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Universal Horizon, CMB, and C

  1. Jun 27, 2011 #1
    I have a question sort of related the size of the observable universe.

    I know that we are limited by the speed of light in how far we can see, but what I am wondering is...

    In the early universe when the first light could finally break free and travel in straight lines did it travel in all directions like the light given off by a star, and if so could there be light 13.7 billion light years beyond the horizon and we just cant see it because there is nothing to reflect it back to us?

    Would this have any importance or relevance of any kind?

    And is 13.7 billion light years the distance across the entire universe or just from our position outwards in all directions, making the universe around 28 billion light years across?
  2. jcsd
  3. Jun 27, 2011 #2
    Essentially yes - Photons were emitted in all directions

    13.7 Billion light years is the distance light has had time to travel from the furthest points away from us so (rough age of the Universe), so yes you could draw a sphere 13.7Gy around the earth (roughly) and this would give you the Observable Universe.

    The Observale Universe and Universe are different and it is crucial for you to note the distinction between the two. The OU is the part of the Universe that light has had time to reach us from (13.7Gy) however the Universe is thought to be many many times larger than this.

    Out past the Observable Universe (Past the furthest points we can see) it is assumed is much of the same. So if we could "instantly" travel to the furthest point we could see and look around - we would see much as we do now with only local variance. On large scales the Universe is homogeneous.

    Hope this helps.
    Last edited: Jun 27, 2011
  4. Jun 27, 2011 #3


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    YOUNGDREAMER, please have a look at Ned Wright's calculator. Google "wright calculator" or alternatively "cosmo calculator" to get it. If you have any trouble please ask for help.

    The calculator has a box where you can type in any redshift.
    The oldest light we can see has a redshift of 1090, its wavelengths stretched out by a factor of just over 1090. So when you get to Wright's calculator,
    type 1090 into the box, and click on the 'general' button to get the distance.

    It will tell you that the material that radiated the oldest light we can see (which by now has its wavelengths stretched by almost 1100) is currently 45 billion lightyears from here IF YOU COULD STOP THE EXPANSION PROCESS to give yourself time to measure it. Defining distance is tricky in cosmology because it would be changing while you were measuring, however you imkagine measuring it. And the expansion rate has varied enormously over the history of the U. So distance has no simple relation to light travel time.

    The most common figure for the radius of the observable part of the universe (the part from which we have received light etc.) is around 45 billion lightyears proper distance.

    That is how far away the stuff is at this moment which 13.7 billion years ago radiated the light which we are now receiving as CMB---the oldest light which we can see.

    That is, if you could freeze the expansion process so as to give yourself time to measure the distance as it is today, you would find that the farthest stuff we are now looking at is 45 billion lightyears from here.

    This freeze-frame distance is called proper distance, and it is the type of distance used in the standard model of the cosmos and the type used in formulating the most basic law--Hubble Law relating distance to recession speed.

    In his comment, Cosmo Novice confuses lookback time or light travel time (13.7 Gy) with distance in lightyears. There is no simple relation to the distance we actually use, say in Hubble Law.

    13.7 billion light years is the distance light has had time to travel NOT COUNTING THE EFFECT OF THE EXPANSION OF THE UNIVERSE in a time period of 13.7 billion years.

    This is definitely correct! 13.7 Gy is not a distance however, it is a TIME called a "lookback time" an it corresponds to about 45 billion lightyears proper distance.
    The reason cosmologists do not normallly work with lookback time as a measure of distance is that it has no simple relation to proper distance---the actual physical distance at this moment if you could stop expansion and measure it, and the distance used to describe the physical model.

    That is also correct according to the standard picture. We have no firm evidence that the U as a whole is homogeneous and the standard model based on that assumption gives an excellent fit to the data.

    COSMO NOVICE, Most of what you say impresses me at least as very helpful. The only point that seems unhelpful is where you tacitly equate lookback time (13.7 Gy) with the actual current distance (45 Gly). We have to work to bring out that distinction clearly and could use your help in making it explicit.
    Last edited: Jun 27, 2011
  5. Jun 27, 2011 #4


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    We have a FAQ entry related to this: Why is the radius of the observable universe in light-years greater than its age? https://www.physicsforums.com/showthread.php?t=506987 [Broken]
    Last edited by a moderator: May 5, 2017
  6. Jun 27, 2011 #5
    My apologies YOUNGDREAMER, I did indeed make the fatal error of confusing the two, your explanation has helped clear that up for me but now I am thinking it is only logical - posted a little too late and after too many an ale!

    I think it fair to say we will never know whether the entirety of U is homogeneous, I think the only likelihood of a heterogeneous topology would lie in a spatially flat infinite U, but then the more I think about it then the more I think spatial curvature could lead to heterogeneity on a larger scale. Ona larger scale than an OU (Which in comparison to U, even if finite, could be very small).

    Marcus thankyou for correcting my assumptions - I had a feeling I was slightly missing something and it is now pretty obvious to me. Also thankyou for the LQG arxiv paper link, I read the one page .pdf and it didnt say anything new so delved into the paper. It is going very steadily due to time restraints and I am constantly having to reference content to other sources but I am steadily understanding the general idea. I will start a post on it when I get further in and have a few questions.

    Again thanks for the corrections.
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