Inflationary gravitational waves in consistent D → 4 Einstein-Gauss-Bonnet gravity

Abstract

We study the slow-roll single field inflation in the context of the consistent D → 4 Einstein-Gauss-Bonnet gravity that was recently proposed in [1]. In addition to the standard attractor regime, we find a new attractor regime which we call the Gauss-Bonnet attractor as the dominant contribution comes from the Gauss-Bonnet term. Around this attractor solution, we find power spectra and spectral tilts for the curvature perturbations and gravitational waves (GWs) and also a model-independent consistency relation among observable quantities. The Gauss-Bonnet term provides a nonlinear k4 term to the GWs dispersion relation which has the same order as the standard linear k2 term at the time of horizon crossing around the Gauss-Bonnet attractor. The Gauss-Bonnet attractor regime thus provides a new scenario for the primordial GWs which can be tested by observations. Finally, we study non-Gaussianity of GWs in this model and estimate the nonlinear parameters fs1s2s3NL, sq and fs1s2s3NL, eq by fitting the computed GWs bispectra with the local-type and equilateral-type templates respectively at the squeezed limit and at the equilateral shape. For helicities (+++) and (−−−), fs1s2s3NL, sq is larger while fs1s2s3NL, eq is larger for helicities (++−) and (−−+).