3.2 Cosmic Inflation 35 by We will define what we will call a flat spacetime slice as the one where We define the term δN ( x , t ) to denote the number of e-foldings between the flat slice and a slice of uniform energy density at time t . Therefore, we reach what is called the δN formalism which thus allows us to calculate the primordial curvature perturbation from calculating the difference in the number of e-foldings of expansion between a flat slice and a latter uniform energy density slice, which is extremely useful when considering the perturbation that is produced from a particular inflation model. Inflation models most often assume that the content of the Universe during and immediately after inflation is dominated by the presence of one or more 3.2.2 Scalar Fields δN ( x ) = δ ∫ t$_{2}$ t$_{1}$ 1 a ( x , t ) d a ( x , t ) d t d t (3.25) = ψ ( x , t$_{2}$ ) − ψ ( x , t$_{1}$ ) (3.26) ψ ( x , t ) = 0 (3.27) ζ ( x , t ) = δN ( x , t ) (3.28)