On the Anisotropy of the Gravitational Wave Background from Massless
  Preheating
  When a light scalar field is present during inflation, its value will vary on
superhorizon scales, modulating the preheating process at the end of inflation.
Consequently, the amplitude of the gravitational wave (GW) background produced
during preheating will also be modulated. The observed energy density of this
background will therefore appear 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 \approx 3x10^{-4},
which represents a strong anisotropy of order 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 should be able to detect
this effect. This could eventually become a powerful tool to discriminate among
inflationary and preheating scenarios.
