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As the title says, we know that the universe is expanding, with it I am assuming that all the related quantum fields are also expanding so that we have the "same" conditions everywhere in space.

Q1: If that is the case is the zero-point energy getting less dense like a balloon (the bigger it gets the less dense / more stretched everything becomes)?

Q2: If however the zero-point energy has the same density as space expands (creating energy), doesn't that break the laws of thermodynamics?

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  • $\begingroup$ Doesnt the expansion of the universe break classical thermodynamics?So since there is something which breaks apart classical thermodynamics I dont see why there shouldnt be another thing. $\endgroup$ Commented Apr 24 at 22:40
  • $\begingroup$ "the zero-point energy" What zero-point energy? $\endgroup$
    – hft
    Commented Apr 25 at 2:12

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The zero point energy is defined as the minimal energy a system may have. In vacuum, it "should" be 0 according to classical physics. In quantum mechanics, it is still a controversial subject. Quantum mechanics is yet unable to predict its value. The trouble is we can calculate only energy differences. The "absolute" value of energy doesn't have a meaning.

In relativity theory, things are more subtle since the absolute value of energy should create a gravitational field. General relativity is unable to calculate its value (it's not its domain, say). But it could accept a non-zero value, as a cosmological constant. In cosmology, this constant may be interpreted as the absolute vacuum energy, and it's ... a constant, even when space is expanding.

General relativity is compatible with thermodynamics. So a constant vacuum energy (i.e the cosmological constant) is not in conflict with thermodynamics (which can't predict its value anyway).

Hope this helps a bit.

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  • $\begingroup$ "Absolute value" is kinda an ambiguous term. $\endgroup$ Commented Apr 25 at 2:52
  • $\begingroup$ Of course, this is why we only calculate "relative" or variations of energy. Only in general relativity could we give some sense to the "absolute energy". $\endgroup$
    – Joshua
    Commented Apr 25 at 4:02
  • $\begingroup$ I mean "absolute value" usually means |x|. $\endgroup$ Commented Apr 26 at 17:29

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