The Influence of Hubble Expansion on the Future Merger of the Milky Way and Andromeda
Abstract
Most existing idealized simulations of galaxy mergers neglect Hubble expansion, as the local Hubble flow is curved by overdensities and the expansion effect is weak at these scales. However, even in the gravity-dominated Local Group (LG), the expansion itself still persists, and it has been shown important in some LG-related studies. Here we take the future merger of the Milky Way (MW) and Andromeda (M31) as a case study to investigate the differences between simulations with and without Hubble expansion. We perform a suite of idealized N-body simulations of the MW--M31 system under pure Newtonian, ΛCDM, and w0waCDM scenarios, testing different masses and proper motions of galaxies informed by current observational constraints. At low M31 tangential velocities, especially in cases where the two galaxies merge within a Hubble time, the expansion effect is negligible; while at high velocities (>66.5 km/s), the ΛCDM simulations introduce a noticeable delay in the merger time compared to the Newtonian ones and may even alter the final fate of the system, but the caveat here is that the isolated-pair idealization and the assumed expansion history break down. The delay is also present but less pronounced in the w0waCDM runs. When M33 and the Large Magellanic Cloud are included, the differences in merger times are reduced. In summary, Hubble expansion can play a noticeable role for galaxy mergers with high tangential velocities, but its actual influence on the MW--M31 merger depends on the precise masses and proper motions to be determined in the future.
Related
- Can cosmic voids ease the Hubble tension? Local expansion in w0waCDM
- A unified cosmographic reconstruction of the Hubble expansion: Bayesian constraints on dark energy and modified-gravity diagnostics
- Observational evidence of the Gaia-Enceladus-Sausage merger's influence on the opposite side of the Galactic Center
- Characterizing robotic positioners under the influence of changing gravity vectors for future spectroscopic surveys
- The solitary star cluster of the Andromeda XXV dwarf spheroidal
BibTeX
@misc{zhou2026influence,
title = {{The Influence of Hubble Expansion on the Future Merger of the Milky Way and Andromeda}},
author = {Yufan Zhou and Tianchi Zhang and Shihong Liao and Zhiyuan Li and Lan Wang and David Benisty},
howpublished = {arXiv},
year = {2026},
month = oct,
doi = {10.48550/arxiv.2610.38490},
eprint = {2610.38490},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2610.38490},
note = {CC BY 4.0},
}
RIS
TY - UNPB
TI - The Influence of Hubble Expansion on the Future Merger of the Milky Way and Andromeda
AU - Yufan Zhou
AU - Tianchi Zhang
AU - Shihong Liao
AU - Zhiyuan Li
AU - Lan Wang
AU - David Benisty
PY - 2026
DA - 2026/10/28
PB - arXiv
DO - 10.48550/arxiv.2610.38490
UR - https://arxiv.org/abs/2610.38490
AB - Most existing idealized simulations of galaxy mergers neglect Hubble expansion, as the local Hubble flow is curved by overdensities and the expansion effect is weak at these scales. However, even in the gravity-dominated Local Group (LG), the expansion itself still persists, and it has been shown important in some LG-related studies. Here we take the future merger of the Milky Way (MW) and Andromeda (M31) as a case study to investigate the differences between simulations with and without Hubble expansion. We perform a suite of idealized N-body simulations of the MW--M31 system under pure Newtonian, ΛCDM, and w_0w_aCDM scenarios, testing different masses and proper motions of galaxies informed by current observational constraints. At low M31 tangential velocities, especially in cases where the two galaxies merge within a Hubble time, the expansion effect is negligible; while at high velocities (>66.5 km/s), the ΛCDM simulations introduce a noticeable delay in the merger time compared to the Newtonian ones and may even alter the final fate of the system, but the caveat here is that the isolated-pair idealization and the assumed expansion history break down. The delay is also present but less pronounced in the w_0w_aCDM runs. When M33 and the Large Magellanic Cloud are included, the differences in merger times are reduced. In summary, Hubble expansion can play a noticeable role for galaxy mergers with high tangential velocities, but its actual influence on the MW--M31 merger depends on the precise masses and proper motions to be determined in the future.
ER -