Gauss-Bonnet dark energy

TL;DR

The paper investigates late-time cosmic acceleration in a scalar-tensor theory with a Gauss-Bonnet coupling inspired by string/M-theory. By deriving the modified Friedmann equations and studying exponential couplings and potentials, it identifies regimes where the effective equation of state $w$ crosses the phantom barrier and where Big Rip singularities can be avoided, including de Sitter attractors. The analysis yields exact and asymptotic solutions, showing that the GB term can dominate at high curvature to tame singularities, and that the sign of the kinetic term ($\gamma$) crucially controls the existence of phantom phases. The work demonstrates that stringy corrections can significantly influence late-time cosmology and motivates further pursuit of consistent string-inspired dark-energy models.

Abstract

We propose the Gauss-Bonnet dark energy model inspired by string/M-theory where standard gravity with scalar contains additional scalar-dependent coupling with Gauss-Bonnet invariant. It is demonstrated that effective phantom (or quintessence) phase of late universe may occur in the presence of such term when the scalar is phantom or for non-zero potential (for canonical scalar). However, with the increase of the curvature the GB term may become dominant so that phantom phase is transient and $w=-1$ barrier may be passed. Hence, the current acceleration of the universe may be caused by mixture of scalar phantom and (or) potential/stringy effects. It is remarkable that scalar-Gauss-Bonnet coupling acts against the Big Rip occurence in phantom cosmology.

Figures (1)

• Figure 1: The qualitative behavior of $\phi_0^2$ versus $h_0$ from (\ref{['GB18']}).