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Help wanted with accelerated expansion and gr

  1. Dec 3, 2006 #1

    Rip

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    When I first read about accelerated expansion in 1999 I immediatelly thought of gravitational repulsion between matter and antimatter. I know it is an old idea, and very much out of the mainstream, but when you read reviews about the arguments against it, it really makes you think. It is a case not as clear as most people seem to think. So blessed by sheer ignorance (I am a professional physicist but working on a different field), I have developed my own ideas on cosmology and general relativity. I would very much appreciate if someone could show me why this paper (http://arxiv.org/abs/gr-qc/9906012) is complete garbage. I would then immediately withdraw it and live happily ever after.

    Cheers

    Rip
     
  2. jcsd
  3. Dec 5, 2006 #2

    Wallace

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    I only had a quick read of your paper, so apologies if I've misunderstood your argument but one problem that strikes me is that while this idea may be able to explain accelerated expansion, it dosn't explain the decelerating era. According the Supernovae data, and consistent with all other cosmological probes, the universe was decelerating till roughly redshift of 1 and has been accelerating since then at an increasing rate. My reading of your theory would imply that the acceleration rate would be constant (i.e. the deceleration parameter q would be constant). I may be mistaken about this but I don't see your theory, as I understand it from your paper, as explaining the nuances of the expansion history we observe.

    It's an interesting idea though, I wish I had time to investigate non-standard models more. It's good to have people constantly questioning accepted ideas (as long as it's done with good reason, which your paper seems to be).
     
  4. Dec 5, 2006 #3

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    Thanks

    Thank you very much for your reply. I was not aware of the so called decellerating era, and haven't though of it to any extent, but my intuition is that our neighbourhood is composed of future-pointing matter just as we are and at longer distances past-ponting matter might be found, so at small redshifts attractive gravity forces would be deccelerating expansion while at longer distances repulsive forces might be accelerating expansion, and from what you say I understand that is exactly what is being observed.
     
  5. Dec 5, 2006 #4
    Although wrong, this paper and theory is pretty interesting. It has always struck me as odd that anti-matter is supposed to have a positive mass, yet all of its other charges get a sign changing. It seems like we are singling out one property and treating it differently to the all of the others for no immediately obvious reason.

    It would seem that you have shown that mass/energy must also obey this rule if we follow the postulate that anti-matter is "time-reversed" matter. I'm not very good at spotting errors in maths, so there might be a mistake in there somewhere... but it looks correct to me.

    I don't quite understand why you seperated this from the other charge conjugations though... if you were to "time-reverse" charged particles in a similar fashion wouldn't this effect appear anyway. i.e. [itex]\tau^+[/itex] particles with mass and charge [itex]m[/itex] and [itex]q[/itex] should become [itex]\tau^-[/itex] particles with mass and charge [itex]-m[/itex] and [itex]-q[/itex]?

    Interesting read anyway. :)
     
  6. Dec 5, 2006 #5

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    Oooops. Wallace: sorry I got it the other way around, it is for larger redshifts that the deceleratoin occurs, so I have no explanaition for it.
     
  7. Dec 5, 2006 #6

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    Wallace: I guess that in my model, if you can call it that, (it is more a vague idea than a model) future-pointing clusters are contracting and being repelled by past-pointing clusters, and that should give you different relative speeds and accelerations in different regions of space, so I would not say that the observations are in conflict with my proposal.
     
  8. Dec 5, 2006 #7

    I would say one reason(maby naive, Im sure there are more convincing reasons) antimatter has to have positive mass is because when a particle annihilates with a antiparticle you get 2 rest masses in energy released.
    If a antiparticle had negative mass you would get zero energy during a particle antiparticle annihilation.
     
  9. Dec 5, 2006 #8

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    Azael: That is why in my paper I do not talk about antimatter, I talk about matter with past-pointing 4-velocity. It is a proven fact that in electrodynamics matter with past-pointing 4-velocity can not be distinguished from matter with the opposite charge and positive mass (antimatter), but such a thing has never been proved in the context of gravity.
     
  10. Dec 5, 2006 #9

    Wallace

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    But the point is that if this was the case then the 'future pointing' clusters would always have been contracting, and the 'past-pointing' clusters would always have been moving away from us. This is not what we observe with any cosmological probe, so your theory is in conflict with every observation we have made.

    Instead what we see, isotropically, is that the universe was initially decelerating and at some point it started accelerating. Note that in terms of GR this is not because of non-inertial forces between particles, it's not so Newtonian. To be clear, we say that a galaxy we observe is moving away from us since we observe it to be red shifted (lets not get into the expanding space vs recession speed issue here). By mapping the luminosity vs redshift relation of Supernovae we can also map changes in redshift with distance (essentially giving us speed vs distance) and hence can discover information about how a galaxy at a given redshift is accelerating. But the big thing to note is that F does not equal MA, i.e. even though we can say the galaxy is accelerating with respect to us it does not feel a force yanking it towards or away from us. If you were on that galaxy, you would say it is we who are accelerating, yet we do not feel a force towards or away from that galaxy.

    Therefore we cannot postulate cosmic acceleration to be due to 'repulsion' between particles. If this was the case we would feel like we were on a rollercoaster, pressed against the surface of the earth (or flung off it) as we were repelled by a 'past-pointing' region.

    Even if we ignore this problem, explaining the observed acceleration vs time of our universe is not feasible in your model. What we see is a smooth transition with redshift of the acceleration parameter q (defined as -(\ddot(a)*a/\dot(a)^2) ). We see this smooth transition isotropically. In your model then, to get this behavior we would have to be in a galaxy that is precisely at the center of a spherically symmetric region of space where the relative density of future pointing and past pointing matter was a smooth function of the radial distance from us. Such a ridiculously contrived model might be able to explain supernovae data, but it would not explain the observed clustering history (how 'clumpy' is the universe as a function of redshift) as well as not explaining the cosmic microwave background, as such a model could not have a big bang.

    The problem with radically new ideas (and the reason I don't spends as much time on them as I would like) is that while you might be able to explain one aspect on the universe in a neater way, to be credible, an idea needs to explain all aspects better. Unfortunately your concept fails at most hurdles when confronted with what we observe.

    All that being said their might be some usefulness in your idea and I encourage you to try and develop it further. With respect however I would advise you read up a little more on the current theory of cosmology, as you seem to be lacking in understanding some of the basics. It's clear from your paper you have skills in physics, but in order to come up with a better cosmological model than the current, you will need to know more about the current model than you seem to.
     
  11. Dec 5, 2006 #10

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    Wallace:

    Thank you again for providing me with some feedback. I really appreciate it.

    > But the point is that if this was the case then the 'future pointing' clusters would
    > always have been contracting, and the 'past-pointing' clusters would always have been > moving away from us.

    You do not seem to have understood an important point of the paper. Most future-pointing clusters would be moving away from us because of past-pointing clusters being in the way. That is what the analogy with the Madelung model of an ionic crystal is all about.

    > so your theory is in conflict with every observation we have made.

    Every observation? I would say that my model might have difficulty with the deccellerating era (that is the only thing you mention), but to check for that would require very complex numerical simulations, the rest is hand waving.

    > Instead what we see, isotropically, is that the universe was initially decelerating and at > some point it started accelerating.

    We do not “see” that at all. We “see” a tiny bit of what is out there. All we have is a handful (7 at the present time I think) of high z supernovae showing decceleration rather than acceleration. Most people talk about the currently prevalent cosmological model as if it was an observation, a fact, rather than speculation. I can´t say much about whether my ideas are compatible with all observations, but I try to explain those observations using only the equations of GR as Einstein wrote them, not inventing arbitrary new concepts like “dark energy” or quintessence.

    I do not expect my extremely simple model to explain every fact of life, but it might help to explain the following:
    - why we do not see antimatter around us.
    - why there seems to be an arrow of time.
    - why there is accelerated expansion.
    - why the universe is flat on a large scale.
    - why the cosmological constant is zero or nearly zero while quantum field theory predicts it to be huge.
    - why quantum gravity has problems with renormalization and how to overcome that.
    And it does so only on the basis of GR, and the assumption of time reversal symmetry on a large scale.

    You say “Unfortunately your concept fails at most hurdles when confronted with what we observe.” but all you seem to mention is the deccelerating era.

    Leaving cosmological speculation aside I make some very strong statements about GR itself and you do not comment on those.

    You are right, I know very little about cosmology. I know very little about GR. I just try to follow the equations of GR in small steps and they lead me to very shocking consequences. I would like you or someone else to tell me exactly where in the path I did get lost, if I did.
     
  12. Dec 5, 2006 #11

    SpaceTiger

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    Rip, has this been published in any professional journal?
     
  13. Dec 5, 2006 #12

    Wallace

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    As a quick response we have many more than 7 supernovae from the decelerating era, and the important point is that every other observation we have (Galaxy cluster surveys, Lyman Alpha Forest, weak lensing, the CMB are the main ones) are in agreement with the model that fits the Supernovae data. If the universe did not have a decelerating era then Galaxies would not be clustered the way they are, the acoustic peaks would be an different angular scales on the CMB. There are many ways to 'see' an effect and they all tell the same story.

    This dosn't mean the current model is set in stone and as you point out requires the somewhat magical dark energy and dark matter, but this is no more of a conceptual leap than past pointing anti-gravity particles!

    On the future pointing clusters moving away due to past pointing clusters being in the way I urge you to consider my previous post. If the was due to repulsion from the past-pointing cluster we would feel an non-inertial force, which we clearly do not.

    I will have another closer read of your paper but I do not see how it explains most of the list of things you claim it does. Explaining why the universe initially decelerated is at least as important as explaining why it is currently accelerating. I note you still havn't explained how your model can give this behaviour. It dosn't require complex simulation at all, it's really quite trivial to see what the effects would be. I explained how it can, requiring a very contrived local universe, which would still only explain one of the three main cosmological probes.

    To say your theory is good because it uses GR without dark energy, despite not reflecting anything we observe is clutching at straws.
     
  14. Dec 6, 2006 #13

    Wallace

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    I've had another read of your paper. Unfortunately I've found another error rooted in a common misconception.

    You claim that having equal amounts of tau_plus and tau_minus matter would lead to flatness on a large scale as the positive curvature of one cancels the negative curvature of the other. The misconception is that the current model predicts a flat universe. It does not. It predicts (and we observe...) a spatially flat universe, which is different to a flat Minkowski space. The full contraction of the Riemann tensor is not zero. A spatially flat Freidmann-Robertson-Walker metric in fact requires a specific non-zero energy density, known as the critical density. What you propose, i.e. no curvature at all, since the tau_minus and tau_plus matter cancel each others effects out, is in fact what is known as the empty universe model, or the Milne model, which is equivalent to Minkowski space through a suitable change in co-ordinates.

    To get the observed spatial flatness we see, you would need to have an excess of tau_plus density over tau_minus density precisely that of the critical density. This would mean that we would see far more tau_plus matter than we would expect and would be currently scratching our heads as to how we seem to have more matter than the critical density yet appear to be in a spatial flat universe.

    I'm sorry, but while your paper presents a nice toy model, it's in complete contradiction to everything that we observe, including the accelerated expansion of the universe, since you predict a constant rate of acceleration which is not what we observe.
     
  15. Dec 6, 2006 #14

    Garth

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    Hi Wallace!

    Conformal transformations are angle preserving. The CMB power spectrum WMAP data is angular in nature, therefore the conclusion from that data that the universe is "spatially flat" should be generalised to "conformally spatially flat", this does allow other models to be concordant with the data.

    Note, we do not observe an accelerated expansion of the universe, we actually observe Type Ia SN that are fainter than predicted by the non-accelerating model.

    Cosmological expansion is a conclusion dependent on the assumption that Type Ia's are standard candles over cosmological time, this may not be the case.

    In any case the empty model actually is a good fit to the Type Ia data, at least out to z ~ 1. See the Perlmutter et al. paper Measurements of [itex]\Omega[/itex] and [itex]\Lambda[/itex] from 42 high-redshift supernovae : Figure 2 pages 23/24
    Garth
     
    Last edited: Dec 6, 2006
  16. Dec 6, 2006 #15

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    Space tiger:

    Nope it is unpublished. I tried a few years ago. The referee got really angry and basically said it as all junk, but did not at all explain why it was so. People get really angry when you tell them they have been wrong all along.

    Wallace:

    I have read a bit about those other pieces of evidence, and my impression is that there must be thousands of imaginable ways to account for that data, specially if anything goes, "dark energy", quintessence, super strings, you name it. I think we should go back to the equations we can relly on, and look at what we can predict from them wihtout making any previous assumptions (such as "since we are going forwards in time, time whole universe is").

    I really do not get your argument about "non inertial forces". My paper only talks about the equations of GR, not about any other exotic force. Since our solar system is free falling in the field created by both future-pointing and past pointing matter, there is no way we can "feel" the field from past-pointing matter.
     
  17. Dec 6, 2006 #16

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    Wallace:
    I forgot to say thank you again. You are helping me a lot to refine my ideas. Specially with the problem with the deccelerating era. I was not even aware of that!
    Garth: Thank you!
     
  18. Dec 6, 2006 #17

    Wallace

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    Okay Garth, there's several problems here. If an alternative theory gives identical results, then how could we ever tell the difference and why would we care? Unless your 'conformally related' theory gives the same CMB angular result, but would predict differences in say structure growth, in which case it could be confronted with the evidence we have. LCDM gives a pretty good fit to ALL the data sets we have, not just the CMB.

    Secondly the SN results you present are hopelessly out of date. We've made a lot more measurements since 1998! The empty universe is a terrible fit to current data. Your own posts quotes that the empty universe is very similar to the 'best fit' (0.73,1.32) model which is complety different to what the standard model of (0.24,0.76) is today.

    Pointing out the banal and obvious fact that cosmic expansion is a model dependant conclusion based on the evidence we have says nothing about whether this is a reasonable model. It sounds clever but in fact says precious little :zzz:
     
    Last edited: Dec 6, 2006
  19. Dec 6, 2006 #18

    Wallace

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    So instead of 'anything goes' dark energy, you invent 'anything goes' magical time reversed anti-gravity particles :surprised Dark energy has several very plausible origins. Non zero vaccum energy, a cosmological constant (which just means that there is no real energy there, gravity just works that way, there's not reason not to add a constant into the Einstein equations) or a scalar field (quintessence) are all reasonable theories. Yeah there is plenty of other ones like strings, branes and cosmic monkeys on bicycles. However, here is the crux of the issue. All these theories make testable predictions and will live or die on the strength of observational evidence.

    Your theory also makes testable predictions, and I've shown how the tests all disagree with the predictions it makes. We cannot insist our theory must be valid due to our theoretical prejudice (i.e. dislike of the concept of dark energy) when the weigh of evidence is against it.

    Waving your hands and saying "there must be thousands of imaginable ways to account for that data" dosn't help to show how your one-thousandth and one way of accounting for it does so. The fact is your theory is in conflict with what we observe.

    You havn't commented on the spatial flatness issue either, which is a major flaw in your paper.

    I agree on this, however the wording you use in the paper is confused. Your ionic crystal analogy is what rang alarm bells for me on this, and I find it detracts from your argument. You set the system up as having attractive and repulsive forces, neither of which occur in GR (apart from 4-forces due to EM).
     
  20. Dec 6, 2006 #19

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    Garth has commented on the spatial flatness issue.
     
  21. Dec 6, 2006 #20

    Wallace

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    What Garth said about conformally related theories being equivalent to GR has no bearing on your theory. How on earth is your theory conformally related to LCDM????

    What you havn't realised is that Garth is talking about a theory of gravity that is not GR but is conformally related to it. The whole point of your idea is that you use GR only but allow backwards in time pointing particles. This is just GR, not some new theory conformally related to GR. To have spatial flatness then you need the correct critical density of matter, when your theory predicts zero total density.

    I'm sorry but you are increasingly appearing out of your depth and clutching at straws.
     
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