The Future of the Local Large Scale Structure: the roles of Dark Matter and Dark Energy

TL;DR

This study uses constrained N-body simulations to compare the long-term evolution of large scale structures under LCDM and OCDM models, highlighting the dominant role of matter density in future cosmic web morphology.

Contribution

It provides the first detailed simulation-based comparison of the future evolution of large scale structures in LCDM and OCDM cosmologies, emphasizing the impact of matter density.

Findings

Local Group's position becomes fixed in comoving coordinates.

Milky Way and Andromeda will merge before the scale factor increases sixfold.

Large scale structure remains similar in comoving coordinates, with clusters more isolated and filaments thinner.

Abstract

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 (LCDM) 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 increased by a factor of 6 (29 Gyr from now in LCDM; 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 a LCDM 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 filaments will become thinner. This implies that the long-term fate of large scale structure as seen in comoving coordinates is determined primarily by the matter density. We conclude that although the LCDM model is accelerating at present due to its Dark Energy component while the OCDM model is non accelerating, their large scale structure in the future will look very similar in comoving coordinates.