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Is the universe accelerating it's expansion?

  1. Apr 26, 2010 #1
    Using red shift, we determine that the farther out that we look that the faster the galaxies are traveling, and so prove that our universe is expanding and accelerating. But really aren't we saying that a long time ago they were moving very fast? After all the light arriving now originated not long after the big bang. Suppose I was driving a car extremely fast and it took a long time for the light to get to you, and when you last saw me i was going very fast but since then ran out of gas and have come to a stop. Is it appropriate to assume that I am still going as fast as your last observation a long time in my past? If we assume that all the laws of the universe are being applied universally, then if I look to galaxies nearby and I see them moving at a slower speed than those at huge distances, can't I infer that all the galaxies in the universe are doing the same thing, moving slower than they once were going, even though the light to prove it hasn't gotten to me yet? Or was I just the slowest galaxy in the universe (lucky me lol), and got left behind.
    Last edited: Apr 26, 2010
  2. jcsd
  3. Apr 26, 2010 #2
    Nope. You cannot conclude that the universe is accelerating using this information only. In fact you would also observe more distant galaxies receding faster even if the expansion were slowing down.
    This is simply a consequence of the fact that the universe is expanding homogeneously: the distance between any galaxy and any other galaxy grows with time (if we neglect peculiar motions), so if you have three galaxies in line, the distance between the first and the second will grow, but also the distance between the second and the third will grow. So the distance between the first and the third will grow even faster: the more distant a galaxy is, the faster is its recessional speed.
    Last edited: Apr 26, 2010
  4. Apr 26, 2010 #3
    The thing is that the more distant galaxies are receding faster than you would expect from a constant expansion. In drawing this conclusion the fact that the light you see now was emitted long ago is taken into account.

    Very basically you can ask "what would the recessional velocity as a function of distance be if the expansion rate of the universe was in the past the same it is today?" and then compare this model with observations.
    This was done in the late 90's by observing distant supernovae explosions...
    Last edited: Apr 26, 2010
  5. Apr 28, 2010 #4
    My question is really basically this:

    A long time ago these other galaxies were travelling faster than our galaxy is today, but today are those galaxies still going as fast? 13 billion years ago were we also travelling at the same speed as those distant galaxies we see today? And have we since slowed? Have those galaxies slowed since then? Can we infer that what we observe locally is actually the current condition throughout the whole unieverse at this moment in time?
  6. Apr 28, 2010 #5
    I'm sure that one of the experts will come and reply but this is my understanding of it:
    1: The Universe is expanding at an accelerating rate.
    2: Our local cluster of galaxies is close enough that the collective gravity overcomes the acceleration of space.
    3: We are not the center of the solar system, galaxy, or universe so you can assume that what we're observing is what everyone is observing.

    In other words, all galaxy clusters close enough together are rushing together due to gravity. These galaxies will all eventually merge into super galaxies. These super galaxies will merge if they are close enough together. Once all sufficiently large clusters have merged, no more galaxy merger can occur and all you will see are other galaxies red shifting away until it looks like the only thing in space is our super galaxy - every location outside of the galaxy you look, you'll see nothing. Eventually the acceleration will increase to the point it starts to pull the galaxies apart then the systems and eventually the atoms. This is called "the big rip".

    I don't think enough is known about spacetime to say with certainty what would happen when it becomes infinitely large with no density. One would hope another big bang but that's well beyond anything I've read about.

    Now that I think about it though, if you follow through the "we're not special" logic, one would have to make the assumption that we are not the first nor the last big bang. It just "feels" wrong to believe that this is it. History has shown that anytime we assume we're special, it's proven wrong. It's just hard to believe that we're the first or the last big bang. Of course that's personal speculation based on my understanding of history and science. To prove this will take research well beyond anything we can do today.
    Last edited: Apr 28, 2010
  7. Apr 29, 2010 #6
    Perhaps each region of space is irregular and a budding off is occurring and is seen as receding? Everything is relative to the observer/s.
  8. Apr 30, 2010 #7
    First of all, velocities are always defined relative to an observer. Our velocity is high vith respect to a distant galaxy, but is low relative to galaxies in our proximity. This is true now as it was in the past.
    What we know for certain is that the recessional velocity is proportional to the distance (Hubble's law).
    We also know that the Universe looks homogeneous (and we know it by counting the number density of galaxies in each spatial bin).
    This points out that the Universe is expanding in a self-similar way: all the distances are increased by the same scale factor, independent of location (the shape of the universe is preserved).

    If this is correct then, on average, what we see today is seen at the same time by another observer located anywhere in the universe.
    And we have undergone the same expansion we see for Galaxies billions of light years away.

    Since the rate of expansion was much higher in the past, a given galaxy had a much higher velocity with respect to us in the past than what we observe today for the same object.

    I don't know if this answers in part to your question...
  9. Apr 30, 2010 #8
    But then you would also see blueshifted objects, or not?
  10. Apr 30, 2010 #9
    A decrease in wavelength is called blue shift and is seen when a light-emitting object moves toward the observer.
    redshifts and blue shifts can be characterized by the relative difference between the observed and emitted wavelengths/frequency of an object. It is customary to refer to this change using a dimensionless quantity called z. If λ represents wavelength and f represents frequency (note, λf = c where c is the speed of light).
  11. Apr 30, 2010 #10


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    Don't get confused between cosmological redshift and the redshift due to relative motion.

    The redshift that we are seeing from these distant stars is due to the actual expansion of space which is stretching out the wavelengths of these photons. It's not the same as the redshift we see from say a car moving away from you since that's due to actual motion. What we are measuring with these cosmological redshifts is an expansion in space, not a recessional velocity in the local sense.

    One can see this clearly in that cosmological redshifts can be greater than 1, while relative motion redshifts can never be greater than 1 (or even equal 1) because nothing can go as fast as the speed of light.
  12. Apr 30, 2010 #11
    "One would hope another big bang but that's well beyond anything I've read about."

    Why should/would we care if there would be another Big Bang? You could participate perhaps?
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