Early Universe with modified scalar-tensor theory of gravity

TL;DR

This paper investigates the early universe's inflationary phase using a modified scalar-tensor gravity theory with a curvature squared term, showing it leads to de-Sitter expansion and aligns with observational data.

Contribution

It introduces a modified scalar-tensor gravity model with curvature squared term and demonstrates its quantum dynamics naturally produce de-Sitter inflation consistent with observations.

Findings

Quantum dynamics lead to de-Sitter expansion.

Inflationary parameters match observational limits.

Power-spectrum remains similar to minimally coupled models.

Abstract

Scalar-tensor theory of gravity with non-minimal coupling is a fairly good candidate for dark energy, required to explain late-time cosmic evolution. Here we study the very early stage of evolution of the universe with a modified version of the theory, which includes scalar curvature squared term. One of the key aspects of the present study is that, the quantum dynamics of the action under consideration ends up generically with de-Sitter expansion under semiclassical approximation, rather than power-law. This justifies the analysis of inflationary regime with de-Sitter expansion. The other key aspect is that, while studying gravitational perturbation, the perturbed generalized scalar field equation obtained from the perturbed action, when matched with the perturbed form of the background scalar field equation, relates the coupling parameter and the potential exactly in the same manner as the solution of classical field equations does, assuming de-Sitter expansion. The study also reveals that the quantum theory is well behaved, inflationary parameters fall well within the observational limit and quantum perturbation analysis shows that the power-spectrum does not deviate considerably from the standard one obtained from minimally coupled theory.