I've just seen a documentary where they state that particles pop into existence and then annihilate each other all the time in empty space. The experiment discussed that proved this was that two metal plates placed very, very near each other would be pushed together.

It occurs to me that if you placed them very, very far apart, such that more of the universe was between them than was outside them, wouldn't they be pushed apart for the same reasons? Could this be what causes the universe to expand? It seems to me that if there's any pushing at all, there has to be expansion.


1 Answer 1


This is the experiment you cited: Two uncharged parallel plates separated by a tiny distance, on the order of the wavelength of a single photon, will limit the photons popping in and out of existence between them to those whose wavelengths can fit a whole number of times between the plates.

As the plates draw closer together, one should find that the photons popping in and out of existence between them consist only of those with such whole wavelengths, and should exclude photons of all other wavelengths. This should reduce the total vacuum energy between the plates, as compared to the vacuum energy in space around the plates, which implies that the plates are drawn together by the reduced vacuum energy. The attraction is called the Casimir effect, as Alpha001 notes in his comment.

Another way of looking at the effect is as though the two metal plates are connected by a stretched spring (a sea of virtual photons), and move together as the stretched spring loses energy (limitation of the virtual photons) and contracts. This metaphor was devised by Stephen Reucroft and John Swain.

Moving up in scale to the entire cosmos, quantum field theory views space to be filled with virtual energy that fluctuates in and out of existence. But unless there are boundaries that limit the virtual energy to certain whole wavelengths in a bounded area, there will be no Casimir effect.

A problem with your interesting suggestion is that it is not believed that the Universe is finite in the sense of having a boundary, and is not divisible into one area containing "more" Universe, and another containing "less" Universe, which sum to the entire Universe, as Countto10 points out in his comment.

Another problem is that even if there were evidence that the Universe were finite and divisible into two parts that sum to the whole, General Relativity provides that the difference in vacuum energy between the two parts of the Universe might cause recognizable curvature, which has not been found.

  • $\begingroup$ the vacuum energy, i.e. the dark energy is thought to be there even if there were no matter. In fact our universe is evolving so that after a few trillion years the matter and normal energy density will be negligible compared to the vacuum, i.e., dark energy. The fact that we've not identified the specific way that we get the value of dark energy (i.e. Vacuum energy) we are measuring we have is still an issue, but without that dark energy measure our universe would not expand the way it is expanding today (and measurably so for about the last 5 or more billion light years). $\endgroup$
    – Bob Bee
    Commented Mar 16, 2017 at 1:38
  • $\begingroup$ And in fact, if the universe was finite, which we don't think it is, and if it had edges (which we have not ever noticed), we could divide it up into two (in some region there would be so much energy or vacuum energy, in another a different quantity, all proportional to their volume, as it seems the energy density is constant. Still, who is going to place those cosmological plates out there (they have to be big)? $\endgroup$
    – Bob Bee
    Commented Mar 16, 2017 at 1:41
  • $\begingroup$ @BobBee : The OP was suggesting that vacuum energy was the dark energy that accelerates expansion. I tried to point out that the Casimir effect does not necessarily explain how vacuum energy would do this. My point in the last paragraph is that if there were a difference in energy density between two parts of the Universe, it might cause curvature recognizable from one to the other, the same way that areas of denser mass/energy cause recognizable curvature, even though it wouldn't change the energy density of the entire Universe. I made some changes based on your comments. $\endgroup$
    – Ernie
    Commented Mar 16, 2017 at 2:20
  • $\begingroup$ Got you. Fine, thanks. I agree the Casimir effect is a long way from explaining how vacuum energy could cause expansion. We do think it should, but calculations are off by 120 orders of magnitude. I doubt Casimir explains that $\endgroup$
    – Bob Bee
    Commented Mar 16, 2017 at 2:34
  • $\begingroup$ The answer, then, appears to be: "It ain't anywhere near as simple as that." ;-) $\endgroup$
    – user447607
    Commented Mar 31, 2017 at 15:41

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