Cosmological models with arbitrary spatial curvature in the theory of gravity with non-minimal derivative coupling

Abstract

We investigate isotropic and homogeneous cosmological scenarios in the scalar-tensor theory of gravity with non-minimal derivative coupling of a scalar field to the curvature given by the term $(\zeta /H_0^2)G^{\mu \nu }{\nabla }_{\mu }\varphi {\nabla }_{\nu }\varphi$ in the Lagrangian. In general, a cosmological model is determined by six dimensionless parameters: the coupling parameter $\zeta$, and density parameters ${\Omega }_0$ (cosmological constant), ${\Omega }_2$ (spatial curvature term), ${\Omega }_3$ (non-relativistic matter), ${\Omega }_4$ (radiation), ${\Omega }_6$ (scalar field term), and the universe evolution is described by the modified Friedmann equation. In the case $\zeta =0$ (no non-minimal derivative coupling) and ${\Omega }_6=0$ (no scalar field) one has the standard $\Lambda$CDM-model, while if ${\Omega }_6\ne 0$ -- the $\Lambda$CDM-model with an ordinary scalar field. The situation is crucially changed when the scalar field possesses non-minimal derivative coupling to the curvature, i.e. when $\zeta \ne 0$. Now, depending on model parameters, (i) There are three qualitatively different initial state of the universe: an eternal kinetic inflation, an initial singularity, and a bounce. The bounce is possible for all types of spatial geometry of the homogeneous universe; (ii) For all types of spatial geometry, the universe goes inevitably through the primary quasi-de Sitter (inflationary) epoch when $a(t)\propto e^{h_{dS}(H_{0}t)}$ with the de Sitter parameter $h_{dS}^2=1/9\zeta -8\zeta {\Omega }_2^3/27{\Omega }_6$. The mechanism of primary or kinetic inflation is provided by non-minimal derivative coupling and needs no fine-tuned potential; (iii) There are cyclic scenarios of the universe evolution with the non-singular bounce at a minimal value of the scale factor, and a turning point at the maximal one; (iv) There is a natural mechanism providing a change of cosmological epochs.