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What accounts for the expansion of the Universe accelerating?

  1. Oct 15, 2007 #1
    If big bang theory is true, what could cause the expansion of the universe to actually speed up AFTER the explosion?

    Wouldn't there have to be some "force" from OUTSIDE the universe for the expansion to speed up after initial conditions were set in motion?

    It is still considered true that the expansion is acceleration, no?
     
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  3. Oct 15, 2007 #2

    SpaceTiger

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    Here's a good explanation for how one can get accelerated expansion:

    http://arxiv.org/abs/astro-ph/0101507

    The paper is talking specifically about inflation, but the general idea works for the cosmological constant and "dark energy" as well. Your question is answered in the first few pages, so you don't have to read the whole thing.
     
  4. Oct 15, 2007 #3

    marcus

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    Always interesting to see what papers people recommend to handle frequently asked questions. I will unpack the abstract on this one:
    http://arxiv.org/abs/astro-ph/0101507
    Eternal Inflation
    Authors: Alan H. Guth (MIT)
    (Submitted on 29 Jan 2001)

    Abstract: The basic workings of inflationary models are summarized, along with the arguments that strongly suggest that our universe is the product of inflation. It is argued that essentially all inflationary models lead to (future-)eternal inflation, which implies that an infinite number of pocket universes are produced. Although the other pocket universes are unobservable, their existence nonetheless has consequences for the way that we evaluate theories and extract consequences from them. The question of whether the universe had a beginning is discussed but not definitively answered. It appears likely, however, that eternally inflating universes do require a beginning.

    Comments: 15 pages, including 2 figures, LaTeX 2.09. Review talk given at the conference on "Cosmic Questions,'' April 14-16, 1999, Washington, D.C., organized by the Dialogue on Science, Ethics, and Religion of the American Association for the Advancement of Science. To be published in the proceedings, The New York Academy of Sciences Press

    =============
    I remember reading that back when!
     
  5. Oct 15, 2007 #4

    marcus

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    I think SpaceTiger answered your second question by implication---to spell it out explicitly, YES it is still considered that the expansion of space is accelerating.

    I don't think there is any proven explanation of how that happens---AFAIK there is no proven mechanism. There are several possible explanations for it. In any case, the expansion is observed to have been accelerating. Observed, like with telescopes. Observations of this for the past ten years seem to be in approximate agreement.

    As far as mechanisms go, the peer-review journal called General Relativity and Gravitation is devoting a special issue to different people's ideas of why there should be this acceleration. I've seen preprints of several of the articles that will be included. It looks to me like there will be roughly about as many explanations proposed as there are papers submitted. This special issue of GRG will be edited by three eminent experts: George Ellis (co-author with Hawking of a book called The Large Scale Structure of Spacetime), Roy Maartens of Portsmouth, Hermann Nicolai of Potsdam Albert Einstein Institute.

    To the best of my ability to interpret, SpaceTiger, in his excellent reply, was careful not to suggest that he was pointing you to the one and only explanation but instead said "here's a good explanation of how one can get accelerated expansion". I believe that means a good explantion of ONE POSSIBLE WAY one can get it. Perhaps it is a way that is currently favored by an overwhelming majority of some sample of professionals.

    Stick around and see what the lineup of explanations is when that issue of GRG comes out. IMHO people have a fair piece to go before they can be sure they understand the observed acceleration, and there may be interesting twists and turns along the way.
     
    Last edited: Oct 15, 2007
  6. Oct 17, 2007 #5

    IMP

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    This explaination is for very large scales. If the expansion rate is forever increasing, will it eventually become noticable on small scales? If I was immortal, will my hand eventually move away from my foot at a speed greater than C?
     
    Last edited: Oct 17, 2007
  7. Oct 17, 2007 #6

    pervect

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    Not unless we have something like a second inflationary episode. You might want to look at http://www.arxiv.org/abs/0707.0380

    section 2.6.2 "is everything expanding"

    The authors then point out that such an object must experience a sort of tidal stretching force that is proportional to the rate of accleration of expansion of the universe and give a detailed calculation, which I'll omit. This calculation assumes that the object has no "self gravity".

    Another good paper on the expansion of the universe is http://arxiv.org/abs/astro-ph/0310808

    though I think that the first paper addresses your point more directly.

    You might also be interested if you have the math to see that De-sitter spacetimes are actually static.

    See for instance http://en.wikipedia.org/w/index.php?title=De_Sitter_space&oldid=150632530#Static_coordinates

    for the "static coordinates".
     
    Last edited: Oct 17, 2007
  8. Oct 17, 2007 #7

    hellfire

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    This is an interesting question. It will not necessarily happen in the standard cosmological model. Although expansion will be always accelerating (the deceleration parameter will be always negative), the Hubble parameter (expansion speed per unit length) will never exceed a maximum value. However, if the dark energy behaves as a phantom energy (equation-of-state parameter w < -1) then the Hubble parameter will increase without any limit.
     
  9. Oct 18, 2007 #8

    IMP

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    Here is a crazy thought

    I removed my post and started a new thread.
     
    Last edited: Oct 18, 2007
  10. Oct 18, 2007 #9
    Dark Energy

    Hi Holocene,

    Perhaps some of these responses go beyond the simple question you asked. According to the most accepted current theory, the acceleration of the expansion of the universe is due solely to the rather non-intuitive effects of Dark Energy, which is an implementation model of the cosmological constant. Wikipedia has a very accessible article on Dark Energy, which contains references to other related articles. Here is a quote from it:

    "Strangely, dark energy causes expansion because it has strong negative pressure.

    A substance has positive pressure when it pushes outward on its surroundings. This is the usual situation for fluids. Negative pressure, or tension, exists when the substance instead pulls on its surroundings. A common example of negative pressure occurs when a solid is stretched to support a hanging weight.

    According to the Friedmann-Lemaître-Robertson-Walker metric, which is an application of General Relativity to cosmology, the pressure within a substance contributes to its gravitational attraction for other things just as its mass density does. Negative pressure causes a gravitational repulsion.

    The gravitational repulsive effect of dark energy's negative pressure is greater than the gravitational attraction caused by the energy itself. At the cosmological scale, it also overwhelms all other forms of gravitational attraction, resulting in the accelerating expansion of the universe.

    One might wonder, how can pushing cause attraction? How can pulling cause repulsion? This sounds like a contradiction. The solution is:

    The pushing of positive pressure (and the pulling of negative pressure) are non-gravitational forces which just move substances around within space without changing space itself.

    But the gravitational attraction (or repulsion) they cause operates on space itself, decreasing (or increasing) the amount of space between things. It is this which determines the size of the universe.

    There is no necessity that these two effects should act in the same direction. In fact, they act in opposite directions."

    You also seem to be asking whether the creation of new mass/energy in the universe is a violation of the laws of conservation of energy. That would be intuitively correct, but it turns out that that is not how the universe actually works. The universe is believed to have gained and lost enormous amounts of mass/energy at various times in its history. The explanation is that this energy is converted to "potential energy", which is the theoretical "negative" counterpart of familiar forms of "positive" energy such as mass, electromagnetic energy, and kinetic energy. Potential energy is viewed as a bottomless bank account of energy that can be "borrowed" against by the universe, through the workings of gravity. By custom, potential energy is viewed as always being a "negative" form of energy. So the more "positive" energy there is in the universe, the more "negative" the borrowing against the potential energy "account" is. The underlying assumption is that "total energy" is always exactly zero, because it is the sum of positive energy and (negative) potential energy. Wikipedia has a good article on potential energy as well.

    Jon
     
    Last edited: Oct 18, 2007
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