Andromeda-Milky Way Collision May Not Happen After All

Artists depiction of what the sky above earth would look like in 3.75 billion years. (NASA, 2012).

    For as long as I have had an interest in astronomy, I have heard of the imminent collision of our Milky Way galaxy colliding with the Andromeda galaxy. The established prediction for the last two decades has been that in ~5 billion years, the Milky Way and Andromeda galaxies will collide to form a massive elliptical galaxy called Milkdromeda (or Milkomeda). We suspected this due to the large blue shift seen when observing Andromeda, meaning that the galaxy is moving quickly in our direction, while most other galaxies appear to be redshifted and moving away. From there several observations over many years were done to see if there was any sideways motion in the galaxy, and the results pointed towards an imminent collision in the next 4-5 billion years (NASA, 2012).


    However, in the years since these conclusions were drawn, our understanding of our Local Group (the galaxies and dwarf galaxies near the Milky Way) has greatly progressed. We now know today that there are 50+ galaxies in the local group. The three main ones in order of size are Andromeda, the Milky Way, and Triangulum, and then there are dozens of dwarf satellite galaxies, which combined have a profound gravitational impact (EarthSky, 2023). This along with the advancement of computer modeling and new data means that a new look can be taken at Andromeda and the Milky Way's trajectories.

The Local Group, with the Milky way depicted at the center (EarthSky, 2023).

    In the last few months, a new study has come out that refutes our previous conclusions. When taking into account the gravitational effect some of the largest other galaxies in the Local Group, the projected paths of the Milky Way and Andromeda over time become less clear, but simulations show that it is very possible that the the galaxies will miss each other entirely and orbit each other, or otherwise collide in ~10 billion years, much later than originally projected. This combined with the many smaller dwarf galaxies that were not included in the simulation mean that there is even more uncertainty in the paths that the two galaxies will take. (Sawala et al., 2024)


Sources:

Sawala, T., Delhomelle, J., Deason, A. J., Frenk, C. S., Johansson, P. H., Keitaanranta, A., Rawlings, A., & Wright, R. (2024). Apocalypse When? No Certainty of a Milky Way - Andromeda Collision. https://doi.org/10.48550/arXiv.2408.00064


NASA. (2012, May 31). NASA’s Hubble shows milky way is destined for head-on collision - NASA science. NASA. https://science.nasa.gov/missions/hubble/nasas-hubble-shows-milky-way-is-destined-for-head-on-collision/


The Local Group is our galactic neighborhood. EarthSky. (2023, October 29). https://earthsky.org/clusters-nebulae-galaxies/what-is-the-local-group/