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Can dark energy perform work?

  1. Feb 13, 2009 #1
    What I keep hearing is that dark energy acts like a sort of 'negative pressure', causing all the objects in the universe to accelerate away from one another, or a 'constant energy density filling space'. Something I have been unable to understand is what the nature of this "negative pressure" or "energy density" is-- i.e. whether it is literal or figurative. In particular, something I can't seem to find a clear answer on is whether dark energy is "usable"-- whether it can be extracted or harnessed, in principle.

    For example: let's say that you tied one end of a rope to the Earth, and the other end to the Earth-sized rock rotating around Gliese 581, 20 light years away.

    - Would the rope experience force tugging at either end of it? If Earth and Gilese 581 are continuously accelerating away from each other, naively one would think it would-- the amount of acceleration times the masses of the weights at the ends, maybe?

    - What would be the magnitude of this force? (I've had trouble working out even what the strength of dark energy is-- wikipedia says the density of "dark energy" is 10^-29 grams per cubic centimeter, but I'm not sure how to interpret a number with those units, given that sounds like mass density, not energy density?)

    - Could this force be harnessed to perform work, or heat a gas or power an electric generator or anything of the sort? (Say, perhaps there's a piston at one end? I'm not sure exactly how one would set this up, but it seems if you have a rope that acts like a force is being exerted from the other end, you should be able to extract energy from that somehow.)

    Never mind that this is utterly ridiculous from an engineering perspective, or that the amount of energy ultimately extracted would be negligible-- in principle setting a system like this up should be physically possible. So what I am trying to figure out is: If you set this system up, is there anything about dark energy which would prevent you, in principle, from extracting energy from it? Am I missing something?
     
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  3. Feb 14, 2009 #2

    Chalnoth

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    Well, the negative pressure is required to drive the expansion. This negative pressure can be understood as a resistance of this sort of energy to change its density as the universe expands. The simplest dark energy model, the cosmological constant, is described as an energy density that is intrinsic to the vacuum: if you remove all matter from a region, there still exists some amount of energy density within that region of space.

    Any force on the rope would be entirely dominated by the relative velocities of the Earth and this other planet. Gliese 581 may be moving towards or away from us right now, for instance, and the orbital periods of our respective planets are not the same. These will be the dominant effects on a hypothetical tether between our worlds.

    Besides, Gliese 581 is trapped within the same gravitational well we are trapped in: the milky way. The effect of dark energy is only felt significantly for far-away objects in different gravitational wells. You would have to go out to at least a few megaparsecs (1Mpc = 3.26 million light years).

    That said, the idea of attaching a hypothetical tether between far away objects to get a feel of dark energy is one I haven't seen before. The distances required, of course, are so absurdly astronomical that it's not something that we could ever actually do. And even if we could perform the task, the far anchor of the tether would be something that is already moving away from us at high velocity, a fact which would dominate the force on the tether.

    Due to mass-energy equivalence, they're the same. What you should interpret from this is that the effect of dark energy is so absurdly minuscule here on Earth that we're unlikely ever to measure it.

    Obscenely unlikely. The energy density is just so low that it's really not feasible.

    Honestly, I don't know. I've never thought it useful to think of dark energy in terms of work that can be extracted from it.
     
  4. Feb 14, 2009 #3

    marcus

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    A more precise figure (if not necessarily more accurate :smile:) is 0.6 nanojoule per cubic meter. Equivalently, 0.6 nanopascal. Since you've tried working it out and might be interested, I'll go thru the steps.

    As it happens, I've considered the similar questions myself here at PF, more generally as thought experiments about harnessing expansion. It seems interesting because GR has no global energy conservation law---largescale conservation is not a theorem. Expansion appears to violate energy conservation---the CMB photons have lost 999/1000 of their original energy, where did it go?, could it have been trapped and used to boil tea?---doubling the volume doubles the "dark energy", where did the putative new energy come from? This was some time back. I think it is worthwhile to think about.

    I can't answer your questions about this. I'm only recalling past speculation and saying I think it's a good sort of question to ask.

    There may be some "catch-22" principle in nature that prevents a local observer from exploiting what appears to be largescale disregard for thermodynamics. That says sorry, no perpetual motion machines...

    Maybe we should consider some schemes and see what if anything goes wrong.

    Meanwhile let's do the calculation:

    You know that the critical density is 3c^2 H^2/(8 pi G) and space is nearly flat so that is nearly the real average density, and H is 71 km/s per megaparsec.

    And you know that dark energy is estimated to be 73 percent. So calculating is easy. You just compute critical density, and it comes out to be about 0.85 nanojoule per cubic meter. And you take 73 percent of that.

    Just to be sure, I put "3 c^2 (71 km/s per megaparsec)^2/(8 pi G)" into google calculator, and it came back with 0.85 nanopascal.
     
    Last edited: Feb 14, 2009
  5. Feb 17, 2009 #4
    We will probably decide before long that dark energy causes gravity and so quite capable of doing work.
     
  6. Feb 17, 2009 #5

    Chalnoth

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    Well, we might discover some interesting things about dark energy, but I can basically guarantee you that that isn't one of them. Dark energy, if it exists, interacts with gravity, to be sure, but it is not gravity.
     
  7. Mar 4, 2009 #6
    This is interesting because I've wondered about what makes gravity tick. It's a force but why ,and from what? It is a cool thought even if it's not correct. Maybe? Who knows for sure? :confused:
     
  8. Mar 4, 2009 #7

    Chalnoth

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    This isn't really any more or less mysterious than any of the other fundamental forces of which we are aware.
     
  9. Mar 5, 2009 #8
    is dark energy the same as the vacuum energy like in the casimir experiment? or is that something different?

    gravity is geometry... or is geometry just the language we use to understand it's effects and it is something else entirely?

    to harness dark energy to do usefull work you would need a fixed frame of reference and there aren't any.
     
  10. Mar 5, 2009 #9
    Wait, wait, this problem exists only in the "birds view", when you look at the expanding baloon from the "outside". For any observer inside the Universe energy is conserved.
     
  11. Mar 5, 2009 #10

    Chalnoth

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    Well, it's the same in that it's basically the same kind of thing. But the magnitude is completely different. The vacuum energy from the Casimir Effect, from naive calculations, is some 10^120 times the magnitude of the vacuum energy that would explain the accelerated expansion. So while they may be the same thing, we don't yet know how that is possible, and on the face of it it seems rather unlikely (as the Casimir Effect only depends upon differences in vacuum energy dependent upon the shape of local matter, we can't actually measure the absolute level).

    Well, as the description of gravity as geometry works so very well, a good way of understanding it is that on some level, whatever it is that causes gravity acts like the geometry of space-time. There are hints that gravity is composed of the collection action of tremendous numbers of particles (gravitons), particles which, on average, come together to cause the curvature of space-time.

    I don't quite understand why you think that would be a requirement.
     
  12. Mar 5, 2009 #11
    First a caveat, I'm not a scientist or physisist, just the average joe trying to get a grip on the concepts.

    If by 'extracting work from the dark energy' you mean somehow taking advantage of the expansion of the universe then consider a simple analogy, that of a flowing river.
    to extract energy you would build a power plant on the banks and put a paddle wheel in the water. the water turns the wheel and you've got something usefull. but that power plant is in a fixed position relative to the water. If you put it on a barge then it just floats down stream and doesn't produce any power.

    Similarly we are imbeded in the expansion of the universe. Going along with the flow so to speak. and therefore can't extract any usefull energy from it.

    at least intuitivly that's what seems to make sense to me, that may be a bad analogy I don't know.

    A question about Marcus's quote..

    "Expansion appears to violate energy conservation---the CMB photons have lost 999/1000 of their original energy, where did it go?, could it have been trapped and used to boil tea?---doubling the volume doubles the "dark energy", where did the putative new energy come from?"


    Why does expansion seem to violate energy conservation? is it because it is expanding when the gravity of the matter and energy in it should be making it contract? and DE is making it expand so DE is energy that is being created out of nothing so that violates conservation?


    if it takes some kind of energy to make the universe expand then could it be the 'lost energy of the cmb photons" ?

    is there realy missing energy? The CMB energy isnt just a fixed amount never increasing or decreasing (conserved) but just diluted by the increasing volume of the universe?
     
  13. Mar 5, 2009 #12
    I agree with TalonD that it would require enough potential energy differences in a relatively close proximity in order to harness enough meaningful energy from DE. I suppose in theory it could be done, but probably not practical. We could probably harness more energy from temperature differences between two random locations (i.e. floor to ceiling).
     
  14. Mar 5, 2009 #13
    Rope tied to a planet (which starts out) in a distant galaxy. Expansion of the universe eventually applies tension to the rope, so the anchored spool will do useful work if allowed to unwind. So that the rope never runs out, part of the work mass-energy can be converted to rope matter. Does this mean heat-death (or big freeze, just not big rip) can be avoided after all?

    EDIT: as the planet gets further away, tension may increase until the rope breaks. Address this by replacing the planet with the weight of the rope itself plus adding a mechanism to trim (abandon) the very end of the rope.
     
    Last edited: Mar 5, 2009
  15. Mar 5, 2009 #14
    Typical perpetual motion fallacy. You would need as much energy to rewind the spool or create more rope as you would get from the expansion unwinding it. It's like trying to lift yourself off the ground by pulling up on your own boot straps. to paraphrase an old expression.
     
  16. Mar 5, 2009 #15
    Oh really? It only takes a fixed (and finite) quantity of mass-energy to add a new metre of rope to the spool. But if space keeps expanding then, by choosing the rope to be sufficiently long to begin with, the force on the spool (and hence the amount of energy obtained when it unwinds the metre) can be made arbitrarily large.
     
  17. Mar 5, 2009 #16
    Now that I think about it, I see your point. But it would have to be an accelerating expansion, which ours is. It wouldn't work with a fixed rate expansion. The excess energy would come from the acceleration.
     
  18. Mar 6, 2009 #17
    But Nickelodeon didn't say that dark energy is gravity. He said:
    To which CozmicScott replied:
    To which TalonD replied:
    To which Chalnoth replied:
    To which I'll add my two cents:
    Suppose we start by assuming that Nature is fundamentally waves and wave interactions across a hierarchy of media -- particles, particulate media, and other more or less persistent objects being bounded wave structures.

    Then, suppose we model the boundary of the universe as a wavefront (say, a more or less spherical shell) created by and expanding away from some disturbance in a pre-existing medium -- and we take this as the fundamental dynamic, the archetype, of any and all motion.

    It seems to naturally follow that the energy of any and all behavior -- the force behind any and all motion -- inside the expanding universal wavefront is also a byproduct or subset of the kinetic energy imparted via the disturbance that originated the expansion.

    Variable universal expansion rates might be produced by altering the properties of the pre-existing universal medium.

    Can the complexity of our world, of our universe, the vast hierarchy and range in scale of particulate media, be produced via countless iterations and interactions governed by a single fundamental wave dynamic?

    Anyway, to respond to the original question? Yes, dark energy is performing work in the sense that, in at least one view, it's the energy imparted via the origin of the universe and it's the fundamental driving force underlying all motion.

    Can we harness it directly -- no, for reasons given by other posters.
     
  19. Mar 6, 2009 #18

    Chalnoth

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    That makes no sense. There is no boundary to the universe. Not that we can see, anyway.
     
  20. Mar 6, 2009 #19
    Maybe our Universe has a boundary, maybe it doesn't. It's just a possible approach to modeling our Universe that might unify the observed fundamental forces and explain their emergence in terms of an underlying fundamental wave dynamic.

    I can't help thinking that the expansion of our Universe is one of the most important discoveries of modern science. The fact of the expansion suggests to me a beginning. A beginning suggests to me a pre-existing medium. And, taken together and considered along with other observations suggesting a wave nature, then modeling our Universe as a bounded complex wave structure seems to me, at least conceptually, sensible enough.
     
  21. Mar 6, 2009 #20

    Chalnoth

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    Not likely. Our forces, as we know them, are fundamentally mathematical quantities. Any attempt at unifying the forces that is not fundamentally mathematical in nature is doomed to failure.
     
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