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Observable universe abuse

  1. Sep 23, 2008 #1
    Am I needlessly annoyed or is this an incorrect interpretation of the term observable universe? It’s not the first time I’ve seen this from space.com. I can understand using “Light Travel Time” as the distance but the only reason 13.7 billion light years is a limit is because you can’t see earlier than the Big Bang. But we can see things that are moving away faster than light. Take an object with a redshift of Z=7, which we have seen. Such an object is moving away from us a roughly 2C. The light has been travelling for 12 billion years and the object is now 30 billion light-years away. Once we get better telescopes, we will see farther. I know of no stuff that came out of the Big Bang that we won’t be able to see given the right equipment. So is space.com misrepresenting the term observable universe?

    Mysterious New 'Dark Flow' Discovered in Space
    http://www.space.com/scienceastronomy/080923-dark-flows.html" [Broken]
    Last edited by a moderator: May 3, 2017
  2. jcsd
  3. Sep 23, 2008 #2


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    The current range of observation is about 45 billion LY. when we receive the CMB signal we are detecting light emitted by matter which is now 45 billion LY from us.

    Normally science journalists and popularizers screw up. You are right to point this out.

    They will always say stupid things like we can only see stuff that is 13.7 billion LY away because the universe is only 13.7 billion years old.
    But distances to stuff have been increasing.

    the main gist of what you say is right AFAICS. but there is a detail that you might want to think about
    we don't yet know whether the bigbang occurred at a finite location or spread over an infinite region. many astrophysicists assume the bigbang was infinite, and space is infinite.

    (it is only journalists and popularizers who give people the impression that the big bang occurred in a small volume, that might be true but we do not KNOW it, and many astronomers think not.)

    So there may be limits to how much of the universe we will ever be able to see. Part of the difficulty is we don't know how big it is. Part of the difficulty is that the expansion of distance is accelerating. the acceleration itself imposes a kind of horizon.

    so that's an extra detail to think about.
    Last edited: Sep 23, 2008
  4. Sep 24, 2008 #3
    Hmm, surface of last scattering, I hadn't thought of that. Good point. The article makes more sense now.
  5. Sep 24, 2008 #4
    I understand what they were trying to convey, they just did a lousy job of doing it. It's not really an easy concept to explain in one paragraph. Each sentence, taken individually, are factually correct. However, combining them leads to an ambiguous conclusion.

    With that said, I still see the 156Gly diameter of the universe being perpetuated around the internet. That was another SDC article where the interpretation was completely misleading.
  6. Sep 29, 2008 #5


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    I got that from an earlier Lineweaver and Davis paper, if I recall. I thought that was more or less where things were at now, although I understand that the actual value (92 G ly was mentioned in a later article) may change as better calibration and modeling occurs.
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