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Dark Energy candidates, ideas?

  1. May 15, 2006 #1
    Greetings,

    I've never liked the idea of Quintessence (although I admit I don't understand it fully).

    1. As for Dark energy, what are the leading theories on how it may be caused?

    2. Also, I have an idea about a possible cause for the Universe's acceleration. Perhaps, the fringes of the Universe are of such a great density that they gravitationally pull the Universe outwards. Can you criticize this basic concept? I'd like to know why this theory could not be true according to known physics, please tear it apart.

    Thanks.

    P.S. Zapper stay away!
     
  2. jcsd
  3. May 15, 2006 #2

    Garth

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    1. Distant Type Ia supernova at z ~ 1 appear fainter and therefore they may be further away than expected. If this is the case then the universe must have expanded faster in the past than the plain GR prediction, in fact it appears to have accelerated rather than declerated in its expansion. DE is a hypothetical form of negative pressure that could cause such cosmic acceleration.
    2. The COBE/ BALLOON/WMAP data sets are consistent with a flat universe wherein the total matter content is only about 27% of the density required for a flat universe, therefore 73% is in some form of energy density.

    The hypothesis of DE fills the bill nicely.

    But what is it?

    Its nature may be deduced from its equation of state and that may be deduced from the expansion history of the universe.

    The leading candidate is a non-zero cosmological constant (CC), which has a pressure that is simply minus its density.

    The zero-point energy (ZPE) of the vacuum also has this equation of state and may therefore be the DE observed cosmologically, i.e the QM ZPE may be the GR CC. However there is no natural way of integrating QM effects in GR gravitation and the density of all this vacuum energy is of the order 10120 too large for the CC. This is known as the 'Lambda Problem'.

    Furthermore the proportion of total density of this 'CC' DE would grow with time, yet at the present time its density parameter is approximately equal to the matter density parameter (73% as against 27%), why? This is known as the 'coincidence problem'.

    It requires us to be living at a 'special time'.

    Your second point has been seriously suggested and discussed in PF, see How to probe the homogeneity of the universe ?

    However, this would require us to be living in a 'special' place.

    An alternative point of view for 1) above is that the faintness of distant Type Ia supernovae may have another explanation, such as the model for early Type Ia S/Ns may not be properly understood, or perhaps, curvature and expansion, (R(t)), effects may be incorrectly applied to the prediction of high-z magnitudes.

    An alternative point of view for 2) above is that the universe may not actually be flat as the data sets are also consistent with a 'conformally' flat universe. (Note this would also affect the expansion effect, (R(t)), applied to S/N magnitudes).

    (And what do you have against ZZ? Your ideas put as questions are perfectly reasonable and I have always found ZZ very reasonable!:smile:)

    Garth
     
    Last edited: May 15, 2006
  4. May 15, 2006 #3
    Garth I have been reading up your SCC concept and I still like the concept of time variation as driving factor. If the universe expands its overall density decreases and therefor its gravity. If this is the case then surely in line with relativity wouldn't time accelerate?
     
  5. May 15, 2006 #4

    Garth

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    What is time accelerating with respect to? How are you measuring this acceleration?

    In SCC the time drift is between time measured by an atomic clock and one measured by the frequency of a 'standard' CMB photon - and this is also ephemeris time as well thus explaining the Pioneer anomaly.

    This time drift, or clock acceleration, is caused by the mass of the atoms changing with time.

    Garth
     
  6. May 15, 2006 #5

    SpaceTiger

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    Garth did a nice job summarizing the dark energy problem. You can also find some discussion of dark energy here:

    Dark Energy
     
  7. May 15, 2006 #6

    marcus

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    that's funny, lil bro,
    just from my brief contacts with both you and Zapper
    I like you both very much.
     
  8. May 15, 2006 #7

    marcus

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    if you want to know about the field of CANDIDATES for explaining the acceleration

    then I would suggest checking out this new MOND proposal
    that was posted yesterday. I will get the link

    Here is a thread about it
    https://www.physicsforums.com/showthread.php?t=120877

    Here is the paper

    http://arxiv.org/abs/astro-ph/0605322
    Dark energy, MOND and sub-millimeter tests of gravity
    I. Navarro, K. Van Acoleyen
    6 pages, to appear in proceedings of the 51st Rencontres de Moriond

    "We consider modifications of General Relativity obtained by adding the logarithm of some curvature invariants to the Einstein-Hilbert action. These non-linear actions can explain the late-time acceleration of the universe giving an expansion history that differs from that of a pure cosmological constant. We show that they also modify the Newtonian potential below a fixed acceleration scale given by the late-time Hubble constant times the speed of light. This is exactly what is required in MOND, a phenomenological modification of the Newtonian potential that is capable of explaining galactic rotation curves without the need to introduce dark matter. We show that this kind of modification also predicts short distance deviations of Newton's law at the sub-mm scale and an anomalous shift in the precession of the Moon's orbit around the Earth, both effects of a size that is less than an order of magnitude below current bounds."

    Ignacio Navarro is at Cambridge.

    Navarro and van Acoleyen have a bid to

    1. replace Lambda or dark energy
    2. replace dark matter and get a better fit to the rotation curves
    3. achieve rapid testability---falsification attainable with only a factor of 10 improvement in the accuracy of measurements that are already made

    I think it would certainly be foolish to start spouting one's own opinion about this proposal, at this point.
    It would sound kind of clueless to begin saying "Oh yes, I think that might work." or "Oh no that will never work, I can tell because of these and these reasons."

    AFAICS the whole point is whether it is TESTABLE. If it is a testable model that might explain dark energy effects then EVEN THOUGH IT MIGHT VERY WELL BE WRONG if there is any merit at all it should be tested.
    And maybe it can be PROVEN wrong.

    Anyway, that is my attitude about these things. I see progress as a process of elimination where you learn something by eliminating interesting potentially valid theories.

    I don't think Ignacio Navarro is a dummy. I think the Rencontres Moriond is a classy venue where this was presented. I think the DAMPT at Cambridge, where he is, is a great outfit. So even if this sounds totally weird I would like to see it tested.

    Others may differ in their assessment :smile:

    ========
    BTW I think there is another theory about Lambda which is also promising. Lambda plays a natural role in a spinfoam model being developed by Laurent Freidel and others. this may be a more fundamental way to look at Lambda (the Navarro way may be comparatively superficial: more of an "effective" theory dealing with appearances)

    in this theory there is no need for a "dark energy" particle or field. Lambda is a feature of the quantum geometry that has both largescale and microscopic effects.
    =======

    maybe the lesson I learn from these things is just that it aint over till its over. surprises waiting about Lambda. maybe it is a particle or field, maybe not. still might be some new ideas
     
    Last edited: May 15, 2006
  9. May 15, 2006 #8
    http://www.arxiv.org/abs/astro-ph/0603084
    On the similarity of Information Energy to Dark Energy

    this shows that there may be a direct correlation between Entropy and DE- from a computational view it could be that the dark energy is a direct result of entropy increasing and thus decreasing the amount of useful information that will be shared between regions of matter which emerges as an increasing amount of space between distant regions of matter not bound by gravity and results in a cosmological constant- in fact the paper shows that a spacetime with increasing entropy will result in a total information energy that exerts negative pressure and results in dark energy and expansion of the spacetime
     
  10. May 15, 2006 #9
    The measure is that each unit of time is smaller than the preceeding unit of time, in order for Planck's Constant to remain valid it would need to be about 10-43 smaller than the immediately preceeding unit of time.
     
  11. May 15, 2006 #10
    This is bordering on the concept that I have in mind as the theory of "accelerating time" but I will have to digest this article properly to analyze the similarities.
     
  12. May 15, 2006 #11

    nrqed

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    *very* interesting and informative post, Garth. Thank you for taking the time to write this.

    Just one question: would life be possible in a universe with the fraction of DE much larger than the fraction of matter? (or vice versa)
     
  13. May 16, 2006 #12

    Garth

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    Life would not be impossible in principle, I think, but that would depend on how the DE become so dominant.

    If its equation of state [itex]p = \omega \rho[/itex] where [itex]\omega < -1[/itex] then at some time the universe would be torn apart in the Big Rip by Phantom Energy and this may yet happen to us in ~ 20 Gyrs - don't hold your breath!

    If DE was already dominant and growing such a demise may be nearer.

    With dominant, albeit well behaved DE, say with [itex]\omega = 1[/itex], which causes masses to repel each other at large ranges, the question would be: "Could structures, galaxies and solar systems, gravitationally form within such a universe?

    If the answer is no, (and that is by no means certain), then you could answer the coincidence problem by anthropic reasoning. But some would see this argument as a cop out.

    Garth
     
  14. May 16, 2006 #13
    Nice comments, ty.

    2. As for my suggestion that the Universe's acceleration may be caused by fringial densities of sufficient magnitude, that they drag the Universe outwards...

    Consider a theoretical scenario where a perfectly spherical ball of dynamite with uniform density explodes in a point in space. Assume that this point in space is sufficiently removed from all gravitational fields so that the sphere is not acted upon by any force. Also assume for the sake of argument that this sphere can combust perfectly well in this point of space.

    If we now allow the sphere of dynamite to explode (and assume that we can have this explosion start from the exact center of the sphere) we would expect to see an isotropic explosion in which the outermost fringes of the explosion should reach a maximum velocity of ~28,000 feet/ second (explosion velocity of dynamite). We should also expect to see that this fringial velocity of ~28,000 feet/ second remains constant over time as it expands because we have said that its sufficiently removed from any gravitational field that might effect it. If an observer watched this explosion from some distance away with a slow motion camera, he would see the sphere explode (going from its zero initial velocity to its maximum velocity of 28,000 feet/ second) in a very short period of time. He would then see the fringes of this spherical explosion expanding, but not accelerating, outwards at a plateaued speed of 28,000 feet/ second.

    With this in mind, consider the bigbang as an analog of the above scenario. Instead of a sphere of dynamite, the bigbang, we will assume, exploded isotropically from a single point. Once again, an observer with a slow motion camera has the privilege to watch this all occur and notices that the singularity goes from an initial expansion velocity of 0 to ~300,000 km/s (C). We do not need to sufficienly remove this explosion from gravitational effects since there is no gravity in the Universe to speak of, yet. So the observer sees the singularity's fringes expanding, but not accelerating, outwards at a plateaued speed of C.

    Now then, both scenarios are laid out for everyone to see. There is one important difference between the two that I would like to make. In the dynamite scenario, the fringes reach a maximum velocity of 28,000 feet/ second and thus one would expect that they do not contain enough kinetic energy to appreciably increase their gravitational effect on the explosion's inner fragments, or 'innards'. However, in the second scenario, the fringes expand with a velocity of C (or perhaps we will say 0.999999999C, so we don't have to violate relativity). We would then expect that these fringes have sufficient kinetic energy to substantially impact the 'innards' of the explosion by gravitationally dragging them isotropically outwards.

    So then, perhaps the Universe's accelerating expansion is a result of these fringial densities gravitationally pulling the 'innards' (you, me and the milky way) outwards over time.


    Please comment and criticize this theory, ty.
     
  15. May 16, 2006 #14
    Chaos - you need to remember that the gravitational field within a spherical shell of uniformly distributed matter is zero everywhere inside that shell.

    You also need to know that the expansion can occur at greater than c, the special relativistic interpretation of recessional velocities is ruled out (by about 23 sigma) by observations of supernova magnitudes vs redshift. See fig. 5 in Davis and Lineweaver(http://arxiv.org/abs/astro-ph/0310808).
     
    Last edited: May 16, 2006
  16. May 17, 2006 #15

    Chronos

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    Kudos for matt.o. It is impossible to pull the observable universe apart by embedding it in an infinitely larger region - regardless of its density. Unless, of course, matter was not uniformly distributed in that realm. But that proposition raises even uglier questions - like a preferred redshift distribution.
     
  17. May 20, 2006 #16
    Matt.o.

    Yes, I expected those two criticisms to come up but I didn't have time to include them in my post.

    What if the 'fringes' were not of Uniform density but instead were dimpled like a golf ball. If these dimples were diametrically opposed (eg. a single convex dimple on one side of the universe, had a concave dimple directly opposite it on the other side of the universe) would this make any difference in creating a non-zero gravitational effect inside the sphere?

    Is there any possible shape you can think of that the fringial 'membrane' of the universe could have, so that it produces a non-zero gravitational field inside the sphere and also produces no preferred red-shift distribution?

    Also, if the dimples were small enough, would we even be able to detect a preferred red-shift? Do we have enough data to rule out this possibility to a high degree of certainty?
     
  18. May 20, 2006 #17
    Oh yes, one other idea.

    What if gravity's inverse square law was not absolute for very large distances (i'm thinking in terms of MOND, although I know very little about it), might this mean a non-zero gravitational force cancellation within the sphere?
     
  19. May 21, 2006 #18

    Garth

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    In any case you have to solve the GR field equation for the cosmos as a whole with a dstribution of density that increases with distance, on a space-like surface.

    This will not give the Newtonian result if there is pressure as well.

    Garth
     
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