Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

About Our Cosmic Horizon

  1. Jun 4, 2007 #1
    Is it true that if a luminous object is captured inside the cosmic horizon , then it stays in the horizon forever?
    And is it also true that if the accelerating expansion of the universe continues forever , then galaxies outside our local supercluster will move beyond the cosmic horizon and will no longer be visible?
  2. jcsd
  3. Jun 6, 2007 #2


    User Avatar
    Science Advisor
    Gold Member

    Perhaps, but what is your point? I think everything we see will always be visible - albeit a bit redder in the future.
  4. Jun 6, 2007 #3


    User Avatar
    Science Advisor

    This is true for the particle horizon but not for the event horizon. In usual matter dominated models new objects may enter the particle horizon and no visible objects leave it. Only in the limiting case of a de-Sitter expansion (cosmological constant dominated model) no new objects enter the particle horizon, but also no visible objects leaves it.

    In may move beyond the event horizon and its light emitted in future will never reach us.
    Last edited: Jun 6, 2007
  5. Jun 6, 2007 #4
    My question is : if these assumptions about the horizon and the accelerating expansion hold (say for a flat and matter-dominated universe), then isn't there a contradiction between them?
    If not, is it the acceleration that alters this standard feature of the particle horizon?
  6. Jun 6, 2007 #5


    User Avatar
    Science Advisor

    Both assumptions are true if they relate to two different defintions of horizon as I have pointed out above. In the usual expanding models no objects leave the particle horizon. On the other hand models with accelerated expansion have an event horizon. Objects may cross the cosmological event horizon from inside to outside.
  7. Jun 6, 2007 #6
    Okay this makes sense. Thank you for the information hellfire!
  8. Jun 7, 2007 #7

    Chris Hillman

    User Avatar
    Science Advisor

    Suggest a book

    Captured? [EDIT: OK, I guess Hellfire cleared that up!]

    Ditto Hellfire: if you mean by "capture" what I would mean by "capture", I don't see how both statements could be true, but your second statement is true.

    Just thought I'd add that the undergraduate textbook by D'Inverno, Understanding Einstein's Relativity, offers a fine discussion of simple cosmological models including nonzero Lambda FRW models. As he illustrates with a nice diagram, in such models, pairs of galaxies tend to move beyond each others' cosmological horizon.
    Last edited: Jun 7, 2007
  9. Jun 7, 2007 #8


    User Avatar
    Science Advisor
    Gold Member
    Dearly Missed

    Although Ricky's question has been well-answered, I'll comment on a cause of confusion simply in the language used.

    It is the "outside" of the cosmological event horizon that is analogous to the "inside" of a BH event horizon.

    A flash of light which is aimed at us and traveling towards us, but is at this moment outside the cos. event horiz. will never reach us.

    this is analogous to the situation with a flash of light aimed at us from inside a BH event horizon. It also will never reach us.

    Ricky I think your question about the space inside the ev. horiz. "capturing" something suggests that you have inside and outside spaces confused. It is the space outside the EH which captures (opposite from what one pictures happening with a black hole)
  10. Jun 8, 2007 #9


    User Avatar
    Science Advisor
    Gold Member

    A pretty torturous way of explaining cosmological redshift, IMO. Even if an object within our observable horizon suddenly receded at infinite velocity, it would remain observable to us - forever [at least until its photons redshifted beyond detection]. Similarily, an object falling into a black hole never actually 'disappears', it merely redshifts [slows down] as it approaches the event horizon. You will literally wait for eternity before it disappears from view. A BH, in that sense, simulates a cosmological event horizon.
    Last edited: Jun 8, 2007
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?

Similar Discussions: About Our Cosmic Horizon
  1. Cosmic horizon (Replies: 9)

  2. Cosmic horizon (Replies: 2)