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 , . 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 . 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 . 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 . The only known models that fall into this category are the inflationary models , , .