Stochastic inflation and the lower multipoles in the cosmic microwave background anisotropies

We generalize the treatment of inflationary perturbations to deal with the non-Markovian coloured noise emerging from any realistic approach to stochastic inflation. We provide a calculation of the power spectrum of the gauge-invariant comoving curvature perturbation to first order in the slow-roll parameters. Properly accounting for the constraint that our local patch of the Universe is homogeneous on scales just above the present Hubble radius, we find a blue tilt of the power spectrum on the largest observable scales, in agreement with the WMAP data which show an unexpected suppression of the low multipoles of the CMB anisotropy. Our explanation of the anomalous behaviour of the lower multipoles of the CMB anisotropies does not invoke any ad hoc introduction of new physical ingredients in the theory.