The Degeneracy Problem in Non-Canonical Inflation

TL;DR

This paper investigates the degeneracy problem in non-canonical inflation models, especially Dirac-Born-Infeld theories, and how future observations of non-Gaussianity and gravitational waves can help distinguish and reconstruct these models.

Contribution

It quantifies the degeneracy in non-canonical inflation models and analyzes how specific future measurements can improve model differentiation and reconstruction.

Findings

Detection of equilateral non-Gaussianity breaks degeneracy with canonical models.

Measurement of primordial gravitational waves improves non-canonical model reconstruction.

Degeneracy remains significant without these observational breakthroughs.

Abstract

While attempting to connect inflationary theories to observational physics, a potential difficulty is the degeneracy problem: a single set of observables maps to a range of different inflaton potentials. Two important classes of models affected by the degeneracy problem are canonical and non-canonical models, the latter marked by the presence of a non-standard kinetic term that generates observables beyond the scalar and tensor two-point functions on CMB scales. The degeneracy problem is manifest when these distinguishing observables go undetected. We quantify the size of the resulting degeneracy in this case by studying the most well-motivated non-canonical theory having Dirac-Born-Infeld Lagrangian. Beyond the scalar and tensor two-point functions on CMB scales, we then consider the possible detection of equilateral non-Gaussianity at Planck-precision and a measurement of primordial gravitational waves from prospective space-based laser interferometers. The former detection breaks the degeneracy with canonical inflation but results in poor reconstruction prospects, while the latter measurement enables a determination of $n_T$ which, while not breaking the degeneracy, can be shown to greatly improve the non-canonical reconstruction.