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Hubble's law

  1. Apr 4, 2010 #1
    I'm confused a little bit by this and have a few questions. I wasn't sure where to put this because it's kind of a relativity question (time dilation) and a cosmology question combined.

    From what I understand, all galaxies are moving away from us. But isn't the Andromeda galaxy eventually going to collide with the milky way?

    Also from what I understand, some galaxies are moving away from us faster than the speed of light. Is that just from our point of view? Because from their point of view, we would be moving away from them at the same speed, right? But we're not really moving faster than the speed of light, or are we actually moving that fast? But if we're really moving that fast, would that cause time dilation to have an affect on us?

  2. jcsd
  3. Apr 4, 2010 #2


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    On average galaxies are all moving apart (space is expanding) but locally nearby galaxies are also affected by gravity and interact with each other.

    Correct - there's nothing special about us.

    It's possible for space to expand so that distant points move apart faster than light.
    What is forbidden is anything able to transfer information faster than light.
    So you can't set off from a point and move faster than light to another point (you could carry a message) but it's allowed for two points to move apart faster than light (and so forever be out of contact with each other)
  4. Apr 4, 2010 #3
    All of which raises the ugly scenario of a future in which virtually all stellar bodies we now percieve, will pass beyond the cosmological horizon. Relativity really knows how to put a person in their place, but damn it can be depressing as all hell.
  5. Apr 4, 2010 #4
    Thanks for the response.

    So all galaxies are moving with space time faster than c, not moving through space-time faster than c? If they were moving through space-time faster than c, time dilation would cause them to go back in time, correct? But if they're moving with space-time, that means time dilation isn't having that affect? Time dilation only occurs when you move through space-time?
    Does that mean in the distant future, astronomers won't be able to see any galaxies outside the Milky Way-Andromeda galaxy?

    When will that start happening? Has it started? Are distant galaxies starting to disappear already?
  6. Apr 4, 2010 #5
    Well, it would be a future without astronomers, but sure, that's exactly what I'm saying, although it would be our local cluster, not just the Milky Way.

    When will it happen? Well, there is still a question as to IF it will happen. There is still no way of knowing if universal expansion will continue indefinitely. Keeping in mind that barring some kind of gravitic equilibrium, the alternative is the famous "Big Crunch"... which I find only moderately more appealing than dispersing into an arbitrary distance.

    To some degree it has always been happening however: http://en.wikipedia.org/wiki/Horizon_problem

    Granted, this would be a severe case, but you get the idea. This hinges on infinite inflation or expansion, which is uncertain, but seems to be the case since 380K+ years after the BB.
  7. Apr 4, 2010 #6


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    Remember the light left those distant objects 14Bn years ago and has just reached us, the fact that they are now outside our observable horizon doesn't really matter because they don't exist anymore anyway!
  8. Apr 4, 2010 #7
    Ray of sunshine you are :wink: . That said, while those objects no longer exist, presumably progressive generations of stars have formed, all outside of our observable horizon.
  9. Apr 4, 2010 #8


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    Yes, but presumably they are similar to the stars we can see locally so we aren't missing much
  10. Apr 5, 2010 #9
    True, and in the spirit of science that means we really don't need to survey them, but in the spirit of human beings who imagine a future long after we're gone, it's vaguely painful.

    What can I say... it's a little frusterating living on a cosmic tape-delay... it's moreso when you know that there have essentially been edits! Is that rational? No... Then again, it's not rational to care about the stars I see with the naked eye, but I still enjoy doing it. I want to see more... don't you? I accept that I won't, and can't, but some things make that fact grate more than others, and long-dead stars and galaxies in our distant past are one of them.
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