Virtual particle density in an expanding Universe As the universe continues to accelerate in its expansion, does the virtual particle density decrease or remain the same throughout spacetime?
 A: This has been asked before at least twice:


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*Does incidence of appearance of quantum fluctuation particles being lowered due to space expansion?

*Does the density of virtual particles decrease when space expands due to dark energy?
And in both cases the question was closed because it isn't very meaningful.
The problem is that the popular science description of space being full of virtual particles flickering momentarily into existence is fundamentally flawed. This is explored to some extent in Are vacuum fluctuations really happening all the time? but it's a tough concept for the beginner so I'm not sure how accessible the answers to that question will be. Anyway, since virtual particles don't really exist they don't have a density so it doesn't make sense to ask if that density changes as the universe expands.
But there is a sense in which we can answer your question. The virtual particles are used in calculations involving the ground state of interacting quantum fields,though they are just a computational device and shouldn't be taken to mean that there really are countless particles jumping in and out of existence. However what we can say is that as far as we know the ground state of the interacting quantum fields is not affected by the expansion of the universe. That is, the ground state of e.g. the electron field now will be the same as the ground state of the electron field in 11 or so billion years when the universe has expanded by a factor of two.
