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Accelerating or Decelerating Expansion?

  1. Jan 22, 2016 #1
    The farther out we look, the faster the galaxies are moving away from us. Closer galaxies are moving away from us, but not as quickly. We reason that when they're as far away as the more distant ones, they'll be going that fast, too. So we infer that the universe's expansion is accelerating. Sounds perfectly reasonable. But, the farther we look out into space, the farther back we look in time, and so we could just as easily say, the farther in the past we look, the faster the galaxies were moving away from us, and when we look into the more recent past, the galaxies were moving away from us more slowly. Isn't that enough to infer that the universe's expansion is decelerating? Faster billions of years ago, slower millions of years ago. Sounds like deceleration. I'm not really casting doubt on modern physics. I'm just asking how we know it's the one and not the other when distance in space is also distance in time? In fact, how do we know that the Big Crunch isn't already happening, but we don't know it because we're getting all this old light, light that left those galaxies before they stopped moving away from us and starting plunging toward us? Again, I don't actually doubt that modern science can answer this question. I'm earnestly looking for those answers :)
     
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  3. Jan 22, 2016 #2

    Orodruin

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    Cosmological redshift is a measure of the relative size of the universe between the time of emission and the time of measurement. It is not dependent on the expansion rate in the same way as Doppler shift depends on relative velocity.
     
  4. Jan 22, 2016 #3
    So, it's a measure of relative size and it's independent of relative velocity. So it's not telling us that more distant objects are moving away from us faster than closer ones are, because that would be telling us about their relative velocity. Instead, it's telling us that the relative size of the space separating those distant objects from us is exponentially larger than the distance separating the closer objects from us? Do I have that distinction right?
     
  5. Jan 22, 2016 #4

    Orodruin

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    In a global setting, the term "relative velocity" is not even well defined (due to space-time being curved). That being said and oversimplifying the issue a bit, you will have both effects. You have the cosmological redshift due to the universe growing (note that this growth is not really due to things having a relative velocity) and the Doppler shift due to things moving with respect to the cosmological preferred frame.
    Under the assumption that the universe is homogeneous, it was always smaller at earlier times. Therefore it has grown more since the "older" light was emitted and it is therefore more redshifted.
     
  6. Jan 22, 2016 #5
    I think I get it. Doppler effects are still incurred given that the galaxy is still moving around relative to, say, its local group, but the cosmological redshift is something caused by the expansion of space.

    So the redshift is a record of the growth undergone by space since the time of emission to the time of measurement, such that, even if the expansion were to have, hypothetically, stopped and reverted to contraction in the time since the emission of the light and its later measurement, then even that would've left its mark on the light as a detectable (cosmological) blueshift, even though the universe was expanding at the time of emission. Am I getting closer?
     
  7. Jan 22, 2016 #6

    Orodruin

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    Yes, wheter the cosmological shift is a blueshift or a redshift only depends on the relative size of the universe. If the universe would have been larger at the time of emission, it would result in a blueshift. The effect does not depend on how the expansion occurred in between those times, it might have expanded to contract again or just contracted. You need to look at the redshift of objects at varying distances to infer the expansion history. (Note! Of course, our universe has not been contracting, so we do not observe any cosmological blueshift.)
     
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