Clarifying inflation models:: Slow roll as an expansion in 1/Nefolds -: art. no. 023008

Slow-roll inflation is studied as an effective field theory. We find that the form of the inflaton potential consistent with Wilkinson Microwave Anisotropy Probe (WMAP) data and slow roll is V(phi)=NM(4)w(phi / root NMPl), where phi is the inflaton field, M is the inflation energy scale, and N similar to 50 is the number of e-folds since the cosmologically relevant modes crossed the Hubble radius until the end of inflation. The inflaton field scales as phi=root NMPl chi. The dimensionless function w(chi) and field chi are generically O(1). The WMAP value for the amplitude of scalar adiabatic fluctuations vertical bar Delta((S))(kad)vertical bar(2) fixes the inflation scale M similar to 0.77x10(16). This form of the potential makes manifest that the slow-roll expansion is an expansion in 1/N. A Ginzburg-Landau realization of the slow-roll inflaton potential reveals that the Hubble parameter, inflaton mass and nonlinear couplings are of the seesaw form in terms of the small ratio M/M-Pl. For example, the quartic coupling lambda similar to 1 / N(M / M-Pl)(4). The smallness of the nonlinear couplings is not a result of fine-tuning but a natural consequence of the validity of the effective field theory and slow-roll approximation. We clarify Lyth's bound relating the tensor/scalar ratio and the value of phi/M-Pl. The effective field theory is valid for V(phi)<< M-Pl(4) for general inflaton potentials allowing amplitudes of the inflaton field phi well beyond M-Pl. Hence bounds on r based on the value of phi/M-Pl are overly restrictive. Our observations lead us to suggest that slow-roll, single field inflation may well be described by an almost critical theory, near an infrared stable Gaussian fixed point.