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Newbish questions (shape of the observable universe, blueshift, etc.)

  1. Apr 16, 2010 #1

    I've juggled these questions around my mind for a few days now and so far haven't been able to google any useful information:

    1. What is the approximate 3D shape of the observable universe? The Wikipedia article on this subject talks about weird stuff, like a "flat universe" and lists other highly mathematical rabble, while presumably presenting the topology of the universe as a whole, not just the observable part of it. Do we even have means to give a simple answer, and if not, what is preventing this?

    2. What is the ratio of observable blue-shifted objects to red-shifted ones? Is there an upper limit to the possible blue shift an observed object can have (in the lay sense that an object is approaching us at a maximum possible speed of X km/s), and can we draw any conclusions as to the "speed" at which space itself is expanding? My uneducated mind tells me that the faster stuff scurries away from us in all directions, the fewer blue-shifted objects we should be able to observe, hence deriving a "boundary" of sorts where objects appear to stand still, and beyond which space itself is expanding so fast that no blue-shift could possibly be detected.

    There were some other questions lying around in my pumpkin, but they elude me as of now. I might cough them up later, when I've had a beer or two (works wonders sometimes). Thanks for any answers!
  2. jcsd
  3. Apr 16, 2010 #2


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    On average, everything is receding from us; its recession rate is directly linked to its distance (the farther away, the faster the recession). That being said, there is a normal amount of variation in every object's drift. Only at close range (a few million light years) does this individual variance overcome the widespread redshifting to produce some blue-shifting. (There are other variancews, such as whole galaxy clusters and super clusters but, I don't think any of their blueshifting is more than their redshift.)

    So yes, we have a pretty good idea how fast the universe is expanding.
  4. Apr 16, 2010 #3


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    Current cosmological measurements indicate that the universe is geometrically flat to within a percent or so. By 'geometrically flat' one simply means things like: parallel lines don't intersect and the sum of the internal angles of a triangle equals 180. The universe could theoretically also be curved -- like the surface of the earth. On a spherical surface like this, the sum of the internal angles of a triangle total more than 180 degrees, and parallel lines intersect at the poles. And so this measurement applies to the geometry of the universe. The geometry of the universe can be measured locally, i.e. without needing to 'measure' the whole universe.

    The topology of the universe is a different story, and in order to determine it one would need to understand the whole universe globally. We do not currently know the topology of the universe.

    EDIT: I should revise the previous statement. One would not need to understand the whole universe globally to detect nontrivial topology. One would merely need to discover the effects of nontrivial topology at whatever the characteristic scale of the topology is.
    Last edited: Apr 16, 2010
  5. Apr 16, 2010 #4
    Thanks for your answer! Perhaps I tried packing too much in one little question; what I wanted to ask was whether there is a certain distance beyond which any proper motion is minuscule in comparison to the expansion of space, and thus undetectable. I suppose this then ties in to the matter of blueshift from proper motion being detectable only a few Mly around us, as you said.

    I faintly remember reading something about expansion of space being measurable at distances upward of 140 Mly or so, and even then only in amounts of around one percent per corresponding time. Is this approximately correct?
  6. Apr 16, 2010 #5
    Thank you, too!

    To quote Wikipedia:

    "The visible universe is thus a sphere with a diameter of about 28 billion parsecs (about 93 billion light-years)."

    With the universe being as good as geometrically flat, are there any visible "dents" in this observable sphere, where the farthest visible object appears to be over 1% closer or farther away in comparison with the farthest objects in other points of the sky? Or could we chalk up any such inconsistencies to our observations still simply being limited and incomplete, allowing for future discovery of more distant objects that will "fill these dents"?
  7. Apr 16, 2010 #6


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    I'm sorry, I don't understand your question. Why would a flat universe lead to "dents" in the observable sphere?
  8. Apr 16, 2010 #7
    No no, I did not imply any connection there, and maybe I understand this flatness the wrong way. Disregard this part.

    To rephrase, does the visible universe to the best of our knowledge seem to be expanding at an equal rate in all directions, or are there points in the sky where farthest either isn't quite as far – or conversely, appears to be closer – than elsewhere, hence forming dents or bulges on our imaginary sphere?
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