**B. The basic idea**

Inflation is based on the idea that the Universe could have been
dominated at very early times by an unusual type of matter with an
equation of state *p* = *w*
, with *w*
< - 1/3. In most models,
these conditions are achieved by a scalar field
(the
"inflaton") which enters into a state where the potential energy
density (*V*()) dominates over other terms in the stress-energy
tensor. Under these conditions, the inflaton has an equation of state
with *w* close to -1, and the Universe enters a period of
quasi-exponential
expansion called inflation. In most models, the inflaton evolves
classically down the potential *V*; these are called "slow roll"
models.

During inflation, the spatial curvature becomes
negligible, leading to a "flat" universe (with
_{tot} = 1).
Also, the quasi-exponential
expansion pushes field modes from infinitesimal scales
all the way to the size of the observed Universe and even well
beyond that. Specific calculations allow us to follow the "zero
point" quantum fluctuations in these modes out to cosmic scales and
lead to concrete predictions for the primordial perturbations produced by
a given inflationary scenario.

Cosmic Inflation has predictive power because details of the state of
the Universe before inflation are hidden beyond the domain of
realistic observations. The observable features of the Universe after
inflation are specified by the dynamics of inflation and are insensitive
to the initial conditions. To realize this picture, a
minimum number (*N*_{e}) of *e*-foldings of the scale
factor during inflation must be achieved (for example,
*N*_{e} 60 for
inflation at the Grand
Unification scale). After a sufficient period of inflation, energy
must be transferred from the (dominant) inflaton field into ordinary
matter via inflaton decay,
causing the Universe to "reheat".

A crucial aspect of the inflationary scenario is that it radically changes the causal structure of the Universe as compared with the Standard Big Bang. It is only thanks to these changes that one can hope to explain the state of the Universe using causal processes. Thus, inflation is noted for "solving the horizon problem" (in the sense that it makes a Universe that appears homogeneous over the present Hubble scale much more probable, given a variety of initial conditions), in addition to explaining specific features of the observed Universe.