Evading the Lyth Bound in Hybrid Natural Inflation

Abstract

Generically, the gravitational-wave or tensor-mode contribution to the primordial curvature spectrum of inflation is tiny if the field-range of the inflaton is much smaller than the Planck scale. We show that this pessimistic conclusion is naturally avoided in a rather broad class of small-field models. More specifically, we consider models where an axion-like shift symmetry keeps the inflaton potential flat (up to non-perturbative cosine-shaped modulations), but inflation nevertheless ends in a waterfall-regime, as is typical for hybrid inflation. In such hybrid natural inflation scenarios (examples are provided by Wilson line inflation and fluxbrane inflation), the slow-roll parameter $\epsilon$ can be sizable during an early period (relevant for the CMB spectrum). Subsequently, $\epsilon$ quickly becomes very small before the tachyonic instability eventually terminates the slow roll regime. In this scenario, one naturally generates a considerable tensor-mode contribution in the curvature spectrum, collecting nevertheless the required amount of e-foldings during the final period of inflation. While non-observation of tensors by Planck is certainly not a problem, a discovery in the medium to long term future is realistic.

LaTeX, RevTeX 4.1, 7 pages; v3: note on curvaton added, refs added

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Cited by in corpus (16)

• D7-Brane Chaotic Inflation
• D7-Brane Moduli Space in Axion Monodromy and Fluxbrane Inflation
• Towards Natural Inflation in String Theory
• Evading Lyth bound in models of quintessential inflation
• Inflating with Large Effective Fields
• Testing Hybrid Natural Inflation with BICEP2
• Modified Lyth bound and implications of BICEP2 results
• Accidental Inflation from Kähler Uplifting
• The Simple Single Field Inflation Models and the Running of Spectral Index
• Harmonic Hybrid Inflation
• BICEP2, the curvature perturbation and supersymmetry
• General bounds in Hybrid Natural Inflation
• A Permutation on Hybrid Natural Inflation
• GUT-scale inflation with sizeable tensor modes
• Generalized hybrid natural inflation
• The Early Universe with High-Scale Supersymmetry