Squeezing of the quantum electromagnetic vacuum

It is commonly agreed that the electromagnetic vacuum is not empty but filled with virtual photons. This leads to effects like Lamb shift and spontaneous emission. Here we argue that if the vacuum has virtual photons it might mean that it is very weakly squeezed and therefore the electromagnetic field is not in its ground state (vacuum) but in an excited dark state. We suggest a stringent test relying on measuring various properties of the electromagnetic field to exclude this yet-untested squeezing hypothesis. This could be done by measuring the number of photons as a function of frequency and comparing it with the spectrum of electric (or magnetic) field fluctuations. If such squeezing exists, it might shed new light on cosmological phase transitions and give complementary information to the observed microwave background radiation as well as be a possible candidate for dark energy.