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

Red shift due to lensing?

  1. Feb 22, 2015 #1
    Does light get red shifted during lensing, when it passes by an object with great mass such as a galaxy or cluster?

    If so then what are the effects on measurements of the CMBR and most distant red shifted galaxies?

    If true then presumably some light would be red shifted more than others resulting in frequency dispersion?
     
  2. jcsd
  3. Feb 22, 2015 #2

    phinds

    User Avatar
    Gold Member
    2016 Award

    Think about shining a laser horizontally along a road from point A to point B, one mile apart and each one foot off the ground. Now put a 2 foot diameter beach ball in the middle of the path and require that the light beam go from point A to a point just above the beach ball and then on to point B. I don't think the added length of the path would cause any significant red shifting.

    I think the CMB measurements ignore any lensing, and since lensing really is only used to show the effects of GR, it's irrelevant to measurements of distant galaxies (assuming I'm right about the added red shift being insignificant).

    All of this is just what I hope is an educated guess. I haven't done any math on this and if I'm off, I'm sure someone more knowledgeable will jump in and let us know.
     
  4. Feb 22, 2015 #3
    What if the light bends around an object and significantly changes direction?
     
  5. Feb 22, 2015 #4

    phinds

    User Avatar
    Gold Member
    2016 Award

    That might make very slightly more difference, but that is not what happens with galactic lensing.
     
  6. Feb 22, 2015 #5

    Chalnoth

    User Avatar
    Science Advisor

    Doesn't really matter. Lensing just changes the direction of light beams. It doesn't impact their wavelength. It does change light travel time and distance, which may have some impact on the redshift, but it's a very minor impact. I doubt it is measurable.
     
  7. Feb 22, 2015 #6
  8. Feb 23, 2015 #7

    Chronos

    User Avatar
    Science Advisor
    Gold Member

    Light gets blueshifted as it approaches a massive body then redshifted by the same amount as it departs. The net effect is zero - re: http://curious.astro.cornell.edu/ab...hen-it-is-gravitationally-lensed-intermediate. There is, however, a special case called the Sachs-Wolfe effect that occurs when photons pass through a massive extended body. Those photons are slightly less redshifted when they exit such a region.
     
  9. Feb 23, 2015 #8

    Chalnoth

    User Avatar
    Science Advisor

    Just to expand upon the Sachs-Wolfe effect, this occurs as a result of the gravitational potential well changing over the time the photon passes through it. Obviously this will only be a significant effect for extremely large systems, where the dominant cause of changing gravitational potential wells is dark energy, which causes the gravitational potential of large systems to decay slowly over time.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Red shift due to lensing?
  1. Red shift (Replies: 3)

  2. Red Shift (Replies: 24)

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

  4. Explanation of Red Shift (Replies: 43)

Loading...