# The Inflationary Model

In general terms, inflation is a model of the very early universe, which involves a very, very short period of exponential expansion. During the inflationary process, the universe is thought to have increased in size from something much smaller than a sub-atomic proton to about the size of a marble in a fraction of the first second of existence. Through this process, modern cosmology now believes it can better explain why spacetime is both homogeneous and geometrically flat.

Given that the full details of the underlying mathematics of inflation
is beyond the scope of this discussion, the goal is only to provide
a general overview of the concepts and how it fits within the overall
cosmological model predicated on the standard
*Big Bang* model. Based
on the earlier presentation of the *historic
timeline* of developments within cosmology, the basic idea of
inflation is a relatively new one, which has only emerged over the last
30 years or so. However, even in this comparably short time, it has
managed to spawn a large number of variant theories, although we shall
try to remain focused on just the central ideas, which have continued
to evolve primarily because the basic model of cosmology, in isolation,
failed to explain a number of key problems, as listed below.

- Homogeneity problems relate to issues associated with density
perturbations and structure formation

- Isotropy problems are associated with the apparent uniformity
of the universe in all directions and why there appears to be no
overall rotation of the universe.
- Horizon problems relate to the homogeneity problem, but
focuses on the issue of causal contact in order to reconcile the
uniformity of the CMB temperature.
- Flatness problems relate to the issue of why the apparent density
of the universe is so close to its critical density, such that Ω≈1.
- Entropy problems relate to the 2
^{nd}law of thermodynamics in which the entropy of a system will not decrease other than by increasing the entropy of some other system. However, if the lifetime of a closed universe with entropy [S] is defined by the equation S^{2/3 }*10^{−43}seconds, it suggests that the entropy of the universe is huge, but does not explain why. - Structure formation problems relate to the perturbation in the
assumed homogeneity of the universe required to form the large scale
structures of the universe in an appropriate timeframe.

- Monopole problem relates to the absence of magnetic monopoles in the universe.

While these problems are representative of the motivation that drove the earlier development of the inflationary model, they also represent the yardstick by which any other theory, wishing to compete with inflation, must be measured.