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What really is sky?

  1. Jun 29, 2012 #1
    When we look up into the cosmos wt we see at the end is a definite boundary or are we just gazing at empty space?
  2. jcsd
  3. Jun 29, 2012 #2
    Universe is probably infinite and if even it is finite it probably does not have any definite boundary.
    Space is big. Too big. It is mostly empty. Yes, you are gazing at empty space.
  4. Jun 29, 2012 #3


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    When we look in the distance, we also look in the past due to the finite speed of light. The cosmic microwave background is light, emitted approximately 300000 years after the big bang. This is the limit of our view with electromagnetic waves. While it would be possible to look a bit further with gravitational waves, it would just shift the limit, with the big bang as ultimate limit.

    The boundary is related to the finite age of the universe, not the size of the universe (which may or may not be finite, but probably without boundary)
  5. Jun 29, 2012 #4

    D H

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    Certainly not empty space, and depending on what you mean by "definite boundary", not that either. There most certainly is "definite boundary" in a sense, the surface of last scattering. However, I doubt this is what you meant by "definite boundary".

    Look at all frequencies (not just visible) in any direction that isn't blocked by some intervening object and what you will see is a near uniform microwave. This is the cosmic microwave background radiation. The CMBR would be an intense orange-yellow glow all over the sky were it not for the expansion of space. The expansion of space has redshifted this well into the microwave region. We can't see it with our eyes, but we can see it with radar. It was first detected as a strange noise that radar developers just could not seem to get rid of.
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