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

Authors: Narakorn Kaewkhao (IF Naresuan), Burin Gumjudpai (IF Naresuan)
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, $h_{\mu\nu} = f g_{\mu\nu}$ between effective metric, $h_{\mu\nu}$ and the usual metric $g_{\mu\nu}$ where $f \,=\,1 - \kappa{\phi}^{,\alpha}{\phi}_{,\alpha}/2$. In FLRW universe, NMDC coupling constant is limited in a range of $-2/ \dot{\phi}^{2} < \kappa \leq \infty$ preserving Lorentz signature of the effective metric. Slowly-rolling regime provides $\kappa < 0$ 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 $V \propto \phi^2$ and $V \propto \phi^4$, 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 $V \propto \phi^2$ 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 $V \propto \phi^2$ 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.
 Comments: 12 pages, no figures, chaotic inflation potential section and appendices added, minor correction on Planck unit Subjects: General Relativity and Quantum Cosmology (gr-qc) Journal reference: Physics of the Dark Universe 20 (2018) 20 DOI: 10.1016/j.dark.2018.02.004 Cite as: arXiv:1608.04014 [gr-qc] (or arXiv:1608.04014v5 [gr-qc] for this version)

## Submission history

From: Burin Gumjudpai [view email]
[v1] Sat, 13 Aug 2016 18:35:06 GMT (17kb)
[v2] Tue, 28 Mar 2017 16:08:17 GMT (21kb)
[v3] Mon, 17 Jul 2017 10:17:29 GMT (21kb)
[v4] Thu, 1 Mar 2018 17:47:09 GMT (23kb)
[v5] Sun, 18 Mar 2018 17:40:45 GMT (23kb)