Slow-roll inflation is studied as an effective field theory .
We find that the form of the inflaton potential consistent with WMAP data and slow roll is V ( \phi ) = N M ^ { 4 } w \left ( \frac { \phi } { \sqrt { N } M _ { Pl } } \right ) where \phi is the inflaton field , M is the inflation energy scale , and N \sim 50 is the number of efolds since the cosmologically relevant modes crossed the Hubble radius until the end of inflation .
The inflaton field scales as \phi = \sqrt { N } M _ { Pl } \chi .
The dimensionless function w ( \chi ) and field \chi are generically \mathcal { O } ( 1 ) .
The WMAP value for the amplitude of scalar adiabatic fluctuations | { \Delta } _ { k ad } ^ { ( S ) } | ^ { 2 } fixes the inflation scale M \sim 0.77 \times 10 ^ { 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 non-linear couplings are of the see-saw form in terms of the small ratio M / M _ { Pl } .
For example , the quartic coupling \lambda \sim \frac { 1 } { N } \left ( \frac { M } { M _ { Pl } } \right ) ^ { 4 } .
The smallness of the non-linear 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 ) \ll 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 .