Gauss-Bonnet assisted braneworld inflation in light of BICEP2 and Planck data

Abstract: Motivated by the idea that quantum gravity corrections usually suppress the power of the scalar primordial spectrum (E-mode) more than the power of the tensor primordial spectrum (B-mode), in this paper we construct a concrete gravitational theory in five-dimensions for which $V(\phi)\propto \phi^n$-type inflation ($n\ge 1$) generates an appropriate tensor-to-scalar ratio that may be compatible with the BICEP2 and Planck data together. The true nature of gravity is five-dimensional and described by the action $S = \int d^5{x} \sqrt{|g|} M^3 (- 6\lambda M^2 + R + \alpha M^{-2} {\cal R}^2)$ where $M$ is the five-dimensional Planck mass and ${\cal R}^2=R^2-4 R_{ab} R^{ab} + R_{abcd} R^{abcd}$ is the Gauss-Bonnet (GB) term. The five-dimensional "bulk" spacetime is anti-de Sitter ($\lambda<0$) for which inflation ends naturally. The effects of ${\cal R}^2$ term on the magnitudes of scalar and tensor fluctuations and spectral indices are shown to be important at the energy scale of inflation. For GB-assisted $m^2\phi^2$-inflation, inflationary constraints from BICEP2 and Planck, such as, $n_s\simeq 0.9603 (\pm 0.0073)$, $r=0.16 (+0.06-0.05)$ and $V_*^{1/4} \sim 1.5\times 10^{16} {\text GeV}$ are all satisfied for $ (-\lambda \alpha) \simeq (3-300)\times 10^{-5}$.