Dark energy and cosmological solutions in second-order string gravity

TL;DR

This paper investigates cosmological solutions in second-order string gravity with curvature corrections and scalar fields, analyzing stability, observational constraints, and future universe evolution, including avoidance of big rip singularities.

Contribution

It provides a comprehensive phase space analysis of second-order string gravity models, exploring stability and observational viability, and examines mechanisms to prevent big rip singularities.

Findings

Gauss-Bonnet gravity alone may not explain current acceleration.

Big rip singularities can be avoided with curvature corrections and fluid-scalar coupling.

Certain models allow avoiding big rip regardless of fluid equation of state.

Abstract

We study the cosmological evolution based upon a $D$-dimensional action in low-energy effective string theory in the presence of second-order curvature corrections and a modulus scalar field (dilaton or compactification modulus). A barotropic perfect fluid coupled to the scalar field is also allowed. Phase space analysis and the stability of asymptotic solutions are performed for a number of models which include ($i$) fixed scalar field, ($ii$) linear dilaton in string frame, and ($iii$) logarithmic modulus in Einstein frame. We confront analytical solutions with observational constraints for deceleration parameter and show that Gauss-Bonnet gravity (with no matter fields) may not explain the current acceleration of the universe. We also study the future evolution of the universe using the GB parametrization and find that big rip singularities can be avoided even in the presence of a phantom fluid because of the balance between the fluid and curvature corrections. A non-minimal coupling between the fluid and the modulus field also opens up the interesting possibility to avoid big rip regardless of the details of the fluid equation of state.