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Expansion of space and looking back in time

  1. Dec 28, 2009 #1
    when we observe objects that are billions of lights years away are we also observing the expanse of space as it existed then? lets say 10 billion light years ago. could we call it a compressed universe as opposed to the expanded universe we are in now and observing from

    the universe would have only been 3.7 billion light years "big" (estimate base on current observations). it seems odd that we could look back at our universe when it was smaller yet we are still in it.
  2. jcsd
  3. Dec 28, 2009 #2


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    Good question that have also puzzled me. The key to understanding is the expansion of curved spacetime.

    Observable Universe
    First, when talking about how "big" the universe is/was – we must remember to emphasize that it’s the observable universe we are talking about. The whole universe might be infinite, we just don’t know yet. Some scientist’s claims that there is proof of a universe a least a thousand times bigger than the observable universe, and most agrees that the observable universe is 93 billion light-years in diameter.

    Expansion of Curved Spacetime
    This is quite complicated things involving the General Theory of Relativity. However, it can be understood by a layman on a basic level, omitting two dimensions of space, and focusing on one dimension of space and one of time.

    The picture below shows how a light ray (red line) can travel an effective distance of 28 billion light years (orange line) in just 13 billion years. This also reveals the fact that observable universe = calculated visible universe.


    For more info, read http://en.wikipedia.org/wiki/Expanding_universe#Understanding_the_expansion_of_space" on Wikipedia.

    Looking Back at the Universe
    What are we looking at? Well, everything that we see when looking 'back' is right here and now. The photon hitting the camera or the eye is right here, and not 'there'. Confusing? Actually it turns out to be even more 'odd'...

    You refer to objects when the universe was 3.7 billion years old. The fact is that we can photograph the whole (observable) universe when it was only 400 000 years old! And it looks like this:


    This 'baby-picture' of the universe shows the Cosmic Microwave Background (CMB) radiation (the oldest light in the universe) and is produced by The Wilkinson Microwave Anisotropy Probe (WMAP).

    More info at http://map.gsfc.nasa.gov/" [Broken].

    Here’s a schematic picture of the history of the universe, showing CMB at 400 000 years:


    How is this possible!? :confused:

    Get Serious
    Well, now it’s time to get real serious and look at the basic mechanism:

    1) Photons always travel at the speed of light relative to the galaxies near them.

    2) As the universe gets older, the galaxies do not expand, but the distance between the galaxies gets larger.

    [PLAIN]http://www.astro.ucla.edu/~wright/cphotons.gif[/INDENT][/URL] [Broken]

    The animation shows the expansion of space and the evolution of the galaxy density, positions and the photon positions within a '78 billion light year box'. Each black dot represents a galaxy, and the two green dots are galaxies emitting red photons.

    http://www.astro.ucla.edu/~wright/intro.html" [Broken] made this excellent animation, and you’ll find a lot more of useful info at his Home Page:

    http://www.astro.ucla.edu/~wright/photons_outrun.html" [Broken]

    Good luck!​
    Last edited by a moderator: May 4, 2017
  4. Dec 28, 2009 #3
    wow, just wow! thank you for the super informative reply. the resources you supplied should be a great read when i get home. thanks for your time.
  5. Dec 28, 2009 #4


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    You’re welcome. Glad you found it useful. (I was also happy when I got this 'into my head' :smile:)
  6. Dec 28, 2009 #5
    btw the animation is great but it does seem to have a center... just saying
  7. Dec 28, 2009 #6


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    It’s ok. The universe doesn’t have a center. Remove one dimension in space and think of it like the two-dimensional surface of earth. Where is the center on the surface of earth...??

    (The answer is not Greenwich or the North Pole :wink:)

    This is real hard to get into the head when thinking real space 3D, but this is how it is.

    A lot more thoughts and answers about this in PF thread:
    Last edited by a moderator: Apr 24, 2017
  8. Dec 28, 2009 #7


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    I don’t know if this helps. But think of the 'baby-picture' of universe (CMB) applied on a sphere:

    https://www.youtube.com/watch?v=<object width="425" height="344"><param name="movie" value="http://www.youtube.com/v/Aeq_pgirQ14&hl=en_US&fs=1&"></param><param [Broken] name="allowFullScreen" value="true"></param><param name="allowscriptaccess" value="always"></param><embed src="http://www.youtube.com/v/Aeq_pgirQ14&hl=en_US&fs=1&" type="application/x-shockwave-flash" allowscriptaccess="always" allowfullscreen="true" width="425" height="344"></embed></object>

    And then think of this sphere expanding (the white § are galaxies):

    https://www.youtube.com/watch?v=<object width="425" height="344"><param name="movie" value="http://www.youtube.com/v/I9Cjxd4Mjog&hl=en_US&fs=1&"></param><param [Broken] name="allowFullScreen" value="true"></param><param name="allowscriptaccess" value="always"></param><embed src="http://www.youtube.com/v/I9Cjxd4Mjog&hl=en_US&fs=1&" type="application/x-shockwave-flash" allowscriptaccess="always" allowfullscreen="true" width="425" height="344"></embed></object>
    Last edited by a moderator: May 4, 2017
  9. Dec 29, 2009 #8


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    It may sort of look that way, but if you moved anywhere in the animation, it would look exactly the same. Our universe is much like that, at least out to the limits of our vision.
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