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Space time expansion

  1. Jul 20, 2012 #1
    If space is expanding then how did the planets and stars come together to form galaxies?
    Why are the galaxies themselves not expanding and the solar system not expanding?
     
  2. jcsd
  3. Jul 21, 2012 #2

    Chalnoth

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    The expansion is an overall, average effect. However, different parts of the universe early-on were slightly more dense than other parts. The parts that were dense enough had enough self-gravity to stop expanding and collapse inward, even while the overall expansion continued.
     
  4. Jul 21, 2012 #3
    And the red shift seen in the distant galaxies...is this due to the expansion or to the doppler effect?
     
  5. Jul 21, 2012 #4

    Chalnoth

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    Both, sort of.

    There's a Doppler effect from the individual motion of each galaxy relative to the expansion. Some galaxies move as fast as 1000km/s with respect to the overall expansion. This amounts to a redshift/blueshift of roughly z=0.0003, depending upon the galaxy.

    But for most galaxies, this local motion is minuscule compared to the expansion (we've measured galaxies with redshifts greater than z=6), and for many of these far-away galaxies, it just doesn't make sense to interpret their redshifts as due to the Doppler effect.
     
  6. Jul 21, 2012 #5
    so how does expansion create a red shift? what is the mechanism...
     
  7. Jul 21, 2012 #6

    phinds

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    With the Doppler effect, what matters is the speed of the receding object at the time light is emitted. This alone determines the red shift. With metric expansion, however, the distance between the photon and us keeps getting bigger as the photon travels towards us, and this is what causes the red shift (which as Chalnoth pointed out, is for distant galaxies almost entirely due to this effect, NOT to the Doppler effect).

    If one were to say that the entire red shift of distance galaxies was due to the Doppler effect, that would be equivalent to saying that the universe is not expanding.
     
  8. Jul 21, 2012 #7
    Galaxies aren't expanding because they aren't homogeneous and they're too dense. Only homogeneous distributions of matter will allow space to expand.
     
  9. Jul 21, 2012 #8

    mfb

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    Draw a curved line on a sheet of rubber, extend it, and you will see an increased wavelength. As the speed of light is constant, an increased wavelength corresponds to a reduced frequency.
     
  10. Jul 21, 2012 #9
    can you explain this further. I know that the doppler effect can show which direction things are orbiting when viewed from earth.

    Why can't the doppler effect explain the red shift seen for distant galaxies? why would this mean that the universe is not expanding?
     
  11. Jul 21, 2012 #10

    phinds

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    Reread post #4

    EDIT: for that matter, reread post #6. I really don't know how else to say it.
     
    Last edited: Jul 21, 2012
  12. Jul 21, 2012 #11
    So let me see if I get this....
    If the doppler effect was used to interpret the red shifts then it wouldn't make sense because the red shifts calculated would be too large to be explained by the velocity of the galaxies???

    So instead of the red shift being due to the velocity of the galaxy it is due to the expansion of space itself and of the wavelength of the photon?

    am i starting to get it right????
     
  13. Jul 21, 2012 #12

    Chalnoth

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    Rather the inferred velocities don't make much sense.
     
  14. Jul 21, 2012 #13
    right, so the velocities needed to explain the red shifts would be too great.....were these assumed velocities approaching light speed?
     
  15. Jul 21, 2012 #14

    Chalnoth

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    It's the other way around. Too small. These objects measured to be too far away to be explained by their inferred velocities by assuming the redshift is due to the Doppler shift. If we instead infer their velocities based upon their distances, the further away galaxies are and always have been receding at faster than the speed of light.
     
  16. Jul 21, 2012 #15

    phinds

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    Yes you are.

    As Chalnoth said, the problem would be with the velocities. When the ACTUAL event occurred, the items emitting the light were moving at a slow pace relative to c, and then the photons were effectively slowed by the expansion, BUT if you use DOPPLER red shift, then the objects themselves would have had to be moving at 3c which is impossible.
     
  17. Jul 21, 2012 #16
    hmmm....
    so where does the energy go when the photon wavelength expands?
    and is this red shift the only evidence that we have of an expanding universe?
     
  18. Jul 21, 2012 #17
    No. We have a ridiculous amount of evidence for an expanding universe. See 'Observational Evidence' here to see a potion:

    http://en.wikipedia.org/wiki/Big_Bang#Observational_evidence

    Also, see our FAQ:

    https://www.physicsforums.com/showthread.php?t=506993

    The redshift is usually the first mentioned in cosmology because it was the first evidence of an expanding universe.

    In terms if the photons, the conservation of energy isn't exact in cosmology. We also have a FAQ on this:

    https://www.physicsforums.com/showthread.php?t=506985

    Finally, we have a FAQ for your questions about redshift and why it is cosmological. See here:

    https://www.physicsforums.com/showthread.php?t=506994 [Broken]
     
    Last edited by a moderator: May 6, 2017
  19. Jul 22, 2012 #18
    A point not mentioned so far is that in the distant past, the universe was 'matter dominated'....that is, when stuff was close enough together for gravity to be relatively strong between matter. So it was easy for cosmic 'dust' to coalesce into planets, stars,etc
    As space expands, that attraction weakens as distances between, say galaxies, increases....
    As space expands galaxies and planet and star formation will slow down, then eventually stop forming.....it will be cold,dark, 'empty'....

    a brief explanation is here:
    http://en.wikipedia.org/wiki/Matter-dominated_era

    You can also see a little more via 'radiation dominated era' in Wiki.
     
    Last edited: Jul 22, 2012
  20. Jul 22, 2012 #19

    Chalnoth

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    That's not what that means. The era of matter domination was the era where matter (normal matter + dark matter) made up most of the energy density in the universe.
     
  21. Jul 22, 2012 #20
    ok let me read some of this stuff and get back....thanks thus far...
     
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