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Significance of FRW metric predicting cosmlogical red-shift?

  1. Apr 23, 2015 #1
    So from a killing tensor the FRW metric is known to possess, for a massless particle we find the well known result that as the universe expands the frequency of the photons decreases .

    But , what does this do for gr ?
    Was this known to happen before gr ?

    Thanks a lot.

    (I know it is used to show the universe is expanding etc but is it any sort of test for gr ? )
  2. jcsd
  3. Apr 23, 2015 #2


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    The discovery of expansion and cosmological redshift occurred in the first decade of GR's existence. Einstein could have gone down as predicting expansion and cosmological redshift before there was any observational evidence for them except that he thought the universe 'must be' largely static, so contrived (unstable) cosmology models that had this feature (having, in private work found that natural solutions were expanding).

    In any case, it is great confirmation of GR, because it is impossible to construct stable cosmological solutions that are not dynamic, with an expanding phase, in GR. Thus not finding these observation would have been a big strike against GR. There was no prior model that predicted such a thing, and no real competing model to GR for matching these features.
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