  In this thesis we will primarily demonstrate how additional scalar degrees of
freedom - which are motivated from many high-energy embeddings - open up new
observational windows onto the physics of inflation. We construct a Bayesian
framework to statistically compare models with additional fields given the
current astronomical data. Putting inflation to the test, we perform our
analysis on the quadratic curvaton accompanying a range of inflationary
potentials, where we find that only one potential remains as a viable
candidate. Furthermore, if the curvaton mechanism were to be confirmed by
future non-Gaussianity measurements (from large scale structure surveys), the
model could prove to be tremendously informative of the early inflationary
history. The initial conditions given to these scalar fields become apparent
when considering their fundamentally quantum behaviour. Taking this physics
into account leads us to develop detailed models for post-inflationary
phenomenology (namely, the curvaton and freeze-in dark matter models) and to
discover powerful new probes of inflation itself. We further demonstrate how
this theoretical study complements our statistical approach by motivating the
prior information in our Bayesian analyses. The thesis finishes with a
discussion of the future prospects for inflationary model selection. By
hypothesising different toy survey configurations, we forecast different
outcomes using information theory and our newly developed Bayesian experimental
design formalism. In particular, we find that the most likely observable to
optimise model selection between single-field inflationary models, through an
order of magnitude precision improvement in the future, is the scalar spectral
index. We conclude with a summary of the results obtained throughout.
