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The vacuum energy density, or the zero point energy, of fields are said to be constant as space expands. Doesn't this mean that as space expands, more and more energy is being created? Don't particles and virtual particles come from this energy (They are fluctuations), meaning new particles are being created constantly, violating conservation of energy?

Oisin Spain
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  • Yes, energy is not conversed in this case. That’s not surprising because energy isn’t conserved in general relativity. – knzhou Sep 11 '19 at 17:47
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    No, “particles and virtual particles” do not get created as a result by “fluctuations”. This is some mish mash of popsci slogans that’s simply false. – knzhou Sep 11 '19 at 17:48
  • @knzhou What form does the zero point energy have in your opinion? – my2cts Sep 11 '19 at 18:00
  • Who says that the vacuum energy density is constant as space expands? – my2cts Sep 11 '19 at 18:01
  • @knzhou: Yes, energy is not conversed in this case. That’s not surprising because energy isn’t conserved in general relativity. That should be an answer. –  Sep 11 '19 at 19:11
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    related: http://physics.stackexchange.com/questions/2838/total-energy-of-the-universe –  Sep 11 '19 at 19:12
  • @my2cts: Who says that the vacuum energy density is constant as space expands? This is what the $\Lambda$CDM model says. It's natural for it to say that, because a varying $\Lambda$ violates the Einstein field equations. –  Sep 11 '19 at 19:13
  • @Ben Crowell No conservation of energy? All we have to do is harvest the ex nihilo and no more wind Mills or solar cells! – my2cts Sep 11 '19 at 19:59
  • @knzhou So where do particles come from? – Oisin Spain Sep 11 '19 at 21:15
  • @my2cts You know perfectly well why this isn't actually practical. – knzhou Sep 11 '19 at 21:41
  • @my2cts You also know perfectly well that conservation of energy is an empirically observed phenomenon in terrestrial experiments. That gives us reason to believe it should continue to hold up terrestrially, and not much reason to believe it should be true cosmologically. This objection is like being amazed that there isn't air drag in interstellar space. – knzhou Sep 11 '19 at 21:42
  • @knzhou I disagree that physics is limited to terrestrial systems. I find Newton on my side. – my2cts Sep 11 '19 at 21:49
  • @knzhou "energy isn’t conserved in general relativity" - Energy conservation is an expression of the stationary action principle that holds in GR and everywhere else. Can you give one example where energy is not conserved, as measured in the same frame? For instance, there is no cosmological redshift in the frame of the emitter. Similarly, in the frame of the receiver, cosmological photons are emitted already redshifted. Either way there is no loss of energy. I believe the correct statement is that there is no unique definition of energy in GR, but not that energy isn't conserved. – safesphere Sep 11 '19 at 22:37
  • @safesphere have you taken a look at Ben Crowell's link? If not, please do so. Further explanations at https://physics.stackexchange.com/questions/35431/is-the-law-of-conservation-of-energy-still-valid and Motl's blog. In short, energy is not conserved in GR: " There's no way to define "energy" in general (cosmological) situations that would be nonzero, coordinate-choice-independent, and conserved at the same moment.", quote by Motl. – untreated_paramediensis_karnik Sep 17 '19 at 12:43
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    @thermomagneticcondensedboson In both links the correct answer is by Philip Gibbs, the founder of Vixra.com. See the links in his answers and comments. His discussion with Lubos on this topic is published on his blog at Vixra. Here also is his paper: https://arxiv.org/pdf/gr-qc/9701028.pdf - My point is more general though. Energy conservatuon is an expression of the Stationary Action Principle that holds everywhere. Whether or not we can define energy explicitly is a different matter. – safesphere Sep 17 '19 at 14:28
  • Alright, I see now your point of view. It is over my head so I cannot judge. From what I see, there is a controversy! Thanks for clarifying what you had in mind though. Now @knzhou can give his opinion. – untreated_paramediensis_karnik Sep 17 '19 at 16:40
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    @thermomagneticcondensedboson The answer is simply that you can change what "energy" does by redefining what you mean by the word "energy". It is just like how I can turn a lefty into a righty by swapping the definitions of the words "left" and "right". The OP's question, however, is referring to the standard definition of the word in GR, and this quantity is not conserved. – knzhou Sep 19 '19 at 05:15
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    @thermomagneticcondensedboson The linked paper defines a certain conserved quantity whose definition requires the specification of an arbitrary vector field. While it is probably formally correct, that quantity is not usually what we mean by the word "energy". – knzhou Sep 19 '19 at 05:17

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