Within the effective field theory of inflation, an initialization of the classical dynamics of the inflaton with approximate equipartition between the kinetic and potential energy of the inflaton leads to a brief fast roll stage that precedes the slow roll regime. The fast roll stage leads to an attractive potential in the wave equations for the mode functions of curvature and tensor perturbations. The evolution of the inflationary perturbations is equivalent to the scattering by this potential and a useful dictionary between the scattering data and observables is established. Implementing methods from scattering theory we prove that this attractive potential leads to a suppression of the quadrupole moment for CMB and $B$-mode angular power spectra. The scale of the potential is determined by the Hubble parameter during slow roll. Within the effective field theory of inflation at the grand unification (GUT) energy scale we find that if inflation lasts a total number of e-folds $N_{\mathrm{tot}}\sim 59$, there is a 10%–20% suppression of the CMB quadrupole and about 2%–4% suppression of the tensor quadrupole. The suppression of higher multipoles is smaller, falling off as $1/l^2$. The suppression is much smaller for $N_{\mathrm{tot}}>59$, therefore if the observable suppression originates in the fast roll stage, there is the upper bound $N_{\mathrm{tot}}\sim 59$.

• Received 24 July 2006

DOI:https://doi.org/10.1103/PhysRevD.74.123007