On the anisotropy of the gravitational wave background from massless preheating

When a light scalar field is present during inflation, its value varies on superhorizon scales, modulating the preheating process at the end of inflation. Consequently, the amplitude of the gravitational wave (GW) background produced during preheating is also modulated. The observed energy density of this background appears therefore anisotropic at different angles in the sky. We provide a master formula for the angular power spectrum C-l of the anisotropies in the GW background from preheating, valid for any scenario where the anisotropies are due to the superhorizon modulation of a light degree of freedom. Using lattice field theory simulations of massless preheating with g(2)/lambda = 2, we find a flat angular spectrum l(l +1)C-l approximate to 3 x 10(-4), which represents a strong anisotropy of similar to 1% variations on large angular scales. For our choice of couplings, long wavelengths are amplified most strongly during parametric resonance, which is crucial for the development of the anisotropies. If future direct detection GW observatories are capable of detecting backgrounds of cosmological origin, they may also be able to detect this effect. This could eventually become a powerful tool to discriminate among inflationary and preheating scenarios.