OCDM) the comoving position of the Local Group is frozen. Well before that epoch the two most massive members of the Local Group, the Milky Way and Andromeda, will merge. However, as the expansion rates of the scale factor in the two models are different, the Local Group will be receding in physical coordinates from Virgo exponentially in an Lambda CDM model and at a roughly constant velocity in an OCDM model. More generally, in comoving coordinates the future large scale structure will look like a sharpened image of the present structure: the skeleton of the cosmic web will remain the same, but clusters will be more ` isolated' and the. laments will become thinner. This implies that the longterm fate of large scale structure as seen in comoving coordinates is determined primarily by the matter density. We conclude that although the CDM model is accelerating at present due to its dark energy component while the OCDM model is non-accelerating, their large scale structures in the future will look very similar in comoving coordinates.