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Question about observable universe.

  1. Jun 4, 2010 #1
    If the unobservable universe is defined by the region in which light originated in it is incapable of reaching earth due to having the expansion of space rate faster than c, the speed of light, thus causing light unable to travel to earth, than under this definition would not our observable universe diminish since light sources located on the edge of our observable universe is constantly outspeed by the expansion of space, and if our observable universe is not diminishing than i must got something wrong here, can someone explain?

    thanks in advance
    Last edited: Jun 4, 2010
  2. jcsd
  3. Jun 4, 2010 #2


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    That's not the definition of the boundary of the observable universe. It doesn't matter that the "expansion speed" is greater than c. The reason why we can't see things from outside of our observable universe is because the light from them has not yet had time to reach us in the age of the universe. As more time passes, light from more and more distant things eventually reaches us. For this reason, our observable universe (or horizon) is actually expanding, not "diminishing."
  4. Jun 6, 2010 #3
    If the farthest thing we can see is the 3 degree microwave background which was cosmolgically redshifted from 3700 degrees Kelvin( a redshift of over 1000), why can't we see eveything (All galaxies, etc) with red shifts smaller than 1000 if we have a large enough telescope?
  5. Jun 6, 2010 #4


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    Surface of last scattering is the short answer. The universe was optically opaque before the CMB was emitted. A neutrino telescope could penetrate the haze almost back to the big bang. The aperature required is, however, unimaginably large.
  6. Jun 6, 2010 #5
    We have to wait 4 more years.
    http://www.jwst.nasa.gov/" [Broken]
    Last edited by a moderator: May 4, 2017
  7. Jun 6, 2010 #6
    Hmmm... So basically, you have a star on the edge of the expanding universe. And you are saying that because the universe is expanding faster than the speed of light, that we shouldn't be able to see the light from that star? Did I get that correctly?
  8. Jun 6, 2010 #7
    Not quite. If we take Hubble's law V=HD, we can calculate distance at which recession speed becomes superluminal using V=C, thus D= C/H. This defines so called Hubble sphere. Things outside that sphere are receding faster than light, but it is not horizon of any kind.

    If object outside of Hubble sphere emits photon towards us, that photon is going with local velocity of c, but since recession velocity is greater than c , total velocity is away from us.

    Now interesting part. Radius of Hubble sphere is increasing with time. So, some of the photons that were on the superluminal side, cross to the subluminal side, and eventually find their way to us. Thus we can observe things that are receding faster than light.
    Last edited: Jun 6, 2010
  9. Jun 7, 2010 #8
    I wanted to make a point(It is really a question)
    Can every galaxy in the universe be seen by us if we have a large enough telescope obseving at right wavelengths(visible, infrared and microwave)?
    I think the answer is yes.

    My thinking: We can see the universe before the galaxies were formed - the microwave background radiation.
    Any galaxy was formed shortly after that and would also be visible,i.e. the light has reached the earth. There is no galaxy whose light hasn't reached us.

    The only way a distant galaxy's light could not have reached the earth is if it is very distant , say 1 billion light years away AND formed more recently than a billion years ago.
    However, I think, except for collisions, all galaxies were formed in the early universe.
  10. Jun 7, 2010 #9
    Galaxy I Zwicky 18 is less than a billion years old. New galaxies continue to be formed.
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