  The Laser Interferometer Gravitational-Wave Observatory (LIGO) detection of
gravitational waves that take away 5% of the total mass of two merging black
holes points out on the importance of considering varying gravitational mass of
a system. Using an assumption that the energy-momentum pseudo-tensor of
gravitational waves is not considered as a source of gravitational field, we
analyse a perturbation of the Friedmann-Robertson-Walker metric caused by the
varying gravitational mass of a system. This perturbation leads to a modified
Friedmann equation that contains a term similar to the 'cosmological constant'.
Theoretical estimates of the effective cosmological constant quantitatively
corresponds to observed cosmological acceleration.
