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?