  We study a possible gravitational vacuum-effect, in which vacuum-energy
variation is due to variation of gravitational field, vacuum state gains
gravitational energy and releases it by spontaneous photon emissions. Based on
the path-integral representation, we present a general formulation of vacuum
transition matrix and energy-momentum tensor of a quantum scalar field theory
in curved spacetime. Using analytical continuation of dimensionality of the
phase space, we calculate the difference of vacuum-energy densities in the
presence and absence of gravitational field. Using the dynamical equation of
gravitational collapse, we compute the rate of vacuum state gaining
gravitational energy. Computing the transition amplitude from initial vacuum
state to final vacuum state in gravitational collapsing process, we show the
rate and spectrum of spontaneous photon emissions for releasing gravitational
energy. The possible connection of our study to the genuine origin of gamma ray
bursts is discussed. We compare our idea with the Schwinger idea for
Sonoluminiescence and contrast our scenario with the Hawking effect.
