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  1. #1
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    Default Dark Energy is a push me pull you



    the following is based upon my reading of Brian Greene's Fabric of the Cosmos.

    Dark Energy. also called the Cosmological Constant.
    it's similar to the dr. doolittle character, pushmi-pullyu.

    it pulls it pushes. wow. that amazes me no end.

    random thoughts:
    scientists discover that 70% of the stuff in the universe is unaccounted for.
    oh, there it is. Dark Energy. and it of course has a normal attractive gravitational effect. wonderful.

    but wait. whats this? the universe is expanding? and the expansion is accelerating? why?

    once again, because of Dark Energy. you see Dark Energy, also has negative pressure. that means that it has a repulsive gravitational effect. (Greene phrases it this way. I am only repeating what he wrote.)

    up until ~7 billion years ago, the Dark Energy's regular old fashioned attractive gravitational effect was dominant.
    but then the universe expanded so much that Dark Energy's attractive gravitational effect (which of course is dependent on distance) weakened.
    it weakened to the point that the negative pressure repulsive effect (which by the way is not dependent on distance) became dominant.

    this fascinates me. the tug of war that occurred (and occurs) between 2 different features of the same thing.

  2. #2
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    It has all these amazing properties and may not exist.

    When an offer sounds too good to be true, it usually isn't true. That holds in physics as well.

    I enjoy Dr. Greene's books too. But he does have a tendancy to downplay alternative theories.

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    Default

    I think you are confusing two different concepts: dark matter and dark energy. They are unrelated (as far as we know).

    Dark Matter is the 70% if matter that is unaccounted for. It has normal attractive gravity. We just don't know if it consists of WIMPs (Weakly Interactive Massive Particles) or MACHOs (MAssive Compact Halo Objects), with the weight of opinion currently favouring WIMPs.

    Dark Energy is something different and unrelated. It is a repulsive force that is causing cosmological expansion to accelerate. It would be a mistake to call its effect gravitational until there is some evidence that it is related to gravity. So far there is no evidence at all for anything except that "something must be doing it".

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  6. #4
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    Hi Chas, I don't see it as a "tug of war." We start with 4 second order differential equations and solve for gravity. This is typically written as a tensor equation. The question Einstein had was whether or not to keep the constant.

    If we look at the acceleration of, say, a car it is a second order differential equation - d^2 x/dt^2. We can solve that for velocity or distance. If we solve for velocity there may or may not be an initial velocity. If there is, it appears as a constant. If we solve for distance the same is true. If there was an initial distance it appears as a constant.

    So, in the gr solution for gravity we should think of the constant as the initial condition. There was an acceleration causing expansion in the initial condition. We don't know what to call it, so we call it dark energy. Some people aren't convinced it even exists.

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  8. #5
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    Quote Originally Posted by KeithBC View Post
    I think you are confusing two different concepts: dark matter and dark energy. They are unrelated (as far as we know).

    Dark Matter is the 70% if matter that is unaccounted for. It has normal attractive gravity. We just don't know if it consists of WIMPs (Weakly Interactive Massive Particles) or MACHOs (MAssive Compact Halo Objects), with the weight of opinion currently favouring WIMPs.

    Dark Energy is something different and unrelated. It is a repulsive force that is causing cosmological expansion to accelerate. It would be a mistake to call its effect gravitational until there is some evidence that it is related to gravity. So far there is no evidence at all for anything except that "something must be doing it".
    thank you for your reply Keith. Greene says (I am repeating as best as I remember) that there is 5% baryonic matter, 25% dark matter and 70% dark energy.
    chapter 10 of the book has a bunch of stuff about what I am telling you about. I was amazed as I read it. everybody knows that there's no such thing as a repulsive gravity.
    well according to Greene, yes there is. dark energy has positive energy that makes for regular old attractive gravitational effect that slows the epansion of the universe. it also has negative pressure that works to make a repulsive gravitational force (and it is that feature of dark energy which became dominant 7 billion years ago and caused the universe expansion to accelerate).

    Quote Originally Posted by kencrowder View Post
    Hi Chas, I don't see it as a "tug of war." We start with 4 second order differential equations and solve for gravity. This is typically written as a tensor equation. The question Einstein had was whether or not to keep the constant.

    If we look at the acceleration of, say, a car it is a second order differential equation - d^2 x/dt^2. We can solve that for velocity or distance. If we solve for velocity there may or may not be an initial velocity. If there is, it appears as a constant. If we solve for distance the same is true. If there was an initial distance it appears as a constant.

    So, in the gr solution for gravity we should think of the constant as the initial condition. There was an acceleration causing expansion in the initial condition. We don't know what to call it, so we call it dark energy. Some people aren't convinced it even exists.
    thanks for your great replies Ken. I am not happy when I see the word "constant". I must read what you said about constants over some more and learn to understand them.
    I also should learn about diffy Q's.
    I believe what you say about Greene. I believe its certainly possible that there's no such thing as dark energy. there is plenty of interesting stuff in the chapter 10 though about inflation theory and the inflaton field.

    that said, the presentation in chapter 10 of Fabric of the Cosmos definitely (in my book, har har), describes what I would call a tug of war. dark energy multi-tasks. it has 2 faces. it pushes and it pulls. the pull side was dominant for half of the life of the universe. for the last half, the push side has been dominant. so said mr. greenejeans.

  9. #6
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    We all know what good old-fashioned gravity does, but no one knows for certain what it is.
    I think it's mighty early to start making assertions about dark matter and dark energy, which are both still new discoveries.
    This does not preclude theorizing, on the contrary. But one should be very wary of buying into the first assertion made. (The graveyard of wrong cosmological theories is full to overbrimming).
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    Quote Originally Posted by Voyager3 View Post
    We all know what good old-fashioned gravity does, but no one knows for certain what it is.
    I think it's mighty early to start making assertions about dark matter and dark energy, which are both still new discoveries.
    This does not preclude theorizing, on the contrary. But one should be very wary of buying into the first assertion made. (The graveyard of wrong cosmological theories is full to overbrimming).
    I understand that concept very well, I got shot down very quickly on a new theoretical composition of how gravity could be caused, and lost out on the discussional portion of the concept. The debate is the fun part of theories, but anyways, the concept of repulsive gravitation effect is still not the same as expansion causing the gravity, it's a matter of aspect (pun intended). Expansion and repulsion are in fact two different things but still related to attraction in a negative factor. Sounds like I may have to read some more literature for further insight. Do you have a link to where to find the reading material?

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    Quote Originally Posted by Sparky77 View Post
    ... The debate is the fun part of theories...Do you have a link to where to find the reading material?
    Enjoying the exchange of ideas is a very healthy attitude to have.

    As to links and reading material...
    I'm sorry, but I have absolutely no idea!
    These discoveries concerning Dark Matter and Dark Energy are so new (and so complex) that I doubt very much anything definitive will be published before I go the great beyond. I have little interest or time to keep up with competing theories on such new concepts.
    My philosophy is, although I try keeping abreast of new developments in a general way, I don't scour scientific publications for the latest theory (which I doubt I could understand in any case!). I figure if anything truly significant comes up, everyone will be talking about it anyway.
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    read chapter 10 in brian greene's book the fabric of the cosmos.

    the book was published in february 2004.

    no idea what the author thinks about this stuff 8 years later.

    am passing along what I read. this book was very highly recommended.
    I'll try to remember what greene said. I would recommend double checking. I'll probably be wrong about some of this.

    gravity depends on mass. einstein realized that gravity also depends on energy.
    and it also depends on pressure. a compressed spring has slightly more mass than a spring that is not compressed. it also has pressure. positive pressure. positive pressure causes a regular old fashioned attractive gravitational force.
    but there is also negative pressure. negative pressure sucks inward instead of pushing outward.).
    negative pressure contributes to repulsive gravity.
    here's a link to some of the text. scroll to the end of the passage:

    The Fabric of the Cosmos: Space, Time, and the Texture of Reality - Brian Greene - Google Books


    if that link does not work, google " fabric of the cosmos negative pressure".
    go to the google books page (should be the first one.).
    read page 277. unfortunately page 278 is not there.

    looks like the whole book is there.




    Quote Originally Posted by Sparky77 View Post
    I understand that concept very well, I got shot down very quickly on a new theoretical composition of how gravity could be caused, and lost out on the discussional portion of the concept. The debate is the fun part of theories, but anyways, the concept of repulsive gravitation effect is still not the same as expansion causing the gravity, it's a matter of aspect (pun intended). Expansion and repulsion are in fact two different things but still related to attraction in a negative factor. Sounds like I may have to read some more literature for further insight. Do you have a link to where to find the reading material?

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    There are recent studies which are causing problems with the idea that WIMPs are doing the Dark Matter thing. It's not all sorted out but things could get weirder.

    I still have a totally unsupported sneaking suspicion that Black Holes supply "Dark Energy". I delight in the idea that Black Holes are tied into the fabric of the universe at a very fundamental level and when matter enters a black hole and gets mashed down into the mathematically infinitesimally small point that it energetically becomes effectively the "Dark Energy" by incrementally increasing the size or expansion pressure of space-time. Not exactly the Heisenberg Principle, but you get the idea.

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