  We propose that cosmological density perturbation may originate from passive
fluctuations of the inflaton, which are induced by colored quantum noise due to
the coupling of the inflaton to the quantum environment. At small scales, the
fluctuations grow with time to become nearly scale-invariant. However, the
larger-scale modes cross out the horizon earlier and do not have enough time to
grow, thus resulting in a suppression of the density perturbation. This may
explain the observed low quadrupole in the CMB anisotropy data and and
potentially unveil the initial time of inflation. We also discuss the
implications to the running spectral index and the non-Gaussianity of the
primordial density perturbation.
