Speaker
Description
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 $(\zeta/H_0^2) G^{\mu\nu}\nabla_\mu\phi \nabla_\nu\phi$ 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 $\Omega_6\not=0$ -- the $\Lambda$CDM-model with an ordinary scalar field. As is well-known, this model has an initial singularity, the same for all $k$ ($k=0,\pm1$), while its global behavior depends on $k$. The universe expands eternally if $k=0$ (zero spatial curvature) or $k=-1$ (negative spatial curvature), while in case $k=+1$ (positive spatial curvature) the universe expansion is changed to contraction, which is ended by a final singularity. The situation is crucially changed when the scalar field possesses non-minimal derivative coupling to the curvature, i.e. when $\zeta\not=0$. Now, depending on model parameters, (i) There are three qualitatively different initial state of the universe: an {\em eternal kinetic inflation}, an {\em initial singularity}, and a {\em bounce}. The bounce is possible for {\em all} types of spatial geometry of the homogeneous universe; (ii) For {\em all} types of spatial geometry, the universe goes inevitably through the {\em primary quasi-de Sitter} (inflationary) epoch when $a(t)\propto e^{h_{dS}(H_0t)} $ with the de Sitter parameter $h_{dS}^2={1}/{9\zeta}-{8\zeta\Omega_2^3}/{27\Omega_6}$.
The mechanism of primary or {\em kinetic} inflation is provided by non-minimal derivative coupling and needs no fine-tuned potential; (iii) There are {\em 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 {\em change} of cosmological epochs.
| Тематическая секция | Гравитация и космология |
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