Constraints on the speed of sound in the k-essence model of dark energy

TL;DR

This paper constrains the sound speed parameter in a k-essence dark energy model using multiple cosmological observations, finding that data favor a nearly homogeneous dark energy component with very low sound speed.

Contribution

It provides the first tight observational constraints on the constant sound speed in k-essence dark energy models, highlighting the preference for a homogeneous dark energy component.

Findings

Lower values of $c_s^2$ are tightly constrained near zero.

Mean $ ext{log}_{10}(c_s^2)$ is approximately -0.61.

Values of $c_s^2 \, extless\, 10^{-3}$ are more than 3$\sigma$ away from the mean.

Abstract

We consider a particular k-essence scalar field model for the late-time cosmic acceleration in which the sound speed, parametrized as $c_s$ is constant. We compute the relevant background and perturbation quantities corresponding to the observables like cosmic microwave background, type Ia supernova, cosmic chronometers, baryon acoustic oscillations, and the $f\sigma_8$. We put constraints on the $c_s^2$ parameter from these observations along with other parameters. We find lower values of $c_s^2$ which are close to zero are tightly constrained. Particularly, we find mean value of $\log_{\rm 10} (c_s^2)$ to be $-0.61$ and $c_s^2 \leq 10^{-3}$ is more than 3$\sigma$ away from this mean value. This means these observations favor a homogeneous dark energy component compared to the clustering one.