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

Abstract

We study the distinct effects of dark matter and dark energy on thefuture evolution of nearby large scale structures using constrainedN-body simulations. We contrast a model of cold dark matter and a cosmological constant(ΛCDM) with an open CDM (OCDM) model with the same matter densityΩm = 0.3 and the sameHubble constant h = 0.7. Already by the time the scale factor has increased by a factor of 6 (29 Gyr from now inΛCDM; 78 Gyr from now in OCDM) the comoving position of the Local Group is frozen. Well beforethat 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 aΛCDM model and at a roughly constant velocity in an OCDM model. More generally, in comovingcoordinates the future large scale structure will look like a sharpened image ofthe present structure: the skeleton of the cosmic web will remain the same, butclusters will be more ‘isolated’ and the filaments will become thinner. This impliesthat the long-term fate of large scale structure as seen in comoving coordinatesis determined primarily by the matter density. We conclude that although theΛCDM model is accelerating at present due to its dark energy component while the OCDM modelis non-accelerating, their large scale structures in the future will look very similar incomoving coordinates.