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Is There Any Difference Between A Stretching or Expanding Universe?

  1. Nov 23, 2009 #1
    I have been reading Cosmology related topics for about eighteen-months. The other day I was reading from the following website: http://wmap.gsfc.nasa.gov/site/faq.html

    Under Item 2. is this sentence.

    "Astronomers see galaxies moving apart from one another: space in the universe is stretching."

    In my eighteen-months of reading Cosmology subjects, this is the first time that I have ever read the word stretching. Every other occurrence, as I recall, used the term expanding.

    What is the difference, if any, between a stretching or expanding Universe?

    Thank you in advance to those that respond.
  2. jcsd
  3. Nov 24, 2009 #2


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    Just different words to imperfectly describe the true behavior (which is only properly described mathematically).
  4. Nov 24, 2009 #3
    streching, expanding, growing, I would all take to mean the same thing: distances between unconnected objects are growing.
  5. Nov 24, 2009 #4
    ... or light speed (from God's eyes view) is decreasing. As frog observers expect it to be constant, they interpret it as 'distances are growing'

    Just different words to imperfectly describe the true behavior (which is only properly described mathematically). (c)
  6. Nov 24, 2009 #5
    Really? But since cosmic expansion only occurs on large scales, does that mean the speed of light is only slower on large scales? I don't believe that. (sorry, I just can't without more explanation).
  7. Nov 24, 2009 #6
    Cosmic expansion occurs on ANY scale, but small systems (stars, galaxies, clusters) have already 'adapted' to the expansion. However, even such system are affected by the expansion, because H varies in time, and tidal forces change. System, perfectly adapted now will not be perfectly adapted later. But the effect is very tiny.
  8. Nov 24, 2009 #7

    AFAIK no one knows how to apply 'space expansion' at a local scale (galaxies, stars, planet orbits, atoms,...
    "Cosmic expansion occurs on ANY scale,..." (and everything afterwords) is unknown to me.

    "...have already 'adapted' to the expansion..." How is it?
  9. Nov 24, 2009 #8
    There was a really good article about it...

    So Earth is very very very slowly falling on Sun. At the same time, Universe is expanding, making the distance larger. The total effect is 0, so Earth orbit is stable.
  10. Nov 24, 2009 #9
    You should not worry about expansion. What was recedeing in the past will continue to do so in the future. It is the dark energy that you are talking about.
    Equilibrium for any system from electron to a galaxy can not be the same with or without DE.
  11. Nov 24, 2009 #10


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    "Expansion" etc. = Distances increase, everywhere. Nothing more.
    On a local scale, "Distance increases" has a name: velocity.
    "adapted to expansion" then means: no velocity.
    And that's it. Applying the "expansion"-idea to local physics is at least misleading.
    Which does not mean that it is wrong. It just provides a totally wrong intuition, causing people to make errors in their reasoning.
  12. Nov 24, 2009 #11
    Are you saying the speed of light is decreasing universally? I cannot believe this. How have I graduated in physics specialising in cosmology, without hearing of this interpretation?
  13. Nov 24, 2009 #12


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    I fail to understand how that could be gleaned from what Dmitry67 said.
  14. Nov 24, 2009 #13
    It is quite obvious.

    An expanding space with constant c is equivalent to non-expanding space with decreasing c. (from birds perspective)

    In the baloon analogy, just use polar coordinates. c is decreasing in such coordinates.

    Of course, all observers would get the same constant c, but they will see that distant galaxies are receding, because it takes more and more time for the light to go from there.

    So both interpretations are equivalent. But I prefer mine (where c is decreasing) because it does not have a singularity in case of Big Rip.
  15. Nov 24, 2009 #14


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    Any real singularity is independent of your coordinate system.
  16. Nov 24, 2009 #15
    Big Rip - infinite tidal forces, infinitely accelerated inflation... singularity.

    But in my coordinate system it just means:

    birds-eye 'c' -> 0

    So light becomes travel infinitely slow and all objects 'appear' infinitely far away from each other.
  17. Nov 24, 2009 #16


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    Like I said, any real singularity is independent of coordinates. But I don't quite see how the "big rip" is a singularity anyway, as it's not a point of infinite density. It's only a singularity by way of analogy.

    The "big rip" only gives you infinitely accelerated inflation after infinite time anyway.
  18. Nov 24, 2009 #17
    Big Rip happens in finite time


    The Big Rip is a cosmological hypothesis first published in 2003, about the ultimate fate of the universe, in which the matter of the universe, from stars and galaxies to atoms and subatomic particles, are progressively torn apart by the expansion of the universe at a certain time in the future. Theoretically, the scale factor of the universe becomes infinite at a finite time in the future.
  19. Nov 25, 2009 #18


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    Hmmm, perhaps, but it's still not the same sort of singularity. Things do go infinite, this is true, but all you're presenting are different coordinates, not anything that fundamentally changes the behavior. It makes sense to use different coordinates to describe different physical situations.
  20. Nov 25, 2009 #19
    WOW!!!!! Thanks for all the responses. So, in summary, stretching and expanding basically amount to the same concept. However, the mathematical model(s) more accurately depict the reality of the Universe's expansion.

    Since I am obviously not an expert on this subject, I will assume that Hubble's Law (v = H x D) is one of these mathematical models. Are there others?

    Also, as I understand it, based on years of observations, the consensus of the scientific community is that the distance between each (non-gravitationally bound) galaxy in the observable Universe is increasing with time. How exactly does this work? That is, can someone please explain this process in (preferably) non-mathematical language? However, if it is convenient to explain this only in mathematical terms, then please do so. Actually, if there is a website that explains this plainly, I would prefer this option.

    My real hang-up with this concept is how do these galaxies move in three-dimensional space as opposed to the balloon analogy of two-dimensional space. What forces (if any) are involved and how does the SPACE/DISTANCE between these galaxies increase? In the balloon analogy, I understand that the stretching of the balloon's surface as it inflates causes the dots (which represent the galaxies) to all move away from each other. How does the Universe accomplish this same task?

    Thank you in advance to those that respond.
  21. Nov 25, 2009 #20


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    That's basically it, but we need to use General Relativity to determine precisely what the relationship between 'v' and 'd' is in that equation.

    That's basically what the expansion means. All sorts of analogies have been used to explain this. One that we might consider is a rubber sheet: take a large rubber sheet, and draw lots of dots on it. Each dot represent a galaxy. If you stretch the sheet evenly in each direction, all of those dots will get further away.

    This is the same basic idea as the expansion.

    As minor caveat, though, let me just stress that the expansion is an average effect on large scales: on smaller scales, overdense systems tend to collapse, so there are, locally, many galaxies moving towards one another.

    The only force that acts on such large distances is gravity. The basic picture is that the initial conditions were such that they were all moving away from one another to begin with, and gravity modifies how they move apart with time depending upon the contents of our universe. Early-on, the gravitational effect of the matter and radiation caused the expansion to slow. More recently, the matter and radiation have diluted away so much that another form of energy density seems to have taken over and caused an acceleration of the expansion due to how it interacts with gravity (caveat: it may also be because we don't understand how gravity acts on very large distance scales, this is currently under investigation).
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