Chaotic inflation in modified gravitational theories

We study chaotic inflation in the context of modified gravitational theories. Our analysis covers models based on (i) a field coupling ω(φ) with the kinetic energy X = -(1/2)g^μν _μφ_νφ and a nonmimimal coupling ζφ²R/2 with a Ricci scalar R, (ii) Brans-Dicke (BD) theories, (iii) Gauss-Bonnet (GB) gravity, and (iv) gravity with a Galileon correction. Dilatonic coupling with the kinetic energy and/or negative nonminimal coupling are shown to lead to compatibility with observations of the Cosmic Microwave Background (CMB) temperature anisotropies for the self-coupling inflaton potential V(φ) = λφ⁴/4. BD theory with a quadratic inflaton potential, which covers Starobinsky's f(R) model f(R) = R+R²/(6M²) with the BD parameter ω_BD = 0, gives rise to a smaller tensor-to-scalar ratio for decreasing ω_BD. In the presence of a GB term coupled to the field φ, we express the scalar/tensor spectral indices n_s and n_t as well as the tensor-to-scalar ratio r in terms of two slow-roll parameters and place bounds on the strength of the GB coupling from the joint data analysis of WMAP 7yr combined with other observations. We also study the Galileon-like self-interaction Φ(φ)Xφ with exponential coupling Φ(φ)e ^μφ. Using a CMB likelihood analysis we put bounds on the strength of the Galileon coupling and show that the self coupling potential can in fact be made compatible with observations in the presence of the exponential coupling with μ > 0.