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I If the observable universe was visible to the naked eye...

  1. Aug 26, 2016 #1
    What would the Earth's night sky look like? Would our eyes see any dark spots? Is there are way to calculate such a probability?

    This hypothetical is about the observable universe only. 93 billion light-years diameter, isotropic, visible light.

    Not sure what's the most appropriate tag for this...
     
    Last edited: Aug 26, 2016
  2. jcsd
  3. Aug 26, 2016 #2

    Vanadium 50

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    I don't understand. The observable universe is visible to the naked eye. If our eyes were more sensitive, everything would of course be brighter.
     
  4. Aug 26, 2016 #3

    andrewkirk

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    I think the question could be interpreted as follows: Is there any non-degenerate cone, with apex at a point on Earth, whose intersection with the sphere that is the observable universe, contains no radiating matter?

    I think some minimum threshold of radiation density to count as 'radiating matter' might need to be set because any particle can undergo nuclear decay and radiate, which would mean that one isolated particle of Hydrogen would be enough to render a cone 'occupied'.
     
  5. Aug 26, 2016 #4
    Lets assume that all the stars, galaxies, clusters, are all magically equally bright in the night sky to our eyes.

    What I'm really asking is if there would be any line of sight that wouldn't end up on the surface of a luminous object.

    Let me know if I'm not making sense...
     
  6. Aug 26, 2016 #5

    russ_watters

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    Does that include all frequencies or just visible ones? Because if it includes all frequencies, the cosmological microwave background would satisfy your query: it's everywhere.
    Aside from the CMB, which originated when the universe was opaque and therefore was itself a single luminous object, the answer to that is no. The best visual range idea of what the universe might look like with more powerful eyes is the Hubble [Ultra] Deep Field pictures:

    hudf32.jpg

    Note that this is also at a magnification/resolution beyond human sight, so at human resolution this would appear to be a near uniformly bright field, a la some regions of the Milky Way.
     
  7. Aug 26, 2016 #6

    Drakkith

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    It seems to me that the limited resolution of the eye would blur much of the light, so even though the HST sees a large amount of blackness, the sky would still appear nearly featureless and overwhelmingly bright.
     
  8. Aug 26, 2016 #7

    andrewkirk

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    A line of sight won't generate a dark spot. To be a spot, it needs to subtend a nonzero solid angle at the eye - hence the reference to a nontrivial cone in post 3.
     
  9. Aug 26, 2016 #8
    Just visible ones.

    Right, I was wondering about this image. I'm not sure about how deep into the past those galaxies are, though. Is this really the edge of our observable universe?

    Yes, given the eye resolving capability it would probably look like a close to uniformly bright sky.
     
    Last edited: Aug 26, 2016
  10. Aug 26, 2016 #9
    This flies over my head. But I'll read on it. Thanks.
     
  11. Aug 26, 2016 #10

    Drakkith

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    No. Most of those galaxies are much closer than the edge. The tiny, deep red dots scattered about the picture are galaxies which are much, much closer to the edge. We just can't see the vast majority of them because they've been redshifted almost completely out of the visual spectrum and are incredibly dim.
     
  12. Aug 26, 2016 #11
    I see. I was kinda imagining this exercise including all those galaxies that can only be detected outside of the visible spectrum, but as if they magically became visible to us. Like literally make all luminous objects within the observable universe visible to the naked eye.

    But I guess there would still be a lot of 'empty' space between baryonic matter for lines of sight to pass through.
     
    Last edited: Aug 26, 2016
  13. Aug 26, 2016 #12
    Well, excluding the CMB.
     
  14. Aug 26, 2016 #13

    Vanadium 50

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    To a very good approximation (not including redshift), they are. Galactic surface brightness is independent of distance, and is driven primarily by the galaxy's star density. There is a little difference related to color, but this of course averages out over any reasonably sized patch of sky.
     
  15. Aug 26, 2016 #14
    Sorry, I see what you mean now. Imagine that we look through the cross section (arbitrarily small radius) of the intersection between the cone and the sphere to see if there's any 'radiating' matter inside the cone. This is exactly what I mean. (planets, comets, anything bigger than a dust particle also counts)

    I should have said all with equal apparent magnitude. Sorry for the confusion. The question of visibility is one point that is likely cleared out. The other is the cone intersection (arbitrarily small cross section, so pretty much a 'line'), which andrewkirk understood right away.

    My bad for not formulating this right.
     
    Last edited: Aug 26, 2016
  16. Aug 26, 2016 #15

    russ_watters

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    @Vanadium 50 was more on point than you realized. The brightness of point sources of light falls with the square of distance, but galaxies are not point sources, they are extended objects. And their apparent size also gets smaller with the square of distance, making the intensity of the light from that smaller area roughly the same. That's the reason the HUDF can detect galaxies of vastly different distances in the same exposure; their surface brightnesses are similar.

    The caveat of redshift means that in particular with the HUDF pictures, some galaxies become redshifted out of the frequency range of the detector.

    In any case, my understanding is that the HUDF does not miss a large fraction of the galaxies in our observable universe, but I don't have a ready link to a source for that. I'm going to take a somewhat of a guess and say it can see about half.
     
  17. Aug 26, 2016 #16

    Drakkith

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    Sounds like a reasonable guess.
     
  18. Aug 26, 2016 #17

    andrewkirk

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    The point of @russ_watters and @Vanadium 50 about apparent brightness of galaxies per unit solid angle being broadly constant makes me feel that the answer to the question must be Yes - there would be dark spots (empty cones). Otherwise we would have a solid white night sky, like a scaled-down version of Olbers' paradox.

    The only thing that might prevent that conclusion is the red shift issue. My guess is that, if we exclude the CMBR then, even if we count all sources within the observable universe, regardless of the frequency at which their light would arrive here, there would still be plenty of dark spots - ie cones from which the only incoming radiation is from the CMBR. But that is just a guess. Maybe the amount of incoming radiation at very low frequencies is so much more than that in the visible range that it fills in all the holes.
     
  19. Aug 26, 2016 #18
    This is Olber's Paradox
    https://en.wikipedia.org/wiki/Olbers'_paradox
    While there are plausible explanations it remains a paradox ( I think).
    Some would use this as an argument that the Universe in totally cannot be infinite.
     
  20. Aug 26, 2016 #19

    Drakkith

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    The resolution of the paradox lies in the fact that light has a finite speed and the universe has a finite age. Light simply hasn't had time to travel from every conceivable portion of the universe to get to Earth.
     
  21. Aug 26, 2016 #20
    Makes sense, thanks.
     
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