Fibre inflation: observable gravity waves from IIB string compactifications

We introduce a simple string model of inflation, in which the inflaton field can take trans-Planckian values while driving a period of slow-roll inflation. This leads naturally to a realisation of large field inflation, inasmuch as the inflationary epoch is well described by the single-field scalar potential V = V0 (3 − 4 e^{−ϕ/√3}). Remarkably, for a broad class of vacua all adjustable parameters enter only through the overall coefficient V0, and in particular do not enter into the slow-roll parameters. Consequently these are determined purely by the number of e-foldings, N_e, and so are not independent: epsilon ≃ (3/2) eta^2. This implies similar relations among observables like the primordial scalar-to-tensor amplitude, r, and the scalar spectral tilt, ns: r ≃ 6(ns − 1)^2. N_e is itself more model-dependent since it depends partly on the post-inflationary reheat history. In a simple reheating scenario a reheating temperature of T_rh ≃ 10^9 GeV gives N_e ≃ 58, corresponding to n_s ≃ 0.970 and r ≃ 0.005, within reach of future observations. The model is an example of a class that arises naturally in the context of type IIB string compactifications with large-volume moduli stabilisation, and takes advantage of the generic existence there of Kahler moduli whose dominant appearance in the scalar potential arises from string loop corrections to the Kahler potential. The inflaton field is a combination of Kahler moduli of a K3-fibered Calabi-Yau manifold. We believe there are likely to be a great number of models in this class — “high-fibre models” — in which the inflaton starts off far enough up the fibre to produce observably large primordial gravity waves.