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Why the red-shift of distant galaxies is thought to be the effect of expanding space?

  1. Oct 1, 2012 #1
    Why it's not explained just by Doppler redshift caused by faster movement of those galaxies billions of years ago when that light was emitted?

    Would the speeds of the galaxies necessary for Doppler redshift to explain all of the observed galaxies redshift be unreasonable or is there something else that prevents such explanation to be sufficient?

    I noticed that distance to far galaxies are given in terms of their redshift rather than lightyears. Do we have any other way to tell how far these galaxies are?

    What is the farthest data point of distance and redshift that we can use to verify how they should correlate?

    Could you point me to some experimental data of this kind?
     
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  3. Oct 1, 2012 #2

    phinds

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    Re: Why the red-shift of distant galaxies is thought to be the effect of expanding sp

    This subject is brought up here so often that I'm sure others will respond (or you COULD just do a forum search), but I will simply point out that if you argue that all redshift is Doppler redshift instead of Cosmological redshift, you have to argue that distant galaxies are MOVING at much faster than the speed of light, which is silly. Cosmological redshift, the true reason, simply shows that they are receeding at faster than light, not moving at faster than light.
     
  4. Oct 1, 2012 #3
    Re: Why the red-shift of distant galaxies is thought to be the effect of expanding sp

    I tried that. Failed. So I asked.

    I'm very thankful for your answer (even more if the subject is brought here that often).

    Is there some maximum doppler redshift that can be achieved by shooting object away from us at almost the speed of light and we observed galaxies that exceed this limit?

    How fast emmiter would have to move away from us to turn gamma photon into microwave background photon?
     
  5. Oct 1, 2012 #4
    Re: Why the red-shift of distant galaxies is thought to be the effect of expanding sp

    I found some cool material on the subject: http://www.astro.ucla.edu/~wright/cosmo_01.htm

    Calculators for things I don't understand included. :-)

    I don't understand the part that says:

    especially: so v = cz + ...
    because I'd rather think that v = (2cz+cz2) / (2+2z+z2) which gives you speeds lowen than c for all possible z.
     
    Last edited: Oct 1, 2012
  6. Oct 1, 2012 #5

    phinds

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    Re: Why the red-shift of distant galaxies is thought to be the effect of expanding sp

    Google cosmological redshift
     
  7. Oct 1, 2012 #6
    Re: Why the red-shift of distant galaxies is thought to be the effect of expanding sp

    I got my answer here: http://physics.stackexchange.com/qu...es-is-considered-to-be-the-effect-of-expandin

    And it's pretty much "Finally of course you'd have to explain how come Hubble's Law holds." which means that by explaining galaxy red-shifts as doppler red-shifts you have to also explain why galaxies would decelerate at constant rate. And no one has managed to do that so far.
     
  8. Oct 1, 2012 #7
    Re: Why the red-shift of distant galaxies is thought to be the effect of expanding sp

    I understand that Hubble, De Broglie, and Enrico Fermi, passed away without believing in the expansion of the universe because the redshift of galaxies are the same for those that are at the same distance from earth.

    This would imply the earth is at the center of the universe in a big bang expanding universe.

    Vera Rubin measured an anomalous cosmological redshift of peripheral galactic stars.

    Her published claim to have measured anomalous speed of peripheral galactic stars is her personal opinion.


    Also, how does one explain the constant cosmic background radiation? Where does it get it's energy from?

    A number of papers I've come across have established that it should have been absorbed by galaxies and intergalactic dust several billions of years ago because it is very faint.
     
  9. Feb 8, 2013 #8
    Re: Why the red-shift of distant galaxies is thought to be the effect

    I've read that Z=1.4 is the red shift indicating a cosmological separation speed equal to the speed of light, and some galaxies have a red shift of Z = 8. So, does that mean they are moving away from at 5.7 times the speed of light? If so, this conflicts with something I read that said the fastest speed recorded so far is about twice the speed of light.
     
  10. Feb 8, 2013 #9

    phinds

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    Re: Why the red-shift of distant galaxies is thought to be the effect

    Objects at the very edge of the observable universe are receeding from us at about 3c but no speeding tickets are issued because they are really sitting still --- it's just that the distance between us is expanding. Google "metric expansion" for details.
     
  11. Feb 8, 2013 #10
    Re: Why the red-shift of distant galaxies is thought to be the effect

    Yes, I know the difference between cosmological and Doppler red shift. The point is, the one's that are the farthest are Z = 8, and Z = 1.4 indicates a recession at light speed. So, 8 divided by 1.4 = 5.7 Does that mean that the galaxies are receding at 5.7 times the speed of light? Apparently that nust not be the correct way to compare the red shift numbers because you say the fastest are at 3c, not 5.7c.
     
  12. Feb 8, 2013 #11

    marcus

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    Re: Why the red-shift of distant galaxies is thought to be the effect

    that's true! the instantaneous recession (as of today or as of when the light was emitted or at whatever moment) is NOT proportional to the redshift!

    as a rule you need a calculator to get the recession from the redshift, it isn't a simple linear relation.

    The speed that the distance is increasing TODAY happens to be proportional to the DISTANCE today, so there is a very easy way to get it. Just go to Ned Wright's calculator which gives the today distance (labeled comoving radial) and find the distance for z=8 and for z=1.4 and take the ratio of the two distances.

    That ratio of the two distances will be the speed of distance increase, as of today, as a multiple of c.

    You can make that more accurate by using a more precise figure instead of 1.4, but 1.4 is good enough and good to know. More decimal places is not going to change things significantly. Google "wright calculator" and give it a try!
     
    Last edited: Feb 8, 2013
  13. Feb 8, 2013 #12

    Chronos

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    Re: Why the red-shift of distant galaxies is thought to be the effect

    You can, of course, translate redshift into an equivalent special relatistic velocity, but, the modern interpretation is based on general relativity. GR enjoys vast experimental confirmation. In fact, it has never failed a single test. SR simply cannot explain a myriad of evidence that is easily accomodated by GR.
     
  14. Feb 9, 2013 #13
    Re: Why the red-shift of distant galaxies is thought to be the effect

    We determine such distances indirectly: Turns out type 1A supernovas all have the same brightness and we can identify such characteristic brilliance:
    http://en.wikipedia.org/wiki/Type_1a_supernova#Light_curve

    So we determine a standard supernova 1A brightness, correlate with calculated distances to known galaxies and observed redshifts where they are found and then correlate redshifts z = ∆λ/λ to distances of other galaxies. Hubble correlated brightness and the parameter z.


    You might find this Scientific American article of interest:

    Misconceptions about the Big Bang: Scientific American 6/14/08
    http://www.sciam.com/article.cfm?id=misconceptions-about-the-2005-03 [Broken]

    This is when the surface of last scattering,the 'hot plasma', the origin of CMBR we observe, was about 42 mly from us.....it's now about 46Bly distant.

    oops..just noticed this is an old thread....oh well...
     
    Last edited by a moderator: May 6, 2017
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