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Expansion speed of the Universe

  1. Oct 28, 2009 #1
    if the farther a galaxy is from us the faster it is receding from us, and the closer a galaxy is to us the slower it is receding from us, and since the farther a galaxy is from us the further back in time we are seeing it, does that mean the universe is expanding slower now in the present and was expanding faster in the past?

    Also is there a way of observing 2 other galaxies and being able to know or infer the rate at which the universe is expanding in relation to those two galaxies instead of just the rate at which the universe is expanding in relation to our galaxy and other galaxies?
  2. jcsd
  3. Oct 28, 2009 #2
    Hi there,

    Fact is that we do not live in a complete linear Universe. There are some galaxies close to the Milky Way, that do not move away from us. Just take our sister galaxy for example. Adnromeda will collide with the Milky Way in a little while. Therefore, it must be getting closer to us.

  4. Oct 28, 2009 #3
    Yea, thats how galaxies collied, and how we have the local group and all that good stuff. I didn't think I implied in my post that we live in a linear universe
  5. Oct 28, 2009 #4
    I think you plot redshift (a measure of expansion between emission and observation) compared to distance (note: over cosmological distances, not just the local group), and find that, for example, a signal emitted 1bn years ago has more than half the redshift than a signal emitted exactly 2bn years ago, implying that the known amount of expansion occurs disproportionately in recent times.

    Redshift is easy to find from standard candles and using luminosity distances, but I'm not sure how the distance itself is found (there are 4 parameters, redshift, distance, luminosity and observed intensity, and I can only see how 2 are observed)
    Last edited: Oct 28, 2009
  6. Oct 28, 2009 #5
    Distance is determined by finding a standard candle, such as a super nova, in another galaxy then compareing it's known luminosity to it's apparent luminosity since light behaves by the inverse square law I believe, an object twice as far from us will appear 4 times fainter.
  7. Oct 28, 2009 #6
    That would be ok, but for the 1a supernovas which were used as evidence for accelerated expansion, the formula is modified to include the redshift for distances over which cosmic expansion is no longer negligible (eg. the light will be redshifted gravitationally, and as the universe expands, the density of photons falls, giving you a superficially low intensity).

    The distance is (1+z) times closer than you would think in a universe that has expanded by redshift z between observation and emission.
  8. Oct 28, 2009 #7
    you can't infer that from your statement.

    There is nothing special about our earth frame fo referenece....we are just another speck in the universe. It's believed the universe is homogeneous and isotropic so whether you make measurements and observations here or from a galaxy a billion light years away it is expected results will be the same.
  9. Oct 29, 2009 #8


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    Well, no what you've described is just simple expansion. To have an idea of how that expansion has changed over time, you have to look very carefully at how the relationship between speed and distance changes as you look further back into the past.

    Yes. It's just a simple coordinate transformation.
  10. Oct 29, 2009 #9
    Can someone explain in clear terms what the evidence is for an expanding universe? I think I may be close in my above post but don't think I understand where all the evidence comes from. I've tried reading the 1998 paper but it's more experimental than theoretical
  11. Oct 29, 2009 #10


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