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Expansion of the Universe

  1. May 29, 2012 #1

    With the assumption that hubble parameter 'H_0' from v=(H_0)*r is constant,
    I would like to ask you a question.

    Will there be more and more objects that will not be visible as time goes by?
    I ask this because, if some object is initially
    some 'x' light years away, say, "3billion",

    after certain amount of time, because it is moving away from us with velocity v=(H_0)*(x), it will be farther away from us.
    Then, if H_0 is held constant, it will now be moving away from us at a greater velocity.
    v=(H_0)*(x+Δx)... If this goes on until v becomes greater than 'c', the speed of light,
    the light that object will no longer be visible. Therefore the object will be no longer visible. Is my reasoning correct? If it is, does that mean the number of objects visible for us is decreasing in numbers?

    Thank you for your help.
  2. jcsd
  3. May 29, 2012 #2
    That is correct. For constant H, the motion of galaxies becomes exponential in the distant future.

    In the past when the energy density of the universe was dominated by matter (or earlier by radiation), the Hubble Constant wasn't constant, it was inversely proportional to co-moving time but matter density falls with expansion while dark energy density seems to be constant so H is tending towards a constant value.

    It also means there is a "particle horizon" beyond which material can never influence us.
  4. May 29, 2012 #3


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    That would be an event horizon, not a particle horizon. The particle horizon is determined by the distance that light has traveled since the big bang and is therefore growing.
  5. May 29, 2012 #4
    can any one tell me more about the "particle horizon" ? please.....
    Last edited by a moderator: May 30, 2012
  6. May 29, 2012 #5


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  7. May 29, 2012 #6
    Oops, thanks for the correction.
  8. May 30, 2012 #7
    as noted already, yes.....

    Galaxies are separating at an increasing rate due to the acceleration of cosmological expansion. As we move towards a more and more energy dominated phase of our universe, as that expansion rate increases which we are in the beginning stages of already,
    fewer and fewer galaxies will eventually be 'visible' to us. It's going to be REALLY dark out there billions of years from now!!!
  9. May 30, 2012 #8


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    Yes, more objects [galaxies, etc] will become visible over time, but, these will not be 'new', rather they will be evolved versions of structures that already exist in the observable universe. The CMB will always be the most distant EM 'structure' possible to 'see' in the observable universe.
  10. May 31, 2012 #9
    All, Can I ask a couple of follow up questions?

    1. I appreciate that with (c) expansion of space, distant objects are moving beyond the event horizon and thus out of view. But, given that light from those distant objects is travelling toward us (at c) does the expansion of the event horizon not keep these objects within the event horizon (and thus visible to us)?

    2. I have read in other threads about how when expansion is just greater than c, light for these objects can still reach us (i.e. I think that the object is just disappearing over the horizon due to expansion but after a period of time, the light from the object is moving forward through expanding space so that the amount of space in front (towards us) is reducing so that although it is expanding, eventually the light crosses the event horizon and thus eventually arrives). Is this that type of instance?


  11. Jun 2, 2012 #10
    The answers are different in two key regimes. In the "early" universe, the density of matter (and before that radiation) was high enough for the expansion to be slowing. That was the case for roughly the first 6 billion years.

    Like an object fired upward from Earth, as long as it starts with greater than escape speed, it will always be moving away and always slowing. If the speed is exactly escape speed, it is asymptotic to zero.

    In the matter-dominated universe, galaxies were moving away but also slowing down. Light emitted from any source towards us must always be between it and us. Even if the source was moving away at many times the speed of light, because the source is always slowing, eventually the rate of recession must fall below c. That distance is called the Hubble Length and since the Hubble Constant is inversely proportional to time, the Hubble Length was proportional to the comsological age.

    Since the light is closer to us, the local matter it is passing would fall below that rate even earlier and thereafter the light must proceed towards us hence will reach us. Given infinite time, we would see light from any arbitrarily large distance.

    That changes when we include dark energy. It seems to have a constant density so as matter is thinned out by expansion, dark energy comes to dominate. The scale factor a(t) then becomes exponential and since the derivative of that a'(t) is also an exponential, the Hubble Constant which is a'(t)/a(t) will also be constant. The Hubble Length also then becomes constant and the current length is asymptotic to that final value. Any light emitted from beyond that final distance will always be in an environment where the distance to us is increasing faster than c so it will never reach us.

    That is correct though not necessarily "just greater than c". The microwaves we see as the CMB was emitted from material that was 42 million light years away, a distance that was increasing by more than 65 light years per year when it was emitted; that material is now about 45 billion light years away and the space between us is currently expanding by only 3.3 light years per year. The redshift is z=1089.

  12. Jun 2, 2012 #11
    Thanks George. I thought that the questions required just yes / no answers, so I really appreciate the detail. Thanks again.


  13. Jun 2, 2012 #12
    No problem, I'm just learning myself so if my post is not quite right, usually someone will correct it and I learn more.
  14. Jun 2, 2012 #13
    So only the Milky Way (or the local cluster) will become the obervable univers from the position of Earth? How many billion years are we talking about before that happens?

    And if intelligent life evolves in the Milky Way at that time, will they be able to deduce that there are other galaxies beyond the horizon?
  15. Jun 2, 2012 #14


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    Assuming that the standard cosmic model LambdaCDM is right, which is our kind of working assumption that questions are answered based on, then in 100 billion years from now would-be cosmologists will be in a sad fix.

    This is described in lugubrious detail by Larry Krauss and Bob Scherrer in this paper:
    Look on page 4 for the bad news about the CMB its intensity will have gone down by 12 orders of magnitude and its wavelength will have stretched out to about 1 meter!
    No longer "microwave" background and probably too feeble for anything to detect.

    Our view of the cosmos depends so much on our being able to study that one thing the CMB. Also other galaxies (outside our small handful of local group galaxies which are bound together with us) will have gone beyond the cosmic event horizon.

    Parts of that essay are written for general audience, so much of it is quite accessible no matter what your background. Have a look.
  16. Jun 2, 2012 #15
    But then there is this paper

    Disturbing Implications of a Cosmological Constant

    The idea is that as galaxies disappear through the cosmic horizon then this outer horizon may act like the event horizon of a black hole. This will generate something similar to Hawking radiation.

    The disturbing part is that they argue that the total amount of energy in a finite part of the universe will stay finite. This means that if you want long enough (and it's a really, really, really long time). The matter and energy in the universe will randomly reorganize themselves such that the universe repeats itself.

    The other thing is that "anthropic arguments" have gotten somewhat fashionable. For example, there is this idea that a universe in which cosmology was impossible would be one in which intelligent life could not really exist, and therefore we'd find ourselves in some sort of universe in which we can do cosmology.


    Testable anthropic predictions for dark energy
  17. Jun 2, 2012 #16
    Something that cosmologists find interesting is the "cosmic coincidence problem". The problem is that if we did observations in the past, then dark energy would be irrelevant and undetectable. If we did observations in the future, then the entire universe would be dark energy, and matter would be irrelevant.

    So the puzzle is that we are doing observations right at the point at which dark energy is *starting* to matter but isn't dominating the universe. That's a weird coincidence.

    Or is it.....
  18. Jun 3, 2012 #17
    Thanks, that was a very interesting essay.

    100 billion years is not much in the grand scheme of things. So an observable universe where we can observe billions of other galaxies is just a passing phase.
    We really do live in an intersting time.
  19. Jun 3, 2012 #18
    I have been thinking and trying to understand this but have a difficulty. If the event horizon stands still (in relation to the expanding space), I can see how objects / galaxies move over the horizon and out of sight - this I understand. But if the horizon represents the limit from which light has been received, isn't this growing in size at the speed of light, and thus anything this side of the horizon now, will remain visible / inside the horizon indefinitely? If this is the case, nothing would disappear (over the horizon) and so everything would always be visible.

    I appreciate that this is an obvious "flaw" and so I assume that the flaw is actually in my logic ... but I can't figure out where!

    All help greatly appreciated.


  20. Jun 3, 2012 #19


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    You are talking about two different horizons, the cosmic event horizon (about 15 billion LY) and the socalled Particle Horizon (about 45 billion LY).

    the CEH is the distance to a galaxy which if you started for it TODAY at the speed of light you could never reach. Or if, TODAY, somebody sent you a signal, or a star blew up, we would never get the signal or see the flash, no matter how many billions of years we waited around for it.

    the CEH distance is changing but only very slowly. it is approaching a limit where it will stabilize.

    But the CEH is not the limit of the currently observable portion of the universe! That is growing rapidly as light comes in from more and more distant matter. It is called the Particle Horizon and it is the distance TODAY of the matter which emitted light or other radiation (a long time ago) which we are getting today.
    So the PH is the distance of farthest matter we could in principle be seeing today.* The PH is the distance now of matter which we can see as it was earlier. It is 45-some billion LY.

    *To see the whole distance we would need a neutrino camera or neutrino eyes because neutrinos go thru the fog of glowing hot gas. But just with ordinary light we see matter that is ALMOST 45 billion LY away. It emitted what we see as the CMB, the ancient light now redshifted to microwave background wavelengths. That is effectively our particle horizon but in principle the real PH is slightly farther.
    Last edited: Jun 3, 2012
  21. Jun 3, 2012 #20
    Thanks Marcus.

    Can I just check something: the PH is at a greater distance (than the CEH) - but we can't tarval to (or receive light from) the CEH, although we are (just) receiving light from the PH. Have I mixed up something here?


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