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Missing galaxies?

  1. Dec 2, 2005 #1
    please correct me if this is a misconception that i have about "visible Universe" . Is true or not that in the limits of the universe visible to us , galaxies move faster than light and because of that we cannot detect it, and that this occurs continually, ( i mean more and more galaxies every day dissapear of our reach scope?),
    i have other questions related but first need to clarify this
  2. jcsd
  3. Dec 2, 2005 #2
    It is currently considered that the space between us and those distant galaxies is expanding at close to the speed of light. Thus over time these galaxies will disappear over time, along with every other galaxy over a much much greater period of time. However considering how often the wind changes in this particular field, it wouldn't surprise me if the opposite were true. I.E New galaxies showing up all the time.
  4. Dec 2, 2005 #3

    Based on our current understanding yes.

    As we now understand our observations, the expansion of the universe is accelerating, i.e. the universe is blowing itself apart faster and faster. The predicted (not observed) result of this is that, as space expands faster, objects currently at the edge of our vision will move beyond our ability to see them, essentially they'll be moving so fast, their light will never reach us.

    Maybe pervect or Spacetiger can help here, I'm not entirely clear on the relativistic issues of this.
  5. Dec 3, 2005 #4


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    No object currently observable will ever 'blink' out of view [i.e., sever the causal connection with our observable universe]. They may, however, eventually redshift beyond detectable limits.
  6. Dec 3, 2005 #5

    no, the edge of the visible universe is the point that space is expanding at the same rate as the speed of light.

    because the space between galaxies is expanding at an accelerated rate, the say will come when people will look into the sky and only see the stars in the milkyway. the universe will seem a lot smaller at that point, but it will in fact be much much larger.
  7. Dec 3, 2005 #6
    the edge of our observable universe is not limited by objects appearing to recede faster than the speed of light due to the Hubble flow. It is limited by time. ie. has the photon emitted by a galaxy at the edge of the observable Universe had enough time to reach us? It only has roughly 13.5 billion years.
  8. Dec 3, 2005 #7


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    You should distinguish between two concepts: the particle horizon and the event horizon.

    The particle horizon is the location of a light ray sent from our position at t = 0. This is the border of our observable universe and is about 45,000 million light-years, much more than 13.700 million light-years (13,700 million years is the age of the universe) because space has expanded between our position and the light ray during all this time. The particle horizon tells us about objects whose light sent in past may reach us.

    The event horizon is the location of objects whose light will never reach us in future, because space is expanding very fast and light will never be able to overcome the growing distance. The event horizon is more or less located at 13,700 million light-years and this corresponds to the distance at which objects are currently receding at the speed of light d = c / H (the Hubble sphere).

    The particle horizon is always growing, but the event horizon will remain at c/H in the current cosmological model.
  9. Dec 3, 2005 #8
    well, somehow I don't think he was interested in the event horizon, since he won't be around at [tex] t= \infty [/tex]!!!

    We should also keep in mind that the Universe was optically thick before the surface of last scattering (around 13.4 billion years ago).
    Last edited: Dec 3, 2005
  10. Dec 3, 2005 #9


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    It depends on what you (or he) understand with "dissapear of our reach scope". For example, a galaxy located at z = 2. We may receive light from it as it was in past, but we will never receive the light it is currently emitting. Of course this does not mean that in future we will see less galaxies than now, because our particle horizon is always growing. However, that galaxy, or the light it emits now, is dissapearing from our (future) reach.

    Yes, but the difference in the distance between the last scattering surface and the particle horizon is less than 2%. Note that if the age of the universe is 13,665.8 million years, the last-scattering surface was emitted 13,665.4 million years ago.
    Last edited: Dec 3, 2005
  11. Dec 3, 2005 #10
    This is completely true. However, sometimes adding extra info can confuse the questioner.

    oh well, my little computer program gave me 13.4Gyr for z=1087.
  12. Dec 3, 2005 #11


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    Perhaps this paper will help clarify some issues for the more technically inclined:

    "[URL [Broken] Confusion:
    common misconceptions of cosmological horizons
    and the superluminal expansion of the universe[/URL]

    In the current model, all objects beyond redshifts of ~1.5 are receding faster than the speed of light. However, we have already observed objects out to redshifts of > 6, so clearly this doesn't set the boundary of the observable universe. The boundary is actually set by the particle horizon, which hellfire mentioned in one of his posts.
    Last edited by a moderator: May 2, 2017
  13. Dec 4, 2005 #12
    Indeed. I think that paper should be read by all who are interested in cosmology!
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