content_image/310159.jpg content_image/310160.jpg content_image/310161.jpg content_image/310162.jpg
by 

<img src='content_image/310159.jpg'>

<img src='content_image/310160.jpg'>

We will define what we will call a flat spacetime slice as the one where 

<img src='content_image/310161.jpg'>

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 

<img src='content_image/310162.jpg'>

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. 

## 3.2.2 Scalar Fields 

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 

