The dynamics of the local group as a probe of Dark Energy and Modified Gravity

TL;DR

This study investigates how different cosmological models beyond General Relativity affect the dynamics of the Local Group, revealing significant deviations in velocities and angular momentum that can serve as observational signatures.

Contribution

It introduces a method to compare Local Group dynamics across various modified gravity theories using observable kinematic data, highlighting potential small-scale signatures of these theories.

Findings

Modified gravity models increase pairwise velocities by up to 60%.

LG angular momentum and energy are significantly affected in alternative theories.

Some models exceed observational constraints, indicating testable differences.

Abstract

In this work we study the dynamics of the Local Group (LG) within the context of cosmological models beyond General Relativity (GR). Using observable kinematic quantities to identify candidate pairs we build up samples of simulated LG-like objects drawing from $f(R)$, symmetron, DGP and quintessence N-body simulations together with their $\Lambda$CDM counterparts featuring the same initial random phase realisations. The variables and intervals used to define LG-like objects are referred to as Local Group model; different models are used throughout this work and adapted to study their dynamical and kinematic properties. The aim is to determine how well the observed LG-dynamics can be reproduced within cosmological theories beyond GR, We compute kinematic properties of samples drawn from alternative theories and $\Lambda$CDM and compare them to actual observations of the LG mass, velocity and position. As a consequence of the additional pull, pairwise tangential and radial velocities are enhanced in modified gravity and coupled dark energy with respect to $\Lambda$CDM, inducing significant changes to the total angular momentum and energy of the LG. For example, in models such as $f(R)$ and the symmetron this increase can be as large as $60\%$, peaking well outside of the $95\%$ confidence region allowed by the data. This shows how simple considerations about the LG dynamics can lead to clear small-scale observational signatures for alternative scenarios, without the need of expensive high-resolution simulations.