  A technique is given to derive the well known Bell-Szekeres solution of the
Einstein-Maxwell vacuum field equations describing the space-time and the
Maxwell field following the head-on collision of two homogeneous, plane,
electromagnetic shock waves. The analogue of this technique is then utilized to
construct the space-time model of the gravitational field following the head-on
collision of two homogeneous, plane, gravitational shock waves. The latter
collision, which is followed by a pair of impulsive gravitational waves and a
pair of light like shells traveling away from each other, provides a mechanism
for generating a cosmological constant which may be important in the
theoretical description of dark energy.
