  The estimation of light velocity dispertion caused by quantum fluctuations
influence on electromagnetic wave propogation in four-dimensional space-time is
presented. Analytical cosmological solutions for the flat and open Universe for
the case of cosmic vacuum model are obtained. The time delay of cosmic
gamma-ray burst radiation is calculated. The delay is explained by propogation
of dispersive electromagnetic wave in the expanding Universe. It is shown that
the delay value depends on a model of the Universe. We conclude that to discern
a model of the Universe measurement accuracy of parameter \Delta t / \Delta
E_{\gamma} should be better than 10^{-5} s / MeV.
