  The notion of vacuum fluctuations of the gravitational field plays important
role in cosmology. The strong variable gravitational field of the very early
Universe amplifies these fluctuations and transforms them into macroscopical
cosmological perturbations. Since the underlying process is the parametric
amplification, the perturbations are placed in squeezed vacuum quantum states.
It is possible that we already see the manifestations of these processes in
certain cosmological observations.
