**3.1. Friedmann's Equation
Exponential
Expansion**

One way to describe inflation is that during inflation, a
_{inf}
term dominates Eq. 11. Thus, during inflation we have,

(22) (23) (24) (25) (26) |

where *t*_{i} and *R*_{i} are the time and
scale factor at the beginning of inflation. To get Eq. 26 we have assumed
0
*t*_{i} << *t* < *t*_{e}
(where *t*_{e} is the end of inflation) and we
have used Eq. 22.
Equation 26 is the exponential expansion of the Universe during inflation.
The e-folding time is 1/*H*. The doubling time is (*ln* 2) /
*H*. That is, during every interval
*t* =
1/*H*, the size of the Universe increases by a factor of
*e* = 2.718281828... and during every interval
*t* =
(*ln* 2) / *H* the size of the Universe doubles.