The speed of gravitational waves and power-law solutions in a scalar-tensor model

TL;DR

This paper demonstrates that in a scalar-tensor dark energy model with specific couplings, power-law solutions can be maintained while satisfying recent observational constraints on gravitational wave speed, through parameter restrictions that cancel anomalous contributions.

Contribution

It shows that kinetic and Gauss-Bonnet couplings can cancel each other’s effects on gravitational wave speed in power-law solutions, aligning with observational bounds.

Findings

Power-law solutions can be compatible with gravitational wave speed constraints.

Kinetic and Gauss-Bonnet couplings cancel anomalous contributions.

Model achieves dark energy contributions of order unity.

Abstract

One of the most relevant solutions in any cosmological model concerning the evolution of the universe is the power-law solution. For the scalar-tensor model of dark energy with kinetic and Gauss Bonnet couplings, it is shown that we can conserve the power-law solution and at the same time meet the recent observational bound on the speed of gravitational waves. In the FRW background the anomalous contribution to the speed of gravitational waves, coming from the kinetic and Gauss-Bonnet couplings, cancel each other for power-law solutions. It is shown that by simple restriction on the model parameters we can achieve a non-time-dependent cancellation of the defect in the velocity of the gravitational waves. The model can realize the cosmic expansion with contributions from the kinetic and Gauss-Bonnet couplings of the order of ${\cal O}(1)$ to the dark energy density parameter. The results are valid on the homogeneous FRW background and the limitations of the approach are discussed.