  The group velocity of gravitational waves in a flat Friedman-Robertson-Walker
universe is investigated. For plane waves with wavelength well inside the
horizon, and a universe filled with an ideal fluid with the pressure to density
ratio less than 1/3, the group velocity is greater than the velocity of light.
As a result, a planar pulse of gravitational waves propagating through the
universe during the matter/dark energy dominated era arrives to the observer
with the peak shifted towards the forefront. For gravitational waves emitted by
inspiralling supermassive black holes at the edge of the observable universe,
the typical shift that remains after the effects of nonplanarity are suppressed
is of order of ten picoseconds.
