Einstein static Universe in hybrid metric-Palatini gravity

TL;DR

This paper investigates the stability of the Einstein static universe within hybrid metric-Palatini gravity, revealing that many stable solutions exist, which could support the universe starting in a static state during inflation.

Contribution

It analyzes the stability of the Einstein static universe in hybrid metric-Palatini gravity using linear homogeneous perturbations, identifying conditions for stability in this modified gravity framework.

Findings

Large class of stable solutions found

Stability depends on derivatives of scalar potential

Supports the universe starting in a static state

Abstract

Hybrid metric-Palatini gravity is a recent and novel approach to modified theories of gravity, which consists of adding to the metric Einstein-Hilbert Lagrangian an f(R) term constructed a la Palatini. It was shown that the theory passes local tests even if the scalar field is very light, and thus implies the existence of a long-range scalar field, which is able to modify the dynamics in galactic and cosmological scales, but leaves the Solar System unaffected. In this work, motivated by the possibility that the Universe may have started out in an asymptotically Einstein static state in the inflationary universe context, we analyse the stability of the Einstein static Universe by considering linear homogeneous perturbations in the respective dynamically equivalent scalar-tensor representation of hybrid metric-Palatini gravity. Considering linear homogeneous perturbations, the stability regions of the Einstein static universe are parametrized by the first and second derivatives of the scalar potential, and it is explicitly shown that a large class of stable solutions exists in the respective parameter space, in the context of hybrid metric-Palatini gravity.