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The universe is expanding. It then seems logical to say that the QM fields are expanding as the universe expands. My question is how does this happen? When I consider an expanding field it forces me to consider the actual properties of the field. Are the QM fields becoming less dense or are these fields only mathematical constructs that enable predicts to be made. If the fields are not only mathematical then does their expansion require the creation of new field “stuff” to fill in the new space being created. QM mathematics is way out of my skill level so an answer that respects my curiosity and understands my limitations is appreciated.

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  • $\begingroup$ There's a good discussion of this in Birrell & Davies, in which a plane-wave solution evolves in a universe with $a^2=A+B\tanh\rho\eta$. $\endgroup$ – J.G. Jan 21 '18 at 21:07
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Photons are quantum particles you get when the electromagnetic field is quantized. So consider the photons which make up the cosmic background radiation. They were generated when the mean temperature was about 3,000 K, but now represent a temperature of about 2.7 K. They didn't "cool off", but instead their wavelengths increased with the cosmic expansion, which can be easily calculated as 1100 = 3,000 K/ 2.7 K, which is the expansion since the cosmos cooled below the hydrogen ionization level.

No fancy math is required, just experimental results.

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    $\begingroup$ Lest anyone overgeneralize this valid example, it gets a lot more subtle when you talk about bound systems such as a hydrogen atom. Spacetime curvature does theoretically have an effect on a hydrogen atom, but the effect is many orders of magnitude too small to be detectable. $\endgroup$ – user4552 Jan 21 '18 at 23:45
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    $\begingroup$ As explained in the movie, Annie Hall: "Brooklyn is not expanding." Obviously, Brooklyn is a bound system. :-) $\endgroup$ – Peter Diehr Jan 22 '18 at 0:56
  • $\begingroup$ So how does the expansion of the universe affect quantum fields? The photons are just electromagnetic waves. What about the other quantum fields? $\endgroup$ – freecharly Jan 22 '18 at 2:21
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    $\begingroup$ @freecharly: photons are the quantized fields associated with the electromagnetic field. As such, they are ideal exemplars of other unbound quantum fields. $\endgroup$ – Peter Diehr Jan 22 '18 at 15:09
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    $\begingroup$ @Lambda: all measurements of the speed of light are consistent with it being a universal value. For more discussion, see math.ucr.edu/home/baez/physics/Relativity/SpeedOfLight/… $\endgroup$ – Peter Diehr Jan 22 '18 at 15:12

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