The observable universe

  1. Hello,

    i'm reading through a book called "Discovering The Universe" and in the chapter about cosmology the familiar idea of the observable universe is discussed.

    It speaks of the radius of the cosmic particle horizon being ~15 billion light years, which makes perfect sense as light from further away has not had time to reach us yet.

    But I have a question. Have we ever seen, or are we actively looking for new stars in the night sky? Am I right in thinking as time goes on, it's possible that light from a never before seen star will reach earth and will effectively 'appear' in the sky.

    Are we likely to notice if one does?

    Thanks!
     
  2. jcsd
  3. phinds

    phinds 8,346
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    The size of the observable universe is increasing slightly over time so yes, new stars will move into it, BUT ... light from a single star at that distance will be too weak to detect. Google the "Hubble Deep Field" for a discussion of how GALAXIES at fairly close to that distance are detectable, but only with some effort.
     
  4. Chronos

    Chronos 9,878
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    I don't think we will be seeing any light that is older than the universe. The CMB is the effective limit on how far we can see into the universe in EM wavelengths. It is not quite as old as the universe itself. To see any further we would require neutrino or gravity wave detectors. These are a work still in progress.
     
  5. Low-Q

    Low-Q 254
    Gold Member

    I assume you mean that we can't see light that is older than the universe as we know it?
    I'm not trying to post personal views here, but it is tempting to believe that the universe doesn't expand faster than light, so that what we can observe (if we could observe the most distant object) in fact is the limit of the scope of the universe (?).

    Vidar
     
  6. phinds

    phinds 8,346
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    The two parts of that statement are mutually contradictory. The universe DOES expand faster than light so to say otherwise is an unsupportable personal theory.
     
  7. Is it still thought to be expanding so fast?

    I know that rapid expansion is part of the accepted cosmological model for the early universe. Hence why the diameter of the observable universe is ~90 billion light years.
     
  8. Chronos

    Chronos 9,878
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    The 90 billion light years thing is a source of confusion. It is true the observable universe is believed to be this large NOW, but, we cannot and will never see the universe as it is NOW. We see the universe at the age it was when photons we detect were emitted. For example, the CMB, the most ancient photon source possible to detect, was a mere ~40 million light years distant when the photons we now detect were emitted. Due to expansion, it took those photon 13.7 billion years to reach us. In the mean time, the CMB has receded to 90 billion light years, also due to expansion.
     
  9. phinds

    phinds 8,346
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    No, it is not THOUGHT to be, it is known to be. Objects at the edge of our observable universe are receding from us at about 3c.
     
  10. Cool - I didn't know that.
     
  11. Low-Q

    Low-Q 254
    Gold Member

    So objects CAN travel faster than light? When was this discovered? Because yesterday I could read that objects cannot travel as fast as light does.

    Vidar
     
  12. phinds

    phinds 8,346
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    No, objects cannot travel faster than light and no one has said they can or do. "Receding" is not the same thing. Google "metric expansion" for an explanation.
     
  13. George Jones

    George Jones 6,363
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    Adding to the confusion :tongue: :
    In more detail:
     
  14. Low-Q

    Low-Q 254
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    Thanks :-)

    Vidar
     
  15. Light would be Red-Shifted if the source is moving away. The universe is expanding and a new star just forming at the edge of the universe would probably be moving away also.
     
  16. phinds

    phinds 8,346
    Gold Member

    Yes, but the fact that is moving away doesn't mean its light wouldn't reach us, it just means that as you say it would be red-shifted. It also would be incredibly faint, both of which combine to mean that it would not likely be detectable. "Not detectable" however, is not the same as "does not exist".
     
  17. Chronos

    Chronos 9,878
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    But, such a star would not be 'new' in the sense of expansion overtaking a hitherto unobservable region of space. Since all photons originate inside the CMB, the region of the universe containing that star has always been observable. Remember too, that all events we observe in the remote universe are also time dilated. So the 'birth' of an object at high redshift would appear to be in slow motion as observed from earth. This why distant SN1a take longer to peak in luminosity than low z supernova.
     
  18. We won't detect any new stars or galaxies at that distance not only because the stars would be too faint, but because we'd be waiting for a very long time to detect much of anything. For example, if an undetected galaxy is relatively close to the edge of the visible universe--let's say 1 million LY--we wouldn't be able to detect it for a million years. The timescale is simply so great that we don't look for changes in things like that.
     
  19. phinds

    phinds 8,346
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    No, if it were 1 million LY from the edge of the observable universe, the light would not arrive here for something like 49 BILLION light years.
     
  20. Allow me to clarify: a million years from now.

    And what do you mean that it would arrive in 49 billion LIGHT-years? lol
     
  21. phinds

    phinds 8,346
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    Yep, I blew that one. I meant 47 billion years, not LY. That's how long it will take a new photon from the edge of the observable universe to reach the Earth.
     
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