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Increasing expantional speed of the universe

  1. Jan 20, 2004 #1
    According to the best current data available to cosmologists today, the universe is not only expanding, but the speed of which it does so is increasing. This is well-known but the cause for it is not.

    There are many different theories, from dark matter and dark energy to ideas revolving around Enstien's Cosmological constant. But the most interesting theories to me are the ones that involve the property of gravity that allows it to be a repelling force rather then a attracting one.

    This brings me to my question. According to GR, gravity's attractive force is caused by a bending of space. My understanding is unfortunely insufficient to determine whether this is a figurative or literal. Is space-time actually bending, or is this just an extraordinarily well manufactored analogy to something that our mind's can not grasp directly?

    If it is space bending could we take it one step further and say that the bending of space has some sort of reaction, such as a recipical and opposite curvature outside the ring of gravities normal effect. And when I say "ring" of effect I am recalling the often used waterbed analogy to gravity in which one imagines a bowling ball placed on a waterbed, the curvature could be thought analgous to gravity.

    When I imagine that analogy I can't help but to think about the parts of the water bed surface that are effected other then obvious depression created. Although the change in curvature in the areas outsite the ring are far less pronounced because they are spread out over greater area could these be responsible for gravity's repulsive force? Can one of those who are far more knowledgeable then myself here share some insight that could disprove or support these ideas?
     
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  3. Jan 20, 2004 #2

    Nereid

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    Welcome to Physics Forums Deeviant!
    No; gravity does not have a 'repulsive force', over the ranges of mass and distances we've observed to date, and that's a mighty big range (<1mm and <1mg, to >100 Mpc and at least ~10 times the mass of the Sun, maybe 1011 solar masses or more).
     
  4. Jan 20, 2004 #3
    Thank for your reply, although its not really what I was hoping for. Yes, I should have said:

    Deeviant wrote: Although the change in curvature in the areas outsite the ring are far less pronounced because they are spread out over greater area could these lead to the possibility that gravity could, at these long distances, be a repulsive force.

    It doesn't seem like you read into much of what I was saying as you addressed none of the questions I had.

    The Big Bang theory would not work without the assumption that under some conditions gravity can be a repulsive force.

    I am not stating gravity repulsion as a fact but asking of the possiblity. It would solve a one of the major questions in modern cosmology. Gravity is the least understood of the 4 primal forces in physics, to state any stone-cold fact about its nature at this point would be foolhardy as our understanding of it, and many other things is incomplete at best. However, if gravity(its possible replusive behavior) was responsible for the speeding up of the expansion of the univers, it would be a simple explanation.

    But anyways, I digress. I'm really just looking for somebody to review my conceptionalization of gravity perhaps provide me further insight into the matter.
     
  5. Jan 20, 2004 #4

    Nereid

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    Excuse my confusion; perhaps some clarity in terminology is in order:
    - cosmology is the study of the universe, its origin, equation of state, etc
    - the BB theory is one cosmological theory; roughly, the universe began small, dense, and hot and expanded
    - gravity is the force between mass, as characterised by Einstein's GR
    - GR has been tested and shown to be consistent with observations to an extraordinary degree
    - GR cannot be a complete theory, because it doesn't incorporate 'the very small'; Quantum theory does, but it too breaks down at very large masses (and very small distances)
    - dark matter has been observed in many regimes, and has properties that are consistent across those regimes
    - 'dark energy' is the mysterious one; OTOH a cosmological constant seems to match the data; OTOH there are wild inconsistencies
    - several ideas about dark energy are around; IMHO, it's not yet entirely clear we need the concept

    Perhaps you could ask again? In particular, why do you think that, if a replusive force of some kind (e.g. 'dark energy') is observed, it has to be gravity? How could you tell?
     
  6. Jan 20, 2004 #5
    I'm sorry but my writing must have been hard to follow, I know my flow is not very smooth so its hard to read the whole thing through.

    I don't make the logical conclusion that since there is a repulsive force acting upon large cosmic bodies, that it must be gravity acting strangly. There are a great many theories as to explain the strange quickening expansion of the universe but none seem to have gained dominance or favor. I simply suggested that it could be caused by gravity.

    The reason for me stating this is the conceptual picture I described in my first post, which is really part that I am most interested in. The rest of the theories and concepts were really just laying down the framework from which I based my hypothesis.

    edit: spelling
     
  7. Jan 21, 2004 #6
    That is incorrect. You're refering to tidal forces. Tidal forces and spacetime curvature are the same thing. However there is no reason to assume that the presence of a gravitational force means that there is a presence of spacetime curvature.
    Spacetime is a mathematical object which we use to describe space and time. When curvature is present the manifold has the properties of a curved 4-dimensional surface. That manifold describes space and time.
    I can't see how your idea is very meaningful since that water bed analogy describes spatial curvature, not neccesarily spacetime curvature, there is a difference. Plus that analogy you have in mind regards a single planet - not the entire universe.

    On a cosmological scale, however, you are ou are correct in that there is a repulsive gravitational force. That is what Einstein's cosmological constant does. It represents gravitational repulsion.

    Alan Guth gave a seminar at MIT on this a few years ago. See
    http://arcturus.mit.edu/8.224/Seminars/guth-talk.pdf
     
  8. Jan 21, 2004 #7
    That was exactely the type and quality of information I was looking for, Thanks Arcon.
     
  9. Jan 22, 2004 #8
    You're most welcome. You may find this an interesting comment. Gravitational repulsion may be explained by a non-zero cosmological constant. Some people used to call this the "cosmical constant."

    I just came across an interesting comment. From On the "Derivation" of Einstein's Field Equations, S. Chandrasekhar, Am. J. Phys., Vol. 40, Feb. 1972, page 234
     
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