  We survey recent calculations probing what constraints decoupling can put on
the influence of very-high-energy physics on the predictions of inflation for
the cosmic microwave background. Using garden-variety hybrid inflation models
we identify two ways in which higher-energy physics can intrude into
inflationary predictions. 1. Non-adiabatic physics up to 30 e-foldings before
horizon exit can have observable consequences for the CMB, including the
introduction of features in the fluctuation spectrum at specific multipoles and
a general suppression of power at large scales (a prediction which was made
before the recent release of WMAP results). Our comparison of simple models
with the data marginally improves the goodness of fit compared to the standard
concordance cosmology, but only at the 1.5-sigma level. 2. Adiabatic physics
can also affect inflationary predictions through virtual loops of very-heavy
particles, but these can only be distinguished from lower-energy effects within
the context of specific models. We believe our conclusions should apply equally
well to trans-Planckian physics provided only that this physics satisfies
decoupling, such as string theory appears to do. (Non-decoupling
trans-Planckian proposals must explain why meaningful theoretical predictions
at low energies are possible at all.)
