Cosmological evolution of viable models in the generalized scalar-tensor theory

TL;DR

This paper analyzes the evolution of viable generalized scalar-tensor theories post-GW170817, identifying key parameters that distinguish subclasses like Horndeski and GLPV, through numerical modeling and parameter distribution analysis.

Contribution

It provides a model-independent numerical framework to study the parameter distributions of DHOST theories and identifies minimal parameters to differentiate subclasses.

Findings

The Planck mass run rate, α_M, is not effective for distinguishing theories.

The kinetic-braiding parameter, α_B, effectively discriminates models from Horndeski.

Higher-order theory parameters, α_H and β_1, are smaller but useful for classification.

Abstract

We investigate the parameter distributions of the viable generalized scalar-tensor theory with conventional dust matter after GW170817 in a model-independent way. We numerically construct the models by computing the time evolution of a scalar field, which leads to a positive definite second-order Hamiltonian and are consistent with the observed Hubble parameter. We show the model parameter distributions in the degenerate higher-order scalar-tensor (DHOST) theory, and its popular subclasses, e.g., Horndeski and GLPV theories, etc.. We find that 1) the Planck mass run rate, $\alpha_M$, is insensitive to distinguish the theories. 2) the kinetic-braiding parameter, $\alpha_B$, clearly discriminates the models from those of the Horndeski theory, 3) the parameters for the higher-order theories, $\alpha_H$ and $\beta_1$, are relatively smaller in magnitude (by several factors) than $\alpha_M$ and $\alpha_B$, but can still be used for discriminating the theories except for the GLPV theory. Based on the above three facts, we propose a minimal set of parameters that sensibly distinguishes the subclasses of DHOST theories, ($\alpha_M$, $\alpha_B-\alpha_M/2$, $\beta_1$).