  With present and future observations becoming of higher and higher quality,
it is timely and necessary to investigate the most significant theoretical
uncertainties in the predictions of inflation. We show that our ignorance of
the entire history of the Universe, including the physics of reheating after
inflation, translates to considerable errors in observationally relevant
parameters. Using the inflationary flow formalism, we estimate that for a
spectral index $n$ and tensor/scalar ratio $r$ in the region favored by current
observational constraints, the theoretical errors are of order $\Delta n / | n
- 1| \sim 0.1 - 1$ and $\Delta r /r \sim 0.1 - 1$. These errors represent the
dominant theoretical uncertainties in the predictions of inflation, and are
generically of the order of or larger than the projected uncertainties in
future precision measurements of the Cosmic Microwave Background. We also show
that the lowest-order classification of models into small field, large field,
and hybrid breaks down when higher order corrections to the dynamics are
included. Models can flow from one region to another.

Astrophysics