Cosmology of non-minimal derivative coupling to gravity in Palatini formalism and its chaotic inflation

Abstract

We consider, in Palatini formalism, a modified gravity of which the scalar field derivative couples to Einstein tensor. In this scenario, Ricci scalar, Ricci tensor and Einstein tensor are functions of connection field. As a result, the connection field gives rise to relation, $h_{μν} = f g_{μν}$ between effective metric, $h_{μν}$ and the usual metric $g_{μν}$ where $f \,=\,1 - κϕ^{,α}ϕ_{,α}/2 $. In FLRW universe, NMDC coupling constant is limited in a range of $ -2/ \dotϕ^{2} < κ\leq \infty $ preserving Lorentz signature of the effective metric. Slowly-rolling regime provides $κ< 0$ forbidding graviton from travelling at superluminal speed. Effective gravitational coupling and entropy of blackhole's apparent horizon are derived. In case of negative coupling, acceleration could happen even with $w_{\rm eff} > -1/3$. Power-law potentials of chaotic inflation are considered. For $V \propto ϕ^2$ and $V \propto ϕ^4$, it is possible to obtain tensor-to-scalar ratio lower than that of GR so that it satisfies $r < 0.12$ as constrained by Planck 2015 \cite{Ade:2015lrj}. The $V \propto ϕ^2$ case yields acceptable range of spectrum index and $r$ values. The quartic potential's spectrum index is disfavored by the Planck results. Viable range of $\k$ for $V \propto ϕ^2$ case lies in positive region, resulting in less blackhole's entropy, superluminal metric, more amount of inflation, avoidance of super-Planckian field initial value and stronger gravitational constant.