Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

B Trying to understand the Andromeda-Milky Way collision

  1. Mar 4, 2016 #1
    Since space is rapidly expanding between galaxies, why are the Milky Way and Andromeda galaxies on a collision course with each other? Do galaxies need to be a certain distance apart for the force of their gravity to be weak enough so that it cannot overcome the expansion of space? Or is it because the Milky Way and Andromeda galaxies are simply moving toward each other as part of their orbits around the Great Attractor?
  2. jcsd
  3. Mar 4, 2016 #2


    User Avatar
    Gold Member

    In general things tend to get further apart due to space expansion, as long as they do not have proper motion, gravitation ally bound to another body.
  4. Mar 4, 2016 #3
    The two galaxies are approaching each other because their combined speed is faster than the rate the universe is expanding. Presently, the best estimate puts the rate of expansion of the universe at 67.80 ± 0.77 (km/s)/Mpc. Since we are measuring the approach of the Andromeda galaxy at ≈110 km/s, that must mean the Milky Way and Andromeda galaxies are actually approaching each other at a combined rate of ≈162 km/s in order to overcome the expansion of the universe which would be ≈52.18 ± 0.6 km/s at the distance of the Andromeda galaxy (0.78 Mpc).
  5. Mar 4, 2016 #4
    OK thanks for that explanation. When you talk about speeds that galaxies are traveling, what is meant by this? Are galaxies moving just because they're orbiting some central point of a galaxy cluster? Are all galaxies moving in a specific direction irrespective of the expansion of space?
  6. Mar 4, 2016 #5
    Think in terms of competing forces - clusters of galaxies, for example are gravitationally bound. The degree to which this force is larger than expansion at any given time calculates how long they remain bound. AFAIK it is not meaningful to consider a "central point" other than the combination of individual vectors.
  7. Mar 8, 2016 #6


    User Avatar
    Science Advisor
    Gold Member

    Expansion is a nonfactor for objects as near as Andromeda. It is currently thought to be about 68 kilometers per second per megaparsec - re: http://phys.org/news/2015-02-fast-universe.html - or 1 km per 2.2E+018 km per second. that translates to around 9 trillionths of a kilometer [9E-012] per second at the distance of Andromeda - disregarding any other considerations. Andromeda is currently approaching the MW at about 300 kilometers per second.
  8. Mar 8, 2016 #7
    Galaxies within a cluster are gravitationally bound and can be moving relative to each other regardless of overall expansion of space.
    A cluster doesn't have a central point around which everything rotates, the movement of individual galaxies is the sum of the momentum from the gas clouds which produced them and subsequent gravitational interactions.
    Afaik it's chaotic, no cluster is particularly similar to another one, other than in the fact of being a cluster.
    Last edited: Mar 8, 2016
  9. Mar 8, 2016 #8
    Assuming the Earth and humanity still exist in 1 trillion years, what would our observable universe look like? Would the other galaxies in our cluster still be around?
  10. Mar 8, 2016 #9
    Assuming the Earth is not consumed when the sun reaches its Red Giant phase ≈5 billion years from now, the Earth would still be rendered into a charcoal briquette. By the time the sun reaches its degenerate white dwarf phase it will lose ≈70% of its mass, which will effect Earth's orbit (as well as the orbits of all the objects in our solar system). Regardless of whether Earth remains in orbit around the white dwarf, or is thrown out of the solar system completely, it will become a frozen ice ball. Either way, humans will have to leave Earth within the next 500 million years, assuming we make it that long. Odds are that humans will become extinct long before that.

    There will still be a few galaxies left in our local cluster a trillion years from now, but that number will be much smaller than the current ≈36. The Andromeda/Milky-Way combined galaxy will be the largest in our local cluster, and most like already include several more galaxies by that time. Where the Earth will be is anyone's guess, but there will not be anything living on it.
  11. Mar 8, 2016 #10


    User Avatar

    Staff: Mentor

    If we extrapolate current cosmological trends, we will not be able to see any other galaxies, as they will have receded beyond our cosmic light horizon and the local group will possibly have merged into one galaxy.

    See here: https://en.wikipedia.org/wiki/Timeline_of_the_far_future
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted