As the universe is expanding is work being done? If so, where is the energy coming from?
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1$\begingroup$ This answer could either be 0 or impossible to answer. If we are looking from inside the universe, the answer would be 0 because work would mean that energy is transferred from or to the 'outside' the universe. But since the universe is isolated system, it must be 0. $\endgroup$– ProscionexiumCommented 2 days ago
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$\begingroup$ The other answer could be impossible to assert if we assume that there exists some kind of 'outside' the universe and we are looking from the outside. In this case, we simply can't even imagine what the supposed 'outside' really is and whether laws of thermodynamics (and nature) apply to it as they apply within the universe. $\endgroup$– ProscionexiumCommented 2 days ago
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2$\begingroup$ I suspect that quite a good answer is "God knows! Fortunately." :-) $\endgroup$– Russell McMahonCommented yesterday
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3$\begingroup$ I think the question means something like this: consider two unit masses separated by a distance d. They have a potential energy of $-G/d$. If the universe is expanding, $d$ is increasing, so the potential energy is going up. It is easy to see how this additional energy could be harnessed. This raises a perfectly natural question about whether the expansion of the universe is adding energy into gravitational systems. $\endgroup$– MattCommented yesterday
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Work is the transfer of energy from one system to another. Energy, in turn, is a conserved quantity associated with the time translation invariance of the physical laws.
At cosmological scales the time translation invariance is lost so there is no conserved energy. With no energy there is nothing to be transferred and with the universe there is no other system to transfer from.
So the concept of doing work on the universe doesn’t make sense as it stands. Of course, you could modify/generalize those concepts to make them apply. But in the course of doing so you could probably arrive at any answer you like. There isn’t currently a standard generalization that would serve.
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1$\begingroup$ I don’t follow. Why does “no conserved energy” (1st sentence, paragraph 2) become “no energy” (next sentence). The way this is written seems to imply there cannot be energy because it is not conserved. $\endgroup$ Commented yesterday
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1$\begingroup$ Energy is the conserved quantity associated with the time translation invariance of the physical laws. So if a quantity is not conserved then it is not energy. $\endgroup$– DaleCommented yesterday
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1$\begingroup$ so maybe I’m just picky on a Sunday night but what u mean is (2nd paragraph) is there “… is no conserved quantity associated with time invariance so no energy in the usual sense” and I would agree with that. $\endgroup$ Commented 23 hours ago