  We investigate the anisotropies in the gravitational wave (GW) background
produced at preheating after inflation. Using lattice field theory simulations
of a massless preheating model, we show that the GW amplitude depends
sensitively on the value of the decay product field chi coupled to the inflaton
phi, with the only requisite that chi is light during inflation. We find a
strong anisotropy in the amplitude of the GW background on large angular
scales, the details of which strongly depend on the reheating dynamics. We
expect similar conclusions for a wide class of inflationary models with light
scalar fields. If future direct detection GW experiments are capable of
detecting the GW produced by preheating, they should also be able to detect
this effect. This could eventually provide a powerful way to distinguish
between different inflationary and preheating scenarios.
