  The Laser Interferometer Gravitational-Wave Observatory (LIGO) detection of
gravitational waves that take away 5 per cent of the total mass of two merging
black holes points out on the importance of considering varying gravitational
mass of a system in the framework of the Einstein general theory of relativity.
We calculate the acceleration of a particle in the non-stationary field of a
quasi-spherical system composed of a large number of objects emitting
gravitational waves. It is shown that reduction of the gravitational mass of
the system due to emitting gravitational waves leads to a repulsive
gravitational force that diminishes with time but never disappears. This
repulsive force may be related to the observed expansion of the Universe.
