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Dark energy and the speed of light

  1. May 7, 2003 #1

    Ivan Seeking

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    What implications if any does the potential existence of dark energy have on the speed of light passing through empty spaces between galactic structures [remote space where dark energy is thought by some to dominate]? How might the permittivity and permeability constants be effected?
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  3. May 7, 2003 #2


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    I don't think dark energy would change the electromagnetic parameters of the vacuum. If it's due to a cosmological constant then it's "dynamical geometry". In Einstein's equations the electromagnetic effects are in the momentum-energy tensor, one term on the right side of the equations. The cosmological constant is another distinct term (multiplied by the metric tensor). This implies they don't interact.

    The other possible explanation for dark energy is that it's some kind of scalar field spread throughout spacetime. The simplest models of that also have no interaction with electromegnetism.

    Generally speaking, if there were any interaction between dark energy and electromagnetism we would see it. Papers are now appearing that constrain the possible graininess of quantum spacetime by the sharpness of images of very distant galaxies. The light from those galaxies has crossed billions of lightyears, and any interference with it along the way would blur the images. But the images are sharp. This means that the light has been pretty much undisturbed on its way across a large fraction of the universe.
  4. May 7, 2003 #3


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    SA if you recall any author name or anything that would
    help me do a keyword search for abstracts of those papers
    please do post it. I had heard that some time back
    no graininess could be detected (and I don't know how they
    were looking) but this is current research you say and I would
    like to know what the current thinking is about that.

    Wait, I just did a search using what you already said and got:


    March 24, 2003 Nature Science News Update. interesting
    article---please let me know if you have some better leads but this
    is fine for starters
    Last edited: May 7, 2003
  5. May 7, 2003 #4
    Actually, electromagnetic fields (and anything electromagnetic) can't be transferred into the brane which causes the illusion of dark energy. Only gravitational fields can be transferred.
  6. May 7, 2003 #5


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    I found an interesting paper on quantum spacetime by Richard Lieu in "arxiv"
    Richard Lieu was author of one of the two papers cited
    in the nature science update article discussing the "no
    graininess" observations you mentioned, namely

    Here is the arxiv link to Lieu's article and the wide-audience discussion of it at nature.com:


    While looking for papers by Lieu about spacetime graininess I accidentally came upon this which was so nice it deserves mention.


    Some people at grenoble dumped cold neutrons down a laundry chute (or whatever) and discovered that they only existed at certain heights during their fall---a neutron would flicker out of existence and then rematerialize at a slightly lower level. Sounds wacky but just what QM would predict like energy levels in an atom except it was energy levels in earth's gravitational field.

    Nesvizhevsky, V. V. et al. Quantum states of neutrons in the Earth's gravitational field. Nature, 415, 297 - 299, (2002).
    Last edited: May 7, 2003
  7. May 7, 2003 #6


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    I havent read anything to suggest that cosmologists know what dark energy is---don't you get the impression they are just beginning to try to find out?

    the simplest and most common model is that it is uniformly distributed----everywhere the same and not changing perceptibly with time. So I do not see how it could have any effect on the speed of light.

    when the speed of light was measured in 1900 there was the same dark energy in the vacuum as when the speed of light was measured in 1950. There has always been this dark energy as a feature of the vacuum and so when people have measured the speed of light "in vacuo" it was present just as now and everywhere. So even if we do not understand the physics of it I do not see how it could be expected to have a novel effect on the speed of light.

    Whatever effect it has (if it even has an effect) it has always had and has always had uniformly everywhere---so the effect (if there even is any) does not seem to matter much.
  8. May 8, 2003 #7
    I think scientists already know what dark energy is hypothetically. Dark energy is the mysterious energy that results from the propagation(is that a word? but you get the idea...) of gravitational forces 'between' branes. Since we can't detect or see anything in the brane which the gravity propagates because only gravitational forces are able to propagate through the brane, and not electromagnetic forces, it is virtually impossible to know what the exact cause and behaviour of the particles are, as far as I know.
  9. May 8, 2003 #8


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    Re: Re: Dark energy and the speed of light

    Hi Majin,
    do you have a link so some, like, astrophysics journal preprint
    about this?

    Like I say, I haven't yet read anything to suggest that cosmo'ists
    know what dark energy is.

    The journal articles I've seen are by people who say they
    don't know----they are beginning to consider various explanations and think of observations that might test them

    If you know of a "brane" explanation there's likely a preprint in the arxiv about it that you could tell me a link to.
    Something like:


    I'd like to see it.
    The idea of interaction between layers sandwiched together
    which is limited to only one particular type of interaction and
    which causes accelerating expansion in our layer is an intriguing
    idea, if that is what you mean. what cosmologists are investigating this idea, do you happen to know?
  10. May 9, 2003 #9
    I read about it in "The Universe In A Nutshell" by Stephen Hawking. There's a whole chapter on it. I'll find a site for you.
  11. May 10, 2003 #10


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    That is a good reference! Thanks. I dont have the book but
    it will be easy for me to find at a library or browse that chapter in a bookstore. I was amazed----Stephen Hawking has an idea of what dark energy could be and I had never heard about it! Had no inkling at all. He thinks that it might be an interaction between
    our universe and some other universes that we are sandwiched together with? Or did I misunderstand what you said earlier?

    It was kind of you to offer to find a website as well and if you do come across one please post it. but the book reference ought to suffice.
  12. May 12, 2003 #11
    Re: Re: Dark energy and the speed of light

    Not if dark energy is representative of some variation on Eistein's reincarnated cosmological constant.

    Nature would argue that the simplest and most common model is flawed, since uniformly distributed energy does not carry forth the continuity of every other form of energy in the universe which aggregates into levels of related but relatively opposing layers.

    If dark energy is representative of a cosmological constant, then it is quite plausible that the negative pressure component is related to mass energy via dark matter if we simply assume that dark energy aggregates into the weakly tangible form of matter which has been observed to "halo" around massive clusters.

    Assuming that the total energy of the vacuum "very nearly" balances with mass energy, then the remaining energy would naturally have to be rarefied beyond the halo region in order to maintain proportionallity, and this would explain why there isn't a whole lot of interference between massive clusters, especially if light's speed "approaches" infinite in energy rarefied regions of space, where permittivity and permeability would have a negative effect, relatively speaking.

    I think that this vacuum could possibly be legitimately supported by the limiting case to SR, since space-time deformation would be virtually non-existent if mediation falls off in gradient fashion between massive clusters, so you end up with a nearly rigid and almost instantaneous linear transfer of information between clusters, thus solving the great "action at a distance" mystery, while remaining within the constraints of relativity.

    FYI: This could reveal a few other minor little details too, like a viable causal mechanism for gravity, at the, uh... "level" that we are currently interested in, for one example... ;)
  13. May 12, 2003 #12


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    Re: Re: Re: Dark energy and the speed of light

    You have unconventional and imaginative ideas---I wish you success with them, but cannot respond to the point.

    At the present moment she is not arguing with us, I think, but smiling enigmatically. We have just been told that 73 percent of the energy in space is something we have no model of or theory about. YOU mr. island may have a theory about it. But people collectively are clueless.

    It seems to be a property of the vacuum and to be extremely evenly spread out and not to clump (or clump very very slowly if at all).

    Nature is probably laughing at us---not arguing.

    Dark matter (which is some 23 percent of total energy in space) is puzzling enough. No agreed-on notion of what it could be. But at least it does cluster and clump around galaxies gravitationally.

    But dark energy is even more of a riddle. Great time in history, no?

    BTW did you know that the MAP orbiting observatory is orbiting the SUN instead of the earth? It is at a lagrange point a couple of million miles further out from the sun than the earth is. So it keeps up with the earth as both go around the sun, flying in formation with the earth. The "L2" point of the earth and sun system.

    It is MAP (microwave anisotropy probe, renamed WMAP recently
    in honor of a mr. W deceased) that we get the 73 percent figure from. Earlier people said 70 percent with a wider plus/minus, ie less certainty.

    I invite you to remember the density of the universe (critical for observed flatness) as 1.8E-123
    and consequently dark energy density as 1.3E-123
    these figures being in c=G=hbar=k=1 units.
    I find I cannot remember these aspects of nature in SI metric terms. something disconcertingly artificial and even slightly pedantic about saying joules per cubic kilometer or eevee per cubic centimeter or whatnot. Maybe your smiling or arguing personification of Nature is trying to tell me something.
  14. May 12, 2003 #13
    Re: Re: Re: Re: Dark energy and the speed of light

    Thank you, but surely it is not my theory, since Nature provides the universal pattern. Alas, I am only the messenger, as patterns are the essence of all math and physics.

    General Relativity says that this cannot be correct, because regions of positive vacuum energy will elicit positive curvature, i.e., are gravitationally "attractive". Vacuum energy is necessarily higher in the region which immediately surrounds massive clusters, since the liklihood of particle pair production increases in condensed regions of matter and radiation, so...

    Indeed... especially if you consider that the discrepancy between theory and observation is an incredible 120 orders of magnitude.

    "orders of magnitude"... hmmmmm, I think that Nature IS trying to tell us something... e.g., Do you see the pattern again?... ;)
  15. May 12, 2003 #14


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    Re: Re: Re: Re: Re: Dark energy and the speed of light

    Let's say it is vacuum energy. Then the argument is that it is repellant rather than attractive. I cant speak as expert on this but I'll tell you what others say, in brief

    one of the two friedman equations, for example, says

    act,ct/a = -(4pi/3)(rho + 3p)

    rho = rhomatter + rholambda
    p = pmatter + plambda

    oops, have to go, can't finish.

    Anyway the lambda pressure is negative so because of the
    three it overwhelms the positive stuff and the RHS of the
    equation becomes positive----accelerating expansion.

    I'm back. I'm simplifying by including the radiation energy density in with the "matter" terms. think of "matter" as including visible and dark matter and also a small contribution from light and the CMB. Dont want to have to write so many terms.

    key thing is the negativity of the the pressure

    Finicky footnote: the equation would be more kosher if I defined a force
    F = c4/G (the natural unit of force) and wrote an F in front of the LHS of the equation. then the units on both sides would agree.

    As it stands the LHS is one over an area (the second derivative
    with respect to ct is reciprocal area) and the RHS is pressure or energy density (dimensionally the same type of quantity).
    Multiplying the LHS by a force would make it a pressure or energy density, agreeing then with the RHS. And it would algebraically boil down to the ordinary friedman equation.

    So to be strictly correct, put the F in, either of two ways

    Fact,ct/a = -(4pi/3)(rho + 3p)

    act,ct/a = -(4pi/3F)(rho + 3p)
    Last edited: May 12, 2003
  16. May 12, 2003 #15
    Re: Re: Re: Re: Re: Re: Dark energy and the speed of light

    This is false. A region of space that has a higher "constant" vacuum energy density than the surrounding space creates a gravitationally "attractive" force, because this isolated region of "comparitively" positive pressure elicites positive curvature, and no expert will say differently. Not that you're aren't NORMALLY correct, but this scenario does not apply to your example, which applies to the more general case of evenly dispersed vacuum energy.

    [edit to elaborate on the following]

    Last edited: May 12, 2003
  17. May 12, 2003 #16


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    Re: Re: Re: Re: Re: Re: Re: Dark energy and the speed of light

    this is very astute of you to point out but you are missing one detail.

    the thing that distinguishes dark energy or vacuum energy or cosm.const or quintessence is that a positive energy density corresponds to a negative pressure

    this makes it different from everything else we know

    and it means that the net contribution of (rho+3p) is negative

    in fact, limiting this to dark energy, no matter terms,
    p = -rho!!!!!!!!!!! (this is for the most commonly considered lambda)

    so (rho + 3p) = -2rho!!!!!!

    Einstein was wise to this so he put a positive lambda in the
    picture in order to keep his universe from collapsing----he wanted a steady state---hadnt hit on the expanding universe model.
    the negative pressure, with its expansive effect on space, canceled the matter term, and gave him
    a total (rho + 3p) = 0. It was his trick
    to avoid collapse.

    It is totally unintuitive that a constant energy associated with the vacuum could represent negative pressure, but I will repeat the explanation I've seen several cosmologists use.
  18. May 12, 2003 #17


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    why the pressure is negative

    island you understand that I do not want to argue with you.
    I can tell you are acute enough to see the inadequacy of this
    intuitive explanation----I wont defend it, I am just bringing it over from eg. Ned Wright's Cosmology FAQ, or half a dozen other
    places. This is folklore and it is a widely used explanation.

    Imagine an energy that is linked to space itself, a vacuum energy, which is constant throughout space and time

    however much energy a cubic meter contains, well, two cubic meters contain exactly twice as much. this is the model
    (imagine this even if you dont think it could be realistic)

    Now imagine a piston and cylinder, looking like a one-cylinder pump, with a cubic meter of vacuum in it,
    (this pump is surrounded by NOTHING not even vacuum. it is
    a highly idealized situation :smile: )
    and imagine pulling out the piston a ways so that there are now two cubic meters of vacuum inside.
    You must have done some work because you have created a certain amount of energy.
    the force you must have exerted is equal to the amount of energy created by enlarging the volume, divided by the distance you had to pull.
    if you think about it, this means that there is negative pressure in the cylinder and it is equal to the energy density of the vacuum.

    I'm not defending this.
    The fact is that cosmologists DECLARE that dark energy is a special something such that p = -rho by definition.
    Or sometimes they play around with the ratio and say p = -(2/3)rho, just to see what effect.
    The relation between pressure and density (typically p =-rho) is
    called the "equation of state" of the dark energy.

    If it has that equation of state then it does not matter how it got it or whether it is perfectly uniformly distributed or whatever.
    If it has that equation of state then it works to accelerate the expansion of the universe (agreeing with observations that show
    that there really is acceleration), and if it works OK, let the physicists figure out what it is! Perhaps I am a bit facetious here but I think you understand.

    Anyway it is negativity of the pressure that is the key distinctive feature here.
  19. May 12, 2003 #18
    See Alan Guth's false vacuum solution to get an idea of what I'm talking about. His idea is different, but the end result is the same. The faq's do not apply to this case, nor does "quintessence".

    PS: I was still in the process of editing when you responded, so the text that you quoted is incomplete, sorry.

    The vacuum, per QM and Einstein's Cosmological Constant, "seethes" with particle potential, which isn't really counter-intuitive if you don't read too much un-necessary detail into it, and understand that to mean that the vacuum is simply a rarefied form of the same energy that makes up a material object. It is out of technological reach, is all, with the exception of inductive reasoning, i.e., the casimir effect implies that the theories are correct in their predictions, for example.

    The "so called", problem with Einstein's static universal model was that it was assumed that any instability would send it into a runaway negative or positive gravitational acceleration. In Einstein's model, the rho of the vacuum is equal to about half of the rho of matter, where "static" means that g=0, but the rho "IS" greater than zero, and so P must be less than zero, P<0.

    A better way to understand this, is to imagine that g=0 represents the "fabric" of space-time without any matter. The metric is often represented by way of a flexible rubber sheet analogy, and so if you were to stick a fork into it and spin it into enough of a knot to form a tangible material object, (rho>0), then the rest of the rubber sheet would have to rarefy or become thinner in order to balance with the metric. This is a good representation of Einstein's vacuum and the cosmological constant as "anti-gravity"... because the metric pulls back when you twist it into a knot. In other words, isolated gravitational pressure forces the negative pressure component into being as the metric pulls back on the effect that a massive object has on it.

    Positive regions of vacuum energy density are under the kind of tension that is described above, but in this case, the "tension" exists with respect to the rest of the vacuum, (positive vacuum energy density equates to an increased mass energy effect, relative to less energy density beyond the "halo" area), and so the effect actually compounds with the already compounded gravitational effect that massive clusters have on space-time, helping to slow down universal expansion, ergo the "attractive" gravitational "force".

    The compounded gravitational effect changes the perspective by... "orders of magnitude"

    If you "twist the fork" enough to overlay the energy by only one "fold", then you will derive that the rho of the vacuum has been rarefied to about half of that of the matter that has been produced from one "layer" of tangible matter, and this also just so happens to near exactly describe Einstein's model, where the rho of the vacuum is equal to about half of the rho of matter!

    Now, the more that you "twist the fork", the more opposed become the two forms of energy, the more "severe" the interaction between them becomes. That is to say... the "pressure gradient" between matter and the vacuum becomes greater as particle pair production leaves more and more "holes" in the vacuum, so the vacuum surrounding massive clusters expands as the boundary contracts due to the increasing tension that is being produced via the increased liklihood of particle pair production in the regions of concentrated mass energy and radiation... like, massive clusters.

    The neat part about this is that Lambda stays near zero, as it is the gradient and the interaction between opposing energy forms which becomes more severe as tension between mass-energy and the vacuum increases, while the total energy remains constant, albeit constantly more active... ;)
    Last edited: May 13, 2003
  20. May 13, 2003 #19
    Does the casimir force "pull" two plates together because vacuum energy density is greater between the plates? No, but by your logic, negative pressure is greater outside the plates becasue vacuum energy density is comparitively higher outside, and so they should be pulled apart, and not pushed together.
  21. May 13, 2003 #20


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    The cas. effect is not due merely to there being lower energy density between the plates than outside.

    More significant is the fact that the energy density between depends strongly (fourth power) on the distance.

    Quote from Physics FAQ article

    [[The Casimir effect is a small attractive force which acts between two close parallel uncharged conducting plates. It is due to quantum vacuum fluctuations of the electromagnetic field.

    The effect was predicted by the Dutch physicist Hendrick Casimir in 1948. According to the quantum theory, the vacuum contains virtual particles which are in a continuous state of fluctuation (see physics FAQ article on virtual particles). Casimir realised that between two plates, only those virtual photons whose wavelengths fit a whole number of times into the gap should be counted when calculating the vacuum energy. The energy density decreases as the plates are moved closer which implies there is a small force drawing them together.

    The attractive Casimir force between two plates of area A separated by a distance S can be calculated to be,

    pi2 hbar c
    ----------- A = force
    240 S4

    where h-bar is Planck's constant over two pi and c is the speed of light.]]

    Notice that the units work out because hbar c is the product of force with area and S4 is the square of area. So after canceling only force remains.

    the Casimir effect is not simply that there is a force but more
    importantly that it falls off as the fourth power of the separation distance.

    I believe after a little scribbling that this means the deficit in energy density
    also declines with the fourth power of the separation. If anyone here knows, please tell me if my impression is mistaken.

    I would hazard a guess that if the energy density did not depend so strongly on the separation, the attractive force might be so weak as to be undetectable.

    What impresses me is not merely that the vacuum energy density between two plates is less than out in the open, which one might guess anyway, and which might not have much effect depending on various things. What impresses me is that by moving the plates a little ways apart one creates a considerable amount of energy.
    This energy created (larger volume with larger density in it)
    must show up as the work one does to pull the plates apart.
    Therefore it must show up as a force that one overcomes in order to pull them apart.
    Last edited: May 13, 2003
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