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When Will the Milky-Way Collide with Andromeda?

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Avi Loeb Medium

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Predicted evolution of the local group of galaxies over the next 10 billion years. Panels (a) and (b) show 100 randomly selected tracks from 10,000 realizations for the future trajectories of the Milky-Way (MW) and Andromeda (M31) galaxies, and panels © and (d) show the same, including the minor galaxies M33 and the Large Magellanic cloud (LMC). In both cases, the left panel is the face-on projection and the right panel is the edge-on projection with respect to the orbital plane of the Milky-Way and Andromeda centers. The green and red lines trace the orbits of the Milky-Way and Andromeda, while the pink and silver lines trace the Large Magellanic Cloud and M33, respectively. Line brightness indicates the relative probability density at any position. (Image credit: H. Wu et al. 2026)

The nearest neighbor of our own Milky-Way galaxy is the Andromeda galaxy, located at a distance of 2.5 million light years. The dark matter halos of the two galaxies, containing about a trillion solar masses each, are currently touching each other as the two galaxy cores are headed towards each other on a collision course.

Alien astronomers in Andromeda might use their own telescopes to monitor the encounter. A communication signal from them will require 2.5 million years to reach us, comparable to the duration of the entire human history on planet Earth.

A merger of these similar galaxies will scramble their stellar disks into a giant elliptical galaxy. I dubbed this merger product “Milkomeda” in a paper (accessible here) that I wrote with my former postdoc T.J. Cox in 2007. Stars within the two galaxies, including the Sun, will not physically collide with each other because of the vast space that separates them. Our computer simulation indicated that during the merger process, the Sun might be carried along with a group of other stars trailing the Milky-Way core. During this period, a terrestrial astronomer would be able to see the Milky-Way and Andromeda from a distance as their stars fill the night sky in evolving patterns. Within 5 billion years, while the Sun is still alive — as its remaining life span is 7.6 billion years, Milkomeda will acquire twice the stellar and dark-matter reservoirs of the Milky Way.

Given that the rest of the Universe is accelerating, within a hundred billion years from now Milkomeda will be surrounded by vacuum in a dark and lonely cosmic environment. The merger between the Milky-Way and Andromeda will constitute the last major event in our cosmic history book, followed by a dull and dark future after that. We might still be able to infer that the universe expands by observing hypervelocity stars ejected by Milkomeda, as I pointed out in a paper published here. This is the only publication in my resume that has a chance of being cited in a trillion years.

Currently, the Milky-Way and Andromeda (also called M31) are falling toward each other at a speed of 110 kilometers per second. However, a paper published here in 2024 argued that the merger is not guaranteed, given the gravitational influence of other galaxies in between them — such as M33 and the Large Magellanic Cloud (LMC). Whereas including M33 increases the merger probability, the orbit of the Large Magellanic Cloud runs perpendicular to the Milky Way — Andromeda orbit and makes their merger less likely. The authors argued that uncertainties in the present positions, motions, and masses of all galaxies allow for a 50% chance that that there will be no merger between the Milky-Way and Andromeda during the next 10 billion years.

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Time evolution of the separation between the Milky-Way and Andromeda centers for 100 representative tracks and a merger fraction of 90% within 10 billion years, computed from 10,000 trajectory samples. White symbols along the bottom mark the corresponding merger times, and the white curve shows the most probable evolution. (Image credit: H. Wu et al. 2026)

Today, a new numerical study (accessible here) used the latest and most precise Gaia-based proper motions for stars in Andromeda to show that the merger probability rises to 90%, with a median merger time of 6.5 (+1.3/−1.5) Gyr, broadly restoring the classical view expressed in the paper that I co-authored with T.J. Cox. Future proper-motion measurements with uncertainties below 2 micro-arcseconds per year will be required to firm-up this conclusion in anticipation of the merger.

Of course, the alternative route for figuring out the expected fate of the Milky-Way is to seek electromagnetic signals from alien astronomers in Andromeda. If they developed telescopes with uncertainties below 2 micro-arcseconds per year a few million years ago, they may have already beamed their answer towards us — announcing what to expect in our mutual party. When they arrive to the vicinity of the Sun, hopefully in 5 billion years, we can all meet and celebrate the event together. Salute! 🥂

ABOUT THE AUTHOR

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(Image Credit: Chris Michel, National Academy of Sciences, 2023)

Avi Loeb is the head of the Galileo Project, founding director of Harvard University’s — Black Hole Initiative, director of the Institute for Theory and Computation at the Harvard-Smithsonian Center for Astrophysics, and the former chair of the astronomy department at Harvard University (2011–2020). He is a former member of the President’s Council of Advisors on Science and Technology and a former chair of the Board on Physics and Astronomy of the National Academies. He is the bestselling author of “Extraterrestrial: The First Sign of Intelligent Life Beyond Earth” and a co-author of the textbook “Life in the Cosmos”, both published in 2021. The paperback edition of his new book, titled “Interstellar”, was published in August 2024.

Professional website:

https://lweb.cfa.harvard.edu/~loeb/

Social media:

https://avi-loeb.medium.com/
https://www.youtube.com/@ProfessorAviLoeb

https://open.spotify.com/show/1zhndXkvSY2b8FdjspFpCd
https://x.com/ProfAviLoeb

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