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Thread: Dark Energy and Gravity

  1. #1 Dark Energy and Gravity 
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    Recently, I attended a lecture at an open day for a university I'm considering about "Particle Physics and Mapping the Invisible with the Most Massive Objects in the Universe". The professor was discussing various aspects of particle physics and cosmology (including the standard model, gravitational lensing in cosmology etc). One topic grabbed my attention particularly, though, it was the hypothesis that gravity is mediated by particles called gravitons (and exhibits wave-particle duality through gravitational waves- as yet unobserved) and that dark energy (theorised to consitute over 70% of the universe compositon) is essentially the opposite of gravity- possibly causing the expansion of space, and accelerating the universe's expansion rate.

    So, after hearing about this I considered the possibility of:

    If gravity is mediated by gravitons, then could dark energy be mediated by anti-gravitons?

    I spoke to the professor for a while after the lecture, bringing up my thoughts on this matter. He stated that there is actually an hypothesis that anti-gravitons could "cause" the dark energy effect- however, we discussed that there are problems with the hypothesis such as- seeing as galaxy and star formation occurs- which would possibly be prevented by "anti-graviton dark energy". However, I still think that the hypothesis makes sense and that the problems with it could be ironed out with further study.

    So, what does everybody else think about this hypothesis? How plausible could it be?


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  3. #2 Re: Dark Energy and Gravity 
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    The default approach to dark energy is that this is just part of how gravity works at large distances. That is, if there are gravitons, then they are part of the mediation of the effect of dark energy.


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    I thought that gravity is asymptotic in the fact that it decreases away from the source, but never drops to zero. So, I don't quite understand how it can cause, essentially, an effect of "negative gravity" at large distances.
    "Nature doesn't care what we call it, she just does it anyway" - R. Feynman
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    One problem with the dark energy as anti-gravitons is that our present model of gravitons would predict that they, like photons, are their own antiparticle.

    Another problem I see is how to explain why gravitons would prevail at close range, holding galaxies and galaxy groups together, while anti-gravitons prevail at large scales.
    "Men are apt to mistake the strength of their feelings for the strength of their argument.
    The heated mind resents the chill touch & relentless scrutiny of logic"-W.E. Gladstone


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    If gravity were wavelike, probably with an extraordinarily long wavelength(just guessing), could there be an interference pattern?
    I was some of the mud that got to sit up and look around.
    Lucky me. Lucky mud.
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    Quote Originally Posted by GiantEvil
    If gravity were wavelike, probably with an extraordinarily long wavelength(just guessing), could there be an interference pattern?
    That is being studied now, by a particular instrument known as the Michelson Interferometer.

    Quote Originally Posted by Janus
    Another problem I see is how to explain why gravitons would prevail at close range, holding galaxies and galaxy groups together, while anti-gravitons prevail at large scales.
    Yes, indeed, that is one problem the professor brought up too. It's a tricky one gravity is!
    "Nature doesn't care what we call it, she just does it anyway" - R. Feynman
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  8. #7  
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    Quote Originally Posted by x(x-y)
    I thought that gravity is asymptotic in the fact that it decreases away from the source, but never drops to zero. So, I don't quite understand how it can cause, essentially, an effect of "negative gravity" at large distances.
    You are thinking of standard Newtonian gravity. There are versions of Newtonian gravity that people played around with that had repulsive forces at large distances, but they didn't amount to anything. Einstein revived this idea, somewhat, when he was trying to create a static, eternal universe model and added a term to the way gravity works in GR so that the farther away things are from each other, the more they, in a sense, feel a repulsive force from each other.

    It is essentially impossible to balance out the repulsive forces of an entire universe with the attractive force of an entire universe, so even with this addition one cannot expect a universe without expansion or contraction.

    One can run the math for this as either a part of how gravity just works or as part of how the energy associated with the vacuum couples with gravity. Either way, the graviton would be the relevant particle, if it exists.

    There are a bunch of hypotheses of things that could be different but act a lot like dark energy. Lawrence Krauss wrote a book, Quintessence, about these. I have it, but I haven't read it.
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    Quote Originally Posted by PhysBang
    Quote Originally Posted by x(x-y)
    I thought that gravity is asymptotic in the fact that it decreases away from the source, but never drops to zero. So, I don't quite understand how it can cause, essentially, an effect of "negative gravity" at large distances.
    You are thinking of standard Newtonian gravity. There are versions of Newtonian gravity that people played around with that had repulsive forces at large distances, but they didn't amount to anything. Einstein revived this idea, somewhat, when he was trying to create a static, eternal universe model and added a term to the way gravity works in GR so that the farther away things are from each other, the more they, in a sense, feel a repulsive force from each other.

    It is essentially impossible to balance out the repulsive forces of an entire universe with the attractive force of an entire universe, so even with this addition one cannot expect a universe without expansion or contraction.

    One can run the math for this as either a part of how gravity just works or as part of how the energy associated with the vacuum couples with gravity. Either way, the graviton would be the relevant particle, if it exists.

    There are a bunch of hypotheses of things that could be different but act a lot like dark energy. Lawrence Krauss wrote a book, Quintessence, about these. I have it, but I haven't read it.
    I see, so it's more a problem of devising a plausible quantum gravity theory which corresponds with GR then?
    "Nature doesn't care what we call it, she just does it anyway" - R. Feynman
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    Quote Originally Posted by x(x-y)
    I see, so it's more a problem of devising a plausible quantum gravity theory which corresponds with GR then?
    It needs to be better than just plausible. It needs to be consistent and contain GR, quantum chromodynamics and the electroweak theory in the appropriate limits.

    This turns out to be extremely hard to do. All attempts at quantum field theories of gravity so far produce anwers of infinity (they are not renormalizable). The attraction of string theory is that the graviton seems to appear naturally. The difficulty is that nobody can yet define what string theory is, or use it to make any testable new predictions.
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    As far as I know, the so called "dark energy" is also peculiar in a sense that it does not dilute with an expanding universe, i.e. its effect and density appears to remain constant, although matter dilutes with expansion. Is such a behaviour really consistent with the idea of an unknown property of gravity?
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    Quote Originally Posted by Dishmaster
    As far as I know, the so called "dark energy" is also peculiar in a sense that it does not dilute with an expanding universe, i.e. its effect and density appears to remain constant, although matter dilutes with expansion. Is such a behaviour really consistent with the idea of an unknown property of gravity?
    It's actually demanded by what we know about gravity.

    The Einstein Field Equation governs the relationship between mass-energy and spacetime geometry. However, the equation is not as general as it could be. Like many of the equations that we use in science, we use only the first term (or the first few terms) of what could be a more general equation.

    To add another geometric term is to add the cosmological constant, which behaves as dark energy does. That is, we add another potential variable into the mix that controls another level of gravity, so to speak. Like if one has a wave pool with a sloping bottom: the waves in the deep end will be controlled by whatever is making the waves, but at the shallower parts of the pool the waves will be influenced by the depth of the pool. Roughly, the gravitational constant determines the way that gravity works out to the inverse square of distance while the cosmological constant term effect increases with distance.

    It is mathematically equivalent to consider that the energy associated with a vacuum state might couple with geometry. Then, rather than add a term on the side of the geometry, we add a term on the side of the mass-energy tensor. We have to add a tensor of a very special sort, since it has to represent the vacuum and it shouldn't undergo certain changes when we consider it in different frames of reference. This means that the particular energy that we are considering ends up having the mathematical equivalent of negative pressure and constant density. After all, one expects that any given volume of vacuum should have the same properties regardless of where or when it is found.
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  13. #12  
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    Quote Originally Posted by PhysBang
    It is mathematically equivalent to consider that the energy associated with a vacuum state might couple with geometry. Then, rather than add a term on the side of the geometry, we add a term on the side of the mass-energy tensor. We have to add a tensor of a very special sort, since it has to represent the vacuum and it shouldn't undergo certain changes when we consider it in different frames of reference. This means that the particular energy that we are considering ends up having the mathematical equivalent of negative pressure and constant density. After all, one expects that any given volume of vacuum should have the same properties regardless of where or when it is found.
    Of course it would help if the best available calculation of the vacuum energy did not overestimate the observed cosmological constant by a multiplicative factor of
    10000000000000000000000000000000000000000000000000 00000000000000000000000000000000000000000000000000 000000000000000000000.
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  14. #13  
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    Hmmmm... Unfortunately, a lot of what you are now discussing is above my current level. But I do plan upon doing a physics degree (September of next year it will begin actually), so I shall understand eventually!
    "Nature doesn't care what we call it, she just does it anyway" - R. Feynman
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    I believe that the spatial medium is a super fluid. And that all quantum particles making up energy and matter emit gravitons in the direction of their acceleration. I believe that gravitons are coiled strings of singularities... that expand when they collide with a remote quantum particle. The gravitational force occurs when a graviton expands (uncoils) directly in FRONT of a quantum particle, creating a tiny little low-pressure area in the spatial fluid into which the receiving particle is squeezed (by the slightly higher pressure behind it). Every time a graviton expands... it adds to the spatial expansion of the universe. The implications of this model are that EVERYTHING affected by gravity has mass. Including things like photons. Space-time is not really "curved" but just SEEMS that way. It also precludes the concept of time travel into the past. So DARK MATTER doesn't exist. Expansion is the aftermath of the gravitational force. All matter and energy eventually decays into spatial fluid. We are literally ICEBERGS melting in a vast superfluid.
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  16. #15  
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    Quote Originally Posted by Quantumerik View Post
    I believe that the spatial medium is a super fluid. And that all quantum particles making up energy and matter emit gravitons in the direction of their acceleration. I believe that gravitons are coiled strings of singularities... that expand when they collide with a remote quantum particle. The gravitational force occurs when a graviton expands (uncoils) directly in FRONT of a quantum particle, creating a tiny little low-pressure area in the spatial fluid into which the receiving particle is squeezed (by the slightly higher pressure behind it). Every time a graviton expands... it adds to the spatial expansion of the universe. The implications of this model are that EVERYTHING affected by gravity has mass. Including things like photons. Space-time is not really "curved" but just SEEMS that way. It also precludes the concept of time travel into the past. So DARK MATTER doesn't exist. Expansion is the aftermath of the gravitational force. All matter and energy eventually decays into spatial fluid. We are literally ICEBERGS melting in a vast superfluid.
    Quantumerik, This sub-forum is for the intent of discussing established physics. Personal theories should be posted in the "New Ideas and Hypotheses" forum.
    "Men are apt to mistake the strength of their feelings for the strength of their argument.
    The heated mind resents the chill touch & relentless scrutiny of logic"-W.E. Gladstone


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  17. #16  
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    Quote Originally Posted by Janus View Post

    Quantumerik, This sub-forum is for the intent of discussing established physics. Personal theories should be posted in the "New Ideas and Hypotheses" forum.
    Janus, no problem, but first explain how "Dark Matter" is remotely considered "established physics?" In which case, the entire subthread should be placed into the other forum you referenced.

    Dark Matter is mainstream speculation... it's a junk term for a variable that hasn't been explained... and I haven't seen a good proposal for its mechanism.

    My comment above addressed the premise of this subthread.
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