Q: If the universe is expanding and all the galaxies are moving away from one another, how is it possible for galaxies to collide?

Physicist: Because the universe isn’t expanding fast enough.  On average all the galaxies are moving apart, but often a given pair will be moving together.

Hubble observed that the farther things are away, the faster they’re receding.  Specifically, in the universe today, v = H_0 d where v is the relative velocity of two objects, d is the distance between them, and H_0 = 22 \pm 2 \, mm/s/Lyr (millimeters per second per light year).  Now this is an averaging thing, since galaxies are free to move however they like.

So for example, this equation says that the Andromeda galaxy which is 2.5 MLyr (million light years) away should be moving away at around 55km/s.  Instead it’s flying at the Milky Way at about 120 km/s.

As a side note: when Andromeda gets here (or we get there, or whatever) the collision of our gas clouds should set off a huge spike in star formations resulting in a liberal peppering of supernovas (bad for everyone).  But we’ve still got another 2.5 billion years, so don’t pack your bags just yet.

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6 Responses to Q: If the universe is expanding and all the galaxies are moving away from one another, how is it possible for galaxies to collide?

  1. Scott says:

    It’s not bad for everyone. Astronomers are always complaining about how few supernovas they get to see. This should shut them right up.

  2. The Physicist Physicist says:

    Fair enough. I should have said “Bad news for everybody except fire-proof, lead-lined astronomers”.

  3. David says:

    Does time have an affect on distance? If two objects are very far from each other (say, 1 billion light years) and are stationary relative to eachother (no velocity is bringing them closer or father apart), how far apart will they be from each other in a very long time (say 1 billion years)?

    What is the farthest distance an object can be such that light could never reach me (due to the expansion of space causing space to expand faster than the light can cover the additional distance, if that makes any sense)?

  4. The Physicist The Physicist says:

    Based on the present rate of expansion of the universe, the most distant object that will ever been seen is about 14 billion light years away (right now).

  5. Joshua says:

    I really find these articles just… Facinating.

  6. Dan-O says:

    First of all, thank you so much for this website. While I’m still having an immensely difficult time wrapping my head around many of the concepts (especially the origin of the big bang and cosmic inflation), it’s great fun visualizing all the stuff surrounding us.

    And now, a relevant question: What is the state of dark matter between us and Andromeda? If it is the prime mover in the universe’s expansion, does that mean it’s lessening between us, or increasing behind us? Does it have to be one or the other?

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