Scalar field dark matter with time-varying equation of state

TL;DR

This paper introduces a scalar field dark matter model interacting with dark energy, revealing a non-zero, evolving equation of state and non-zero sound speed, which could influence cosmic structure formation.

Contribution

It presents a novel fluid-based model of scalar field dark matter with a time-varying equation of state and non-adiabatic effects, highlighting new dark sector interactions.

Findings

Dark matter equation of state becomes negative at late times.

Dark matter has a non-zero sound speed at all scales.

Interactions in the dark sector can affect structure formation.

Abstract

We propose a new model of scalar field dark matter interacting with dark energy. Adopting a fluid description of the dark matter field in the regime of rapid oscillations, we find that the equation of state for dark matter is non-zero and even becomes increasingly negative at late times during dark energy domination. Furthermore, the speed of sound of dark matter is non-vanishing at all length scales, and a non-adiabatic pressure contribution arises. The results indicate that there are still unexplored possible interactions within the dark sector that lead to novel background effects and can impact structure formation processes.