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Time Dilation vs Percieved age of Universe

  1. Oct 26, 2012 #1
    I've pondered a hypothetical problem

    Take as time perspective "A" as being relative time dilation of Earth
    Time dilation "B" being half of "A" slower due to relativistic effects.

    assuming both "A" and "B" were both formed at the same time say 5 billion years ago.

    If you were to measure the age of the universe with "A" getting the result of 13.7 billion years.

    would "B" see the age as 11.2 billion years ? or would "B" still measure the age of the universe at 13.7 Billion years?
  2. jcsd
  3. Oct 28, 2012 #2


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  4. Oct 28, 2012 #3
    The last line of the FAQ is good us amateurs:

    The rest of the FAQ may not translate so easily to the form of this posted question.

    In fact, upon reading this question posed here more closely, I do not understand exactly what is being asked. But we can say is that if one observer is measuring duration relative to being at rest with respect to the CMBR, and another is moving with respect to the CMBR, they will get different cosmological measures....
  5. Oct 28, 2012 #4


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  6. Oct 28, 2012 #5


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    Sound from a supersonic jet can reach you, right? So the fact that the source is going away faster than the signal velocity does not imply that the signal won't reach us.

    Suppose a hare chooses to send signals back to its den via tortoises. Each tortoise that the hare sends home will eventually make it even though the hare is going much faster than the tortoises.
  7. Oct 28, 2012 #6
    phinds....yes..that's a tricky one....Seems off target at first.....
    I did not understand until previous discussions in these forums:

    A simple version: In the past when the cosmological expansion was decelerating, we could see more and more over time; but in the future it is accelerating, so we'll be able to see less and less....eventually we'll not be able to see anything except local stars.

    A more complete answer:

    "I have read in other threads 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?"

    "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 when emitted, 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 now is z=1089.
    Marcus: Assuming that the standard cosmic model Lambda CDM is right… then in 100 billion years from now would-be cosmologists will be in a sad fix..."

    This is described in:

    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.

    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.


    You are talking about two different horizons, the cosmic event horizon (about 15 billion LY) and the so called 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.

    Last edited: Oct 28, 2012
  8. Oct 28, 2012 #7


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    Ah ... right. I think I had actually gotten my head wrapped aound that some time back but had forgotten, and it's not (to me anyway) immediately obvious. Thanks.

  9. Oct 29, 2012 #8
    The problem I was having with the question was of this nature from perspective "A" 5 billion is the age of their planet and looking at planet "B" only 2.5 billion years had elapsed due to relative time view points. From the prespective of "B" 5 billion years is also the age of their planet however time looking at "A" time is moving faster. Light from various locales would arrive at both at the same time.
    I had not realized there was a FAG on this should have looked. I was essentially wondering how "B" would compensate for its time dilation as its time runs slower than "A" and yet light would reach it at the same time as it would on "A". The Fag sort of helps on that

    This type of problem can make head spins so I may be overthinking it and getting twisted around lol
  10. Oct 29, 2012 #9
    A totally different scenario with totally different implications. Is the air through which the jet is flying moving at supersonic velocity wrt observer? An obvious answer.
    Again - totally different scenario. Is the ground the hare is running on moving at supersonic speed wrt den? Obvious answer. I have an admittedly very rudimentary grasp of cosmology in general, but believe the following to be true. The idea of distant recession owing to Hubble expansion assumes it is space that expands faster than c wrt local observer - hence also any distant objects co-moving within that space. On my reading anyway that implies emitted light from such a distant galaxy is and always will be beyond our own horizon - assuming Hubble expansion continues as it has. Things get worse, not better with passing time in that regard. Distant objects now just visible will redshift to invisibility in the future - recede beyond our horizon.
  11. Oct 29, 2012 #10
    I'm sure we have all seen the current measurement for the expansion rate in recent articles

    74.2 ±3.6 kilometers/second/megaparsec

    thought I would post that from sevral articles I've read
  12. Oct 29, 2012 #11

    You'll have to define '.....as it has'.....slowing as in the past or accelerating as recently and the future.....

    You can verify the first few lines of my post #6 here [this one sure
    is more precise]:


    which says:

    edit: add.."Objects at the Hubble limit have an average comoving speed of c relative to an observer on the Earth so that, in a universe with constant Hubble parameter, light emitted at the present time by objects outside the Hubble limit would never be seen by an observer on Earth. That is, Hubble limit would coincide with a cosmological event horizon (a boundary separating events visible at some time and those that are never visible[6])."

  13. Oct 29, 2012 #12

    Now I get it! :wink:
  14. Oct 29, 2012 #13
    Think about the Doppler effect! What would be going on there?

    The tortoise(s) still move at the same speed, but have less energy if I picture it right.
  15. Oct 29, 2012 #14
    Fair point. I meant primarily just extrapolating assuming constant Hubble parameter. That would be a coasting cosmos scenario. Of course accelerating expansion as observed just exacerbates things as per your second Wiki quote. Your #6 did cover the ground pretty well and I must admit to only cursorily reading it. Anyway unless there is some weird definitions involved, it logically follows that if any two galaxies were always receding from each other faster than c, no light can have ever passed from one to the other - i.e. they were always beyond each other's horizon. Otherwise 'receding faster than c' becomes a meaningless phrase.
    [EDIT: Evidently being able to see objects that are and have been receding faster than c gets down to Hubble 'constant' being anything but that when looked at over total BB universe time frame, as reading sect. 3.3 here makes evident. So ok depending on definitions used, one can after all make that statement I objected to.]
    Last edited: Oct 29, 2012
  16. Oct 29, 2012 #15


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    Woah! You mean that hares and tortoises aren't exactly the same as the FLRW metric in every way? :surprised

    I wasn't addressing the FLRW metric or cosmology at all. I was addressing phinds' question about how you can receive a signal when the source is moving faster than the signal propagation speed. I believe that the analogies get at the heart of the question, and they were not intended to get at the heart of cosmology.
  17. Oct 29, 2012 #16
    If you say so. Must have been me way off in thinking that context of phind's query was very much cosmology based - receding galaxies and stuff. Anyway I now appreciate a bit more the subtleties and pitfalls involved in such an inherently non-linear spacetime arena.
  18. Nov 3, 2012 #17

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