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Questions re: the edge of the observable universe

  1. Sep 15, 2014 #1
    I was thinking a bit about the farthest parts of the universe we can see and I came to a few interesting questions that are maybe obvious to more studied physicists but new to me. Is the ~14 billion light year distance we can see in all directions slowly expanding with time? In another billion years will we see 15 billion light years in every direction? If so, does that mean the surface of that observable sphere is always going to look like the very early universe, just after the big bang? That leads to my next thought, from our perspective is the Earth located at the "oldest" point in space we can see?
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  3. Sep 15, 2014 #2


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    The edge of the observable universe is growing and will continue to grow. It's not, by the way 14 billion light years, it's more like 47 billion light years. 14 billion years is how long ago the light we see was emitted but the emitting sources have been moving away all that time and are now receding at about 3c.

    You need to read some basic cosmology and then come back when you have questions. Getting bits and pieces on an internet forum, even one as good as this one, is not really the way to go about studying this stuff.
  4. Sep 16, 2014 #3


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    I understand your questions, yet there are multiple perspectives to consider that offer different answers. The farthest parts of the universe we can see were closer to us when they emitted light that's reaching us now. That's due to expansion of the universe that results in more distance added to the path the light still needs to traverse while it's travelling to us. So there are already two possible answers - how far away they were when they emitted the light we see, or how far away do they appear to be once the light reaches us. Also, the objects that emitted the light are *now* more distant than they appear because of 14 billion years of expansion (if they still exist), so there's another answer although technically not in the here-and-now observable category.

    Regarding, "in another billion years will we see 15 billion light years", that also can be looked at in multiple ways, not only because of similar like above, but also because 'c', the speed of light is a constant, and that creates results that will challenge your intuition.

    There is a good thread that Marcus and Jorrie participated in as Jorrie was developing his cosmological calculator. It's really cool, and I think you might find the thread really interesting and learn about the Hubble constant, Hubble time, and the various "answers" depending on the answer you want. :smile:


    We can't use "oldest" to mean two different things. The oldest light we can see defines our observable universe, from our location. An observer somewhere else would see a different observable area. There isn't any oldest 'location' in the universe. Space changes with time and I've read knowledgeable forum members indicate that this is natural and expected from general relativity. But the expansion we've observed occurs everywhere and at the same rate so there aren't older areas and newer areas in the sense I think you mean.
  5. Sep 16, 2014 #4


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    All observers, regardless of location, will deduce they are at the most ancient point in their observable universe. This is obviously due to the finite speed of light. It is often less confusing to think in terms of age as opposed to distances in the universe. The CMB is a picture of the universe as it appeared when it was about 400,000 years old. That will never change, although the universe will continue to age one year per year.
  6. Sep 16, 2014 #5


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    actual light travel time is not very useful as index of distance because distances are constantly expanding at a constantly changing rate. I suggest you think in terms of "freeze-frame" lightyears, so-called "proper distance". That you would measure if you could PAUSE expansion at some particular moment (like,for example, now) to give time to measure it.

    The most distant matter we can see now or could detect with better instruments is matter which is NOW about 46 billion ly. That is proper distance. It is that size because of how much distances expanded while light was in transit.

    The most distant matter we will EVER be able to see or detect is matter which is NOW about 63 billion ly from us.

    The 46 and 63 are figures from the rightmost column of this table. This gives the PARTICLE HORIZON (the radius of the observable region) multiplied by the scale factor "a". The size to which distances have expanded. So notice that in the bottom row a=100 and the Dpar distance (of the farthest particles whose radiation we can have detected) is about 6300. To get the distance to those particles NOW you have to divide by a = 100. so you get 63.

    But that is far far in future, the radius of the observable is only gradually increasing as radiation from more distant matter arrives. today the most distant matter we could now be detecting is 46 from us, see the Dpar entry in the a=1 row, which refers to the PRESENT DAY situation.
    [tex]{\scriptsize\begin{array}{|c|c|c|c|c|c|}\hline R_{0} (Gly) & R_{\infty} (Gly) & S_{eq} & H_{0} & \Omega_\Lambda & \Omega_m\\ \hline 14.4&17.3&3400&67.9&0.693&0.307\\ \hline \end{array}}[/tex] [tex]{\scriptsize\begin{array}{|r|r|r|r|r|r|r|r|r|r|r|r|r|r|r|r|} \hline a=1/S&S&T (Gy)&R (Gly)&D_{now} (Gly)&D_{then}(Gly)&D_{hor}(Gly)&D_{par}(Gly) \\ \hline 0.001&1090.000&0.0004&0.0006&45.332&0.042&0.057&0.001\\ \hline 0.003&339.773&0.0025&0.0040&44.184&0.130&0.179&0.006\\ \hline 0.009&105.913&0.0153&0.0235&42.012&0.397&0.552&0.040\\ \hline 0.030&33.015&0.0902&0.1363&38.052&1.153&1.652&0.249\\ \hline 0.097&10.291&0.5223&0.7851&30.918&3.004&4.606&1.491\\ \hline 0.312&3.208&2.9777&4.3736&18.248&5.688&10.827&8.733\\ \hline 1.000&1.000&13.7872&14.3999&0.000&0.000&16.472&46.279\\ \hline 3.208&0.312&32.8849&17.1849&11.118&35.666&17.225&184.083\\ \hline 7.580&0.132&47.7251&17.2911&14.219&107.786&17.291&458.476\\ \hline 17.911&0.056&62.5981&17.2993&15.536&278.256&17.299&1106.893\\ \hline 42.321&0.024&77.4737&17.2998&16.093&681.061&17.300&2639.026\\ \hline 100.000&0.010&92.3494&17.2999&16.328&1632.838&17.300&6259.262\\ \hline \end{array}}[/tex]

    Don't get boggled by the other columns of information. All you want here is the "a" column, and the T column (age of U expansion) and the Dpar (radius of observable, particle horizon)

    The link to the Lightcone calculator is in my sig. You can make unnecessary columns go away and simplify the table by using the "column selection" menu.
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