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Description
Until recently the primordial curvature perturbation was considered to originate only from quantum fluctuations of scalar fields. We show that vector fields can generate the curvature perturbation too. If particle production of the vector field is isotropic, such a field can generate the total perturbation in the Universe. Otherwise, vector fields can merely contribute to the total curvature perturbation, in which case there is a clear detectable observational signature - the curvature perturbation is statistically anisotropic, and the non-Gaussianity parameter f_NL is itself anisotropic and correlated with the amount of statistical anisotropy.
