  Vacuum fluctuations of the electromagnetic field induce current fluctuations
in resistively shunted Josephson junctions that are measurable in terms of a
physical power spectrum. In this paper we investigate under which conditions
vacuum fluctuations can be gravitationally active, thus contributing to the
dark energy density of the universe. Our central hypothesis is that vacuum
fluctuations are gravitationally active if and only if they are measurable in
terms of a physical power spectrum in a suitable macroscopic or mesoscopic
detector. This hypothesis is consistent with the observed dark energy density
in the universe and offers a resolution of the cosmological constant problem.
Using this hypothesis we show that the observable vacuum energy density rho_vac
in the universe is related to the largest possible critical temperature T_c of
superconductors through rho_vac = sigma (kT_c)^4/(hbar^3 c^3), where sigma is a
small constant of the order 10^-3. This relation can be regarded as an analog
of the Stefan-Boltzmann law for dark energy. Our hypothesis is testable in
Josephson junctions where we predict there should be a cutoff in the measured
spectrum at 1.7 THz if the hypothesis is true.
