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Light beams, from the present to the time of the big bang

  1. Nov 21, 2015 #1
    Regarding expansion, from the present to the past: Do light beams (generally) converge to a single point, in the remote past, as geometry and matter was also condensed to a single point, prior to the big bang?
    Thank you.
     
    Last edited: Nov 21, 2015
  2. jcsd
  3. Nov 21, 2015 #2

    phinds

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    No, there was no "single point". This is a pop-science misconception that has no bearing on reality as far as we know. The "singularity" does not mean "point" it means "the place where our math model gives un-physical results and we don't know what was really going on."
    The beginning of the universe as we understand it was not a point and may in fact have been infinite in extent.

    By the way, don't feel bad that you got sucked in by that mis-information. There are many dozens, probably hundreds, of posts here with people asking questions based on exactly that misunderstanding.
     
  4. Nov 21, 2015 #3

    DaveC426913

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    Well, yes. Sort of. The CMBR is the light from the earliest time we can see it. Inasmuch as ideal radiation might point in a preferred direction one might argue they would all actually converge from the edge of the observable universe toward the observer. Since the Big Bang happened everywhere, the radiation will come from the edge of everywhere.
     
  5. Nov 21, 2015 #4
    Appreciated. Was the distance between galaxies not smaller in the past?
    If so, does that not imply light beams were closer then they are now?
     
  6. Nov 21, 2015 #5

    phinds

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    Yes, and if you go back far enough there were no galaxies.
    No, light is emitted from celestial objects, generally, radially and the angle between any two rays doesn't change as you get closer to the source.
     
  7. Nov 21, 2015 #6
    Again thanks. Even when viewing several billion light years away, the light paths (travelling toward Earth) are not converging in the remote past?
     
  8. Nov 21, 2015 #7

    phinds

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    The photons that were emitted billions of years ago were closer to their source of origin than they are now, but so what? The photons that leave our sun are all within one solar diameter of each other when they leave and they get farther apart, but "converging" is the wrong way to think about that process in reverse.
     
  9. Nov 22, 2015 #8
    May I ask just one more question? Does expansion separate individual photons, which are massless? I'm not referring to geodesics, or curved space. I'm interested to know if expansion itself can cause two, almost parallel, photons to separate. I suspect that the answer is no.
    Thanks :)
     
  10. Nov 22, 2015 #9

    phinds

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    I think in the sense that you mean (and I may be misinterpreting you) then of course they do because it's all just geometry. Mass has nothing to do with it.

    Take two photons that are traveling "parallel" to each other in that they have started out from sources that are 5 billion light years apart and traveling to targets that are 5 billion light years apart and 8 billion light years away from the respective sources. The geometry of this is a rectangle. BUT ... as spacetime expands, the sides of the rectangle that represent the photons' paths to their targets becomes horn-shaped outward and the photons get farther apart.

    Again, mass has nothing to do with it. Spacetime is a framework and a framework that changes size over time. Google "metric expansion".
     
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