The stability of galaxies in an expanding universe obtained by Newtonian dynamics

TL;DR

This study uses classical Molecular Dynamic simulations with a novel, stable, time-reversible algorithm to explore the long-term stability of galaxies in an expanding universe, suggesting galaxies remain stable for cosmological timescales.

Contribution

It extends a recent time-reversible algorithm to include Hubble expansion, enabling long-term simulations of galaxy stability in an expanding universe.

Findings

Galaxies remain stable for over 25 Gyr in simulations.

Hubble expansion eventually leads to galaxy disassembly.

Galaxies are stable at 13-14 Gyr, matching the universe's age.

Abstract

The dynamics of galaxies in an expanding universe is often determined for gravitational and dark matter in an Einstein-de Sitter universe, or alternatively by modifying the gravitational long-range attractions in the Newtonian dynamics (MOND). Here the time evolution of galaxies is determined by simulations of systems with pure gravitational forces by classical Molecular Dynamic simulations. A time reversible algorithm for formation and aging of gravitational systems by self-assembly of baryonic objects, recently derived (Eur. Phys. J. Plus 2022, 137:99), is extended to include the Hubble expansion of the space. The algorithm is stable for billions of time steps without any adjustments. The algorithm is used to simulate simple models of the Milky Way with the Hubble expansion of the universe, and the galaxies are simulated for times which corresponds to more than 25 Gyr. The rotating galaxies lose bound objects from time to time, but they are still stable at the end of the simulations. The simulations indicate that the explanation for the dynamics of galaxies may be that the universe is very young in cosmological times. Although the models of the Milky Way are rather stable at 13-14 Gyr, which corresponds to the cosmological time of the universe, the Hubble expansion will sooner or later release the objects in the galaxies. But the simulations indicate that this will first happen in a far away future.