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I Cosmic Horizon

  1. Feb 18, 2017 #1
    Ok..I know that at some point, from the Hubble Law , galaxies will seem to moving away from us speed of light,But actaully they cant because the space-time itself expands so it will be like a black hole horizon,which within that radius its , c/H , we can observe things etc.But out of that radius we cant see anything.And objects that getting close there will appear that they never passed the horizon.

    That radius is approximatly is the age of universe and its approx. 13.7 billion light years.So even we can manage to live a billion year. we will see only 13.7 billion light year away.like whatever we do we cant see more then this...
     
  2. jcsd
  3. Feb 18, 2017 #2

    Bandersnatch

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    No. Hubble radius, which is what you've described, is not a cosmic event horizon. Since the rate of expansion (the value of Hubble constant) goes down with time, we will be able to observe emission events from beyond current Hubble radius, just as today we are observing emission events from beyond past Hubble radii.

    Hubble radius is a cosmic event horizon only in universes where Hubble constant doesn't change with time - i.e. in universes devoid of matter, and with only dark energy (so-called 'de Sitter universe').

    Since in our universe matter is constantly diluted, and Hubble constant is asymptotically approaching a set value, in the far future it will look like a de Sitter universe. The future cosmic event horizon is approx. 17.3 Gly away.

    Note that these are emission events. By the time of reception of their signals, the emitters will have receded much further away, so in a sense it is possible to see objects that are farther than the event horizon - this is what is meant when people say that the observable universe is 90 Gly across.

    There should be an insight on cosmic event horizons in PF library, if you look it up.


    Other than that, it's true that a cosmic event horizon exists, and it is impossible to ever see beyond its maximum (future) extent.
     
  4. Feb 19, 2017 #3
    I assumed H is constant for now..but I understand the idea.
    I didnt understand this.we cant see further then CMB and which its 13.7 billion ly away.I looked wiki and here says "The comoving distance from the Earth to the edge of the visible universe is about 45.7 billion light-years in any direction; this is the comoving radius of the observable universe. This is larger than the age of the universe dictated by the cosmic background radiation; see size of the universe: misconceptions for why this is possible." I looked at it but didnt understand...

    and Could you give me the pf insight article url about this,I couldnt find it

    Thanks
     
  5. Feb 19, 2017 #4

    Bandersnatch

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    It's here:
    https://www.physicsforums.com/insights/inflationary-misconceptions-basics-cosmological-horizons/
    There's also a shorter FAQ entry:
    https://www.physicsforums.com/insights/radius-observable-universe-light-years-greater-age/

    This paper might be useful as well:
    https://arxiv.org/pdf/astro-ph/0310808.pdf
    It includes very helpful lightcone graphs, if you can read them.

    And for the absolute minimum, I've answered a similar question here:
    https://www.physicsforums.com/threa...way-if-the-universe-is-only-13b-years.903822/
    using the 'ant on rubber band' analogy.

    After reading at least some of the above, you should be able to see how this:
    is not true in any sense.
     
  6. Feb 19, 2017 #5
    Are the very distant galaxies subject to progressive relativistic contraction and time dilation with distance, or does their being comoving prevent that?
     
  7. Feb 19, 2017 #6
    I understand why its bigger cause universe is also expanding,so cmbr is in 45 billion ly away ? but it happened 13,7 billion years ago ?
     
  8. Feb 19, 2017 #7

    Bandersnatch

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    Yes, the choice of comoving coordinates eliminates SR effects. The recession velocities that remain are a purely GR effect, outside the domain of applicability of SR.

    If a signal was emitted from A to B, which initially are 1 Gly apart, and the distance between them kept increasing as the signal travelled, then it will take more than 1Gly/c to cover the stretching distance.

    In the particular case of CMBR, it was emitted at the proper distance of ~40 Mly, and it took it 13.7 Gy to get here despite the initially close separation.
     
  9. Feb 19, 2017 #8
    oh you were said this one of your posts..40 million years..
     
  10. Feb 19, 2017 #9

    Chronos

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    The observable universe is unavoidably finite due to the finite speed of light. It cannot be otherwise unless it is infinitely old, or you permit the speed of light to vary [VSL]. All existing data suggests the observable universe is not infinitely old and rules out VSL theories.
     
  11. Feb 19, 2017 #10

    Bandersnatch

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    @Chronos all true, but I don't see the relevance. What are you replying to?
     
  12. Feb 19, 2017 #11

    Chronos

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    Addressing the question of why there must be a cosmic horizon.
     
  13. Feb 19, 2017 #12

    Bandersnatch

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    But 'cosmic horizon' discussed here means 'event horizon' - the distance from which no signal can ever reach us. That is not the same as particle horizon. For example, in a non-expanding universe cosmic horizon doesn't exist, despite finite light speed, and regardless of whether or not it is eternal in the past.
     
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