We only can speak about the energy density in a container if its energy content is independent from its dimension -so could, in principle, have any value at all. If in Einstein’s statement “People before me believed that if all the matter in the universe were removed, only space and time would exist. My theory proves that space and time would disappear along with matter” we may replace ‘matter’ with ‘energy,’ then the energy content of the universe is not independent from its volume. Doesn’t this mean that we cannot then speak about the energy density of the universe as a whole?


marked as duplicate by John Rennie, Qmechanic Apr 30 '18 at 8:37

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    $\begingroup$ Didn't you already ask this? The question is basically meaningless. For a start space and time wouldn't disappear along with matter. The solution to Einstein's equations with a zero ASDM mass is simply flat spacetime i.e. the Minkowski metric. $\endgroup$ – John Rennie Apr 30 '18 at 7:51
  • $\begingroup$ @John. What is ASDM mass? Do you mean that in a universe devoid of any energy still is a real, physical universe and that in such universe space and time exist? Though such abstract, empty, mathematical space and time may be interesting; if it only makes sense to speak about the distance and motion of real objects (i.e., which have energy) if it matters, energetically how large their distance and relative velocity exactly is, if there is energy involved in a change of their distance or velocity, then space and time don’t exist in a physical sense in the absence of energy. $\endgroup$ – Anton Apr 30 '18 at 8:50
  • $\begingroup$ @John. Only if we were to conceive of spacetime as something which exists even in the absence of energy, something the properties of which don’t depend on, aren’t affected by the presence of energy -if the meter and second would be defined even in a completely empty universe- might it make sense to speak about the energy density of the universe. $\endgroup$ – Anton Apr 30 '18 at 8:50
  • $\begingroup$ @John, Qmechanic. No, this exact question hasn’t yet been asked and answered. As according to big bang cosmology the expansion rate of the universe (also) depends on the density of the vacuum- / zero-point / dark energy / cosmological constant (of which nobody knows why it is so much smaller than calculated), mine is a legitimate question. $\endgroup$ – Anton Apr 30 '18 at 9:04
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    $\begingroup$ Consider yourself corrected. $\endgroup$ – John Rennie Apr 30 '18 at 9:41