How can we observe and measure interacting vacuum?

[victorvmotti]

When studying the QFT, one considers the vacuum state when the field is not excited and therefore no particles are present.

Now for the matter fields this makes sense to me. But what about the radiation field? Suppose we have an arbitrary small volume of space in the universe without any matter particles inside of it. An empty volume of space. But can we say that there are not any photons inside this empty volume?

I mean regardless of where we pick the empty of matter volume in the universe there is some radiation coming to that volume from some galaxy source. Then what is actually the difference between an empty (of matter) space and the interacting vacuum, an empty of on-shell matter and radiation space, that is talked about in the QFT.

I suppose by QFT vacuum we actually mean even there is no excitation of the electromagnetic field and therefore no on-shell photons. But can we have any volume of space at any time through which a real (not virtual) photon doesn't pass?

How can we observe and measure interacting vacuum value when we don't have a cosmic vacuum?

I guess that the answer is:

Because the distance scales that we talk about and calculate in interacting vacuum and vacuum energy in QFT is much much smaller, like 1/1000 of the proton radius, than the distance scales relevant for the cosmic vacuum and the thermal effects energy like CMB, right?

Actually we subtract all the radiation including the CMB from the energy inside of empty cosmic space to obtain experimental data on the value of interacting vacuum energy and compare with the QFT prediction, right?

 

[anuttarasammyak]

For space filled with CMB, I think its energy is $E=E_{vac}+E_{CMB}$ with vacuum energy as default.