Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Can measurements of red shift be affected by the shape of spacetime?

  1. Jul 1, 2011 #1
    I have read on some threads that there are other possible shapes for spacetime and I wondered could any of these spacetime shapes, or their change over time, affect our measurement of red shift?

    Also does the matter density at the location where a photon is emitted relative to our own present matter density have an affect on our measurement of red shift?

    I was just reading about the recent discovery of the most distant known quasar and it led me to these thoughts about red shift in general as well as the CMBR redshift.

    http://en.wikipedia.org/wiki/ULAS_J1120+0641


    Thanks
    Chris
     
  2. jcsd
  3. Jul 1, 2011 #2

    bcrowell

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    What do you mean by "shapes?" Open versus closed? Nontrivial topologies?

    There is a gravitational time dilation effect. It relates to gravitational potential, not matter density, and it depends on the difference between the potential at the emitting and absorbing locations. If our planet occupies a typical place in the universe (Copernican principle), then it should have no effect on the validity of cosmological models, but if we happen to be in an unusual place, there are calculations that suggest it could seriously skew our results: http://arxiv.org/abs/0709.2044 I think the phrase to look for is "void models." The paper I linked to is a review from 2007, and this is an area of current research, so somebody else may be able to give a better update.
     
  4. Jul 1, 2011 #3
    "What do you mean by "shapes?" Open versus closed? Nontrivial topologies?"

    Ben, yes I think they were using terms like manifolds euclidean space etc. could they, or their change over time, cause changes in the frequency of light? Perhaps this amounts to the same thing when we say that the space has expanded?

    Thanks for the link to the paper. Wouldnt the gravitational potential in the region where the CMBR was first emitted be much different to our present one? The gravitational potential near the event horizon of a black hole in a quasar must be different.

    Thanks
    Chris
     
    Last edited: Jul 1, 2011
  5. Jul 1, 2011 #4

    bcrowell

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Well, no these are all completely different things.

    Hmm...maybe this is relevant? -- http://en.wikipedia.org/wiki/Shape_of_the_Universe I hadn't heard "shape" used the way you're using it, but evidently it's standard enough that it's used in the WP article. I don't know if it's referred to that way only in popularizations...?

    Actually I was not even being technically correct by talking about the gravitational potential, because there is only a well-defined potential in the case of a static spacetime, and cosmological models aren't static. I don't know, maybe it is still valid to talk about a potential as a local approximation, e.g., if we're in a void we'd be at a higher than average potential. But it's definitely not possible to use a potential to calculate the bulk of the cosmological redshift.

    In any case, these are relatively small corrections being posited in the void models. The CMBR was not emitted from near the event horizon of a black hole. There is no controversy about the fundamental soundness of the big bang model, the cosmological interpretation of the CMB, or the general interpretation of cosmological redshifts. What the void models might do is to get rid of the need for a nonzero cosmological constant (dark energy).
     
  6. Jul 1, 2011 #5
    Ben, Re: potential, sorry I meant the gravitational field - which curves space time. Since space time is curved differently at the quasar source to us here can this have an effect on the red shift we measure here?
     
  7. Jul 1, 2011 #6

    bcrowell

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    No, gravitational potential was what I said, and although it wasn't quite right, gravitational field is even more wrong. Curvature is yet another thing. These are all distinct things. You can't just jumble the words together as if they're synonyms. This may help to make things more concrete: http://en.wikipedia.org/wiki/Pound-Rebka_experiment

    I see. You're asking how much of the redshift we see in radiation from quasars is from a gravitational redshift at the source, and how much is cosmological? That's an interesting question. You might want to take a look at this paper:
    http://adsabs.harvard.edu/cgi-bin/n...J...223..747S&db_key=AST&high=3325b47acc08258
    They find that in cases where both the host galaxy and the quasar have measurable redshifts in their visible-light absorption spectra, they are consistent. This makes me think that any gravitational redshift of the visible-light absorption spectra from quasars must be fairly small. Presumably this is because the gas doing the absorbing is relatively far away from the quasar. If you want to see radiation emitted from deep in a quasar's gravity well, I think you want to look at x-rays and gammas...?

    [EDIT] This seems relevant: http://en.wikipedia.org/wiki/Active_galactic_nucleus#Observational_characteristics
     
    Last edited: Jul 1, 2011
  8. Jul 1, 2011 #7
    Ben thanks! one of your links lead me to "gravitational redshift"

    http://en.wikipedia.org/wiki/Gravitational_redshift

    I asked on this forum a few times if there were any other possible causes for red shift and no one offered up this. Thanks!

    Now to find out how much this effect contributes to all red shifts that we observe. Right at the event horizon of a black hole, where light can barely escape at all, I suspect even x-rays would be redshifted almost down to almost zero. I need to confirm.
     
  9. Jul 1, 2011 #8

    bcrowell

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Did you read #6?
     
  10. Jul 1, 2011 #9
    Not just read it but made the calculation! Also 1 Definition:

    "The redshift is evaluated in at a distance in the limit going to infinity. This formula only makes sense when R * is at least as large as rs. When the photon is emitted at a distance equal to the Schwarzschild radius, the redshift will be infinitely large. When the photon is emitted at an infinitely large distance, there is no redshift. The redshift is not defined for photons emitted inside the Scharzschild radius. This is because the gravitational force is too large and the photon cannot escape."


    Now my question becomes, is there any possible way that gravitational redshift could play a role in CMBR and measurements of highly redshifted galaxies? I hope they didnt forget to consider this contribution - no I dont believe it for a minute..

    Chris
     
  11. Jul 1, 2011 #10

    bcrowell

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Here is a more recent paper on void models, which seems to show that they are not viable alternatives to dark energy: http://arxiv.org/abs/1007.3725
     
  12. Jul 1, 2011 #11

    bcrowell

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    I've been trying to sound alarms that you're misunderstanding things, and that the things you're proposing have already been investigated by other people and ruled out. You don't seem to have been listening to the alarms.
     
  13. Jul 1, 2011 #12
    Ben, no problem, I meant it when I said "I hope they didnt forget to consider this contribution - no I dont believe it for a minute.."

    Would still be interested in understanding how it gets ruled out as CMBR redshift contribution though.

    Chris
     
  14. Jul 2, 2011 #13

    Chalnoth

    User Avatar
    Science Advisor

    Well, in principle, yes. In practice, probably not. The difficulty here is that it looks like our universe is so very much larger than the observable universe that any overall shape it has is simply irrelevant to our observations.
     
  15. Jul 2, 2011 #14
    Thanks Chalnoth.
    I presume that the density of matter at time of CMBR radiation was not anywhere near that of a black hole and thus cannot affect red shift.
    Presumably they have to take relativistic redshift into account with quasars though?

    Slightly different subject:

    Do we believe that the CMBR radiation travelled in a straight line to us in the earlier universe and did not go off in random directions due to scattering or recombination?

    Do we have any evidence to suggest the universe is bigger than the CMBR sphere or is it just deduced from the need to satisfy homogeneity and also the fact that there is no edge to the universe?
     
  16. Jul 2, 2011 #15

    Chalnoth

    User Avatar
    Science Advisor

    Well, in that situation it is density contrast that counts, not absolute density, and the universe was pretty uniform at that time.

    Well, when you are looking at a quasar you are looking at the accretion disc around the supermassive black hole. That accretion disc has some width, with some parts closer to and further away from the black hole, and thus you can actually measure how much gravitational redshift matters (and it isn't much).

    Well, we do expect that some of the CMB photons were deflected. But it's a pretty small percentage. The WMAP satellite estimates the percentage of photons that were absorbed/deflected to be about 8.5% or so.

    Mostly it's deduced from the need to satisfy homogeneity. Inflation also leads to a strong theoretical bias towards a universe that is vastly, vastly larger. That and it's incredibly difficult to explain the observed flatness with a universe that is only slightly larger than the observed universe.
     
  17. Jul 2, 2011 #16
    Thanks Chalnoth, in my question about red shift near quasars I meant gravitational redshift. Would Relativistic redshift imply redshift caused by matter travelling near the speed of light?
     
  18. Jul 2, 2011 #17

    Chalnoth

    User Avatar
    Science Advisor

    Yes, that's why I mentioned that the accretion disks have width, with some parts closer to and further away from the black hole. These different parts would have different gravitational redshifts due to their relative proximity to the black hole.

    I don't know exactly what you're asking here, but in General Relativity the answer is unequivocally no, because there is no way of distinguishing between redshift caused by relative velocity and gravitational redshift due to intervening curvature.
     
  19. Jul 3, 2011 #18
    Chalnoth, I was thinking here about matter in close orbit around a Black Hole, which I thought would emit radiation at one frequency whilst moving away from us at 99% of speed of light and another whilst moving towards us at 99% of the speed of light. Would this then be relativistic doppler redshift?

    Thanks
    Chris
     
  20. Jul 3, 2011 #19

    Chalnoth

    User Avatar
    Science Advisor

    Well, as I understand it, that only happens at the very end, right before it enters the black hole, and so doesn't make up a significant fraction of the light coming from quasars.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Can measurements of red shift be affected by the shape of spacetime?
  1. Measuring red shift (Replies: 15)

  2. Red shift (Replies: 3)

  3. Red Shift (Replies: 24)

  4. Red Shift, Blue Shift (Replies: 4)

Loading...