Decoupling, Trans-Planckia and Inflation

TL;DR

This work assesses whether decoupled, high-energy physics can meaningfully alter inflationary predictions for the CMB. It analyzes garden-variety hybrid-inflation models to identify two entry points for high-energy effects: non-adiabatic dynamics of heavy fields and adiabatic, loop-induced corrections from heavy states. Non-adiabatic heavy-field oscillations can imprint oscillatory features and large-scale power suppression in the CMB when they occur up to $10$–$30$ e-foldings before horizon exit, with modest improvements in fit to data. Adiabatic heavy-loop effects introduce logarithmic corrections to the inflaton potential, generating slow-roll modifications even when the heavy mass scale decouples, illustrating a controlled, logarithmic sensitivity rather than a simple $H^2/m_*^2$ suppression. The results suggest that trans-Planckian physics consistent with decoupling can leave observable imprints in inflationary observables, while truly non-decoupling proposals must confront the issue of low-energy predictability.

Abstract

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.)