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Will communication ever become impossible?

  1. Apr 14, 2013 #1
    First, I want to apologize for my ignorance and for misplacing this thread if there is a cosmology forum it would better fit into. My question is as follows. The universe is expanding at an ever-accelerating rate. Will the rate of expansion ever exceed the velocity of virtual photons traveling between particles, making communication impossible? Or will the only form of communication be wave interference with vacuum energy?
     
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  3. Apr 14, 2013 #2

    mathman

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    The accelerated expansion affects distances on a large scale. Anything closer, such as galaxies within the same cluster are unaffected.
     
  4. Apr 14, 2013 #3

    marcus

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    When people talk about "accelerated expansion" they are usually referring to the standard cosmic model, socalled LCDM model.

    What that amounts to (I hope you realize this) is that the expansion speed of a distance of some given length has been slowing down and is predicted to continue slowing down but more and more gradually: if the designated distance is one Megaparsed then the speed is currently around 70 km/s and expected to slow down to around 60 km/s. (but not to zero)

    If you don't understand that this is what they mean when they say "accelerated" then you need to ask questions until you get clear about it.

    According to the latest report (Planck mission) the current expansion rate is around 1/144 of one percent per million years and it is expected to continue declining towards around 1/173 of one percent per million years.

    This would not effect our solar system, or Milky galaxy, or our neighbor galaxies. It will not prevent Andromeda galaxy from continuing to approach Milky, and so on. Our local cluster of galaxies will remain held together by its gravity, as usual.

    So the standard model LCDM leaves lots and lots of room for "communication" out into the indefinite future with its acclaimed and highly advertised "accelerated expansion".
     
    Last edited: Apr 14, 2013
  5. Apr 14, 2013 #4
    Mathman, thank you for the response. I'm not sure why there is such a difference in effects however. I was referring to a "heath death" type of scenario. In other words, after acceleration has progressed to the point where no organization above individual particles exists.


    Marcus, it's interesting to hear that the expansion rate works differently than I had thought. My interpretation of your post is as follows. The distance between any two points expands at a certain percent per year. Thus the distance grows exponentially over time. However the rate is slowing.

    Now I'm curious about something else. Does the rate eventually slow toward zero, the point at which there is no cosmic inflation besides normal particle motion?
     
    Last edited: Apr 14, 2013
  6. Apr 14, 2013 #5

    marcus

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    I want to agree and emphasize what Mathman said. this is well-known. standard cosmology does not pull familiar structures like planets, solar systems, galaxies, local groups of galaxies, apart.

    "Heat death" is something different. It does not have a necessary connection with accelerated expansion. People were describing "heat death" many years before 1998 when the "acceleration" of expansion was discovered.

    Heat death refers to misfortunes like stars using up all their fuel, and stars dying, and life becoming more and more difficult as people run out of ideas for making energy. It is a thermodynamic or entropy notion. It does not require "acceleration". I recall people describing heat death end of life back in 1960s. Not to be confused.
     
  7. Apr 14, 2013 #6
    Thank you for the responses. I think that answers my questions.
     
  8. Apr 14, 2013 #7

    marcus

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    Great! Have a look at this picture of the growth over time of the separation between a pair of observers
    http://ned.ipac.caltech.edu/level5/March03/Lineweaver/Figures/figure14.jpg
    the dark solid line is the standard model case.

    It's like starting a savings account in a bank where the bank is constantly and very gradually reducing the rate of interest it pays.

    Today it pays 1/144%, in the distant future it will stabilize at 1/173%. (see post #3)
    If the percentage rate were constant your account would grow exponentially. But as long as they keep cutting back on the interest rate, it does not grow exponentially. However as long as they reduce the rate gradually enough there will still be almost exponential growth. So there is a kind of tradeoff or tug of war between the two.

    You see in the picture that until around 5 or 6 billion years ago the slope was declining---the curve is convex seen from above. that's because the percentage rate was dropping fast. Then around 5 billion years ago the percentage rate was still declining (as it is projected to keep doing) but so gradually that the slope begins to increase. The curve begins to look concave from above, a bit like exponential.

    Increasing slope means "acceleration" (of a single growing separation watched over a long span of time). So in that sense there is acceleration, even though the percentage growth rate is declining, and the Hubble rate is declining, and the growth speed of any fixed given length, like e.g. a million lightyears, is slowing.
     
    Last edited: Apr 14, 2013
  9. Apr 15, 2013 #8
    Thanks for the additional explanation. So the acceleration is neither exponential nor logarithmic, but almost-exponential. Interesting concept.
     
  10. Apr 15, 2013 #9

    marcus

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    You are entirely welcome! I'd like to see how this works: please tell me if this helps or hinders understanding. It may be too much confusing information all at once.
    Go here:
    http://www.einsteins-theory-of-relativity-4engineers.com/TabCosmo9.html

    Don't worry about the other columns in the table, just look at the 3rd column (time T in billions of years) and the rightmost or 9th column labeled a'Ro

    That rightmost column gives the recession speed, as multiples of the speed of light, of some sample matter which today is 14 billion lightyears from us.

    You can see that the table begins in year 378,000 and goes up to around year 88 billion. It covers a large piece of the the history of the universe.

    Back in the year 378,000 (or 0.378 million = 0.000378 billion as it is written in the table) the matter was receding at a speed of 20c, from "us" i.e. from the matter that eventually became us.

    Do you get that from the table, just looking at the 3rd column (time) and the 9th column (speed)?

    You can see the speed slow down and then start speeding up again.

    Other patches of matter would have been doing likewise, proportionally to their distance. For a patch of matter only half as far away, the speeds would all be half as big. For matter twice as far, the speeds would have been twice what is shown. But they would all show the same slowing down, and then speeding up, proportional to their size.

    The table (just looking at the 3rd and 9th columns) shows the same recession speed behavior as the GRAPH you get with that Caltech link I posted in post #7.
    http://ned.ipac.caltech.edu/level5/March03/Lineweaver/Figures/figure14.jpg
    There you also see slowing down at first, and then speeding up, as the distance grows.

    I'm wondering: is this at all interesting for you to see? Or just confusing?

    You can make the table a little nicer, in my view, if you check the box that says something like "S=1.000" and then press "calculate".

    that will make the table twice as long and the present day will come in the middle row, the one labeled S=1, where the time is around year 13.75 billion (the present age of universe).
     
    Last edited: Apr 15, 2013
  11. Apr 15, 2013 #10
    I see, it's as in the chart you posted earlier. a'R0 first decreases and then increases.

    It's very informative. My very limited knowledge of cosmology has been expanded a lot.

    I wonder what happens when S approaches infinity, the recession rate must have started off at some finite level.
     
  12. Apr 15, 2013 #11

    marcus

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    That's a really interesting question and it's getting increased attention these days.
    The work being done, several lines of research being pursued, is not entirely speculative, since there appear to be ways to test different models by observation.

    At extreme density one expects quantum effects which might overwhelm the normal attraction of gravity. In some models only a finite density is reached. The start of expansion is actually a rebound from a prior contraction. The highest density achieved then depends on the model, how the quantum effects of extreme density are calculated.

    The crucial thing is to be able to test quantum cosmology models of the start of expansion by deriving predictions about potentially observable features of the ancient light background (the CMB, its polarization, its spectrum of temperature fluctuations) that can be looked for. If a model predicts stuff that is not found, then it can be ruled out. Falsifiability is an important merit which some models have and others, as yet, do not. So there is a struggle in progress to construct testable models of start of expansion.

    Not all the models involve a quantum bounce at extreme density. However in case any one is curious here is a rather atypical idea which the authors (Steinhardt and Lehners) say is testable and describe in a paper that just came out.
    http://arxiv.org/abs/1304.3122
    Planck 2013 results support the simplest cyclic models
    Jean-Luc Lehners, Paul J. Steinhardt
    (Submitted on 10 Apr 2013)
    We show that results from the Planck satellite reported in 2013 are consistent with the simplest cyclic models for natural parameter ranges i.e., order unity dimensionless coefficients, assuming the standard entropic mechanism for generating curvature perturbations. With improved precision, forthcoming results from Planck and other experiments should be able to test the parameter ranges by confirming or refuting the core predictions - i.e., no observable primordial B-mode polarization and detectable local non-gaussianity. A new prediction, given the Planck 2013 constraints on the bispectrum, is a sharp constraint on the local trispectrum parameter gNL; namely, the simplest models predict it is negative, with gNL < -1700.
    Comments: 5 pages

    A model like this, if it passes tests and gains credence, would answer your question about the very early history of the expansion rate. The rate would be negative (contraction) and rise thru zero ( at bounce time) and then have an humongous spike of very high rate expansion.

    To be honest, Steinhardt and Lehners model is not a favorite of mine. I am not sufficiently familiar with it to say much, but I admire and respect how up front they are about stating specific ways their idea can be falsified by practical observation (e.g. Planck mission).

    I guess the moral of the story is that nobody can answer your question about the early history of the expansion rate because there are competing models of what was going on right around start of expansion and they all need to be tested and some need to be ruled out--falsified--before the dust settles and smoke clears :biggrin:
     
    Last edited: Apr 15, 2013
  13. Apr 15, 2013 #12
    That would help establish a theory, falsification does not necessarily rule out other explanations though. The simplicity of their model still sounds appealing if it can be made to work with unitless coefficients.
     
  14. Apr 15, 2013 #13

    marcus

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    Agreed! I would be happy to see ANY model pass some observational tests and gain credence. It would always be provisional and cautious optimism. People would be keeping an eye out for alternatives. Understanding the start of expansion has a long ways to go but the quest is getting exciting.
     
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