It was shown in the last section that if the number of e-foldings
exceeds 60, then inflation can solve the horizon, flatness, and
relic (monopole) problems. One generally favors this model over a
Big-Bang model, because of its naturalness. That is to say,
inflation offers a generic scenario for solving the initial value
problems. However, this arbitrariness can also be viewed as a
problem for inflation. For instance, throughout this paper it has
been assumed that inflation necessarily leads to a flat universe
(*k* = 0). However, Hawking, Turok, Linde, and others have shown
that inflation can result in a non-flat universe
[52],
[53].
Models can be created that produce
unwanted or wanted relics and contain inhomogeneities.
Furthermore, we have seen inflation requires an inflaton field to
drive the inflation. Where does it come from and what is its
natural value? Originally Guth had the inflaton as corresponding
to a GUT transition; however, today the preferred energy range is
on the order of the Planck scale. Thus, to fully understand
inflation one needs a full quantum theory of gravity. For these
reasons one may ask; Is inflation a particular type of
cosmological model, or is inflation an arbitrary constituent of
any successful theory of the cosmos?

Inflation's strength today can be seen from its predictions of
large-scale structure. Different models predict different
structure and this can be used to narrow the number of possible
models. One can further constrain the inflationary models by
cosmological parameters. The cosmic background observations,
galaxy surveys, lensing experiments, and standard candles can be
used pin-down the cosmological parameters. In this way,
observational parameters (e.g., *H*,
_{M},
_{}) can be given viable ranges and
inflationary parameters can be determined based on these preferred
ranges. With the cosmological parameters determined, inflation
parameters depend only on the height and shape of the inflaton
potential. The inflaton potential correspondingly yields
predictions about the large-scale structure of the universe and
the anisotropies in the cosmic background radiation.

The study of large-scale structure has been pursued for many years [54]. The problem was that there was an appealing mechanism which could produce the types of perturbations needed to produce the observed large-scale structure. These perturbation types are manifested by the anisotropies in the cosmic background. The anisotropies result from acoustic oscillations in the baryon-photon fluid just before recombination. Therefore, the anisotropy spectrum offers a `snap-shot' of the seeded inhomogeneities that eventually resulted in galactic structure. These inhomogeneities were first discovered when COBE mapped the cosmic background in the early 1990's [55]. In this intimate way, the cosmic background and galaxy surveys predict a scheme by which structure was formed. The type of perturbation that is needed only results from models which predict Gaussian, adiabatic, nearly scale-invariant perturbations [56]. The only known models that fall into this category are the inflationary models [33], [50], [51].