Unifying the dark sector through a single matter fluid with non-zero pressure

TL;DR

This paper proposes a unified dark energy model combining a tachyonic fluid and a scalar field with vacuum energy, leading to a dynamic equation of state that transitions from early universe behavior to late-time acceleration, aligning with observations.

Contribution

It introduces a novel unified dark energy model based on a Murnaghan EoS analogy, incorporating non-zero pressure matter and analyzing its cosmological implications.

Findings

Model exhibits behavior similar to $ m extit{Lambda}$CDM in early universe

At late times, the model behaves as a logotropic or Chaplygin gas

Perturbation growth is enhanced at late times compared to standard models

Abstract

We explore a generalised unified dark energy model that incorporates a non-minimal interaction between a tachyonic fluid and an additional scalar field. Specifically, we require that the second field possesses a vacuum energy, introducing an ineliminable offset due to a symmetry-breaking mechanism. After the transition (occurring as due to the symmetry-breaking mechanism of the second field), the corresponding equation of state (EoS) takes the form of a combination between a generalised Chaplygin gas (GCG) component and a cosmological constant contribution. We reinterpret this outcome by drawing parallels to the so-called Murnaghan EoS, widely-employed in the realm of solid-state physics to characterise fluids that, under external pressure, counteract the pressure's effect. We examine the dynamic behaviour of this model and highlight its key distinctions compared to the GCG model. We establish parameter bounds that clarifies the model's evolution across cosmic expansion history, showing that it, precisely, exhibits behaviour akin to a logotropic fluid that eventually converges to the $\Lambda$CDM model in the early universe, while behaving as a logotropic or Chaplygin gas at intermediate and late times respectively. We explain our findings from a thermodynamic perspective, and determine the small perturbations in the linear regime. At very early times, the growth factor flattens as expected while the main departures occur at late times, where the Murnagham EoS results in a more efficient growth of perturbations. We discuss this deviation in view of current observations and conclude that our model is a suitable alternative to the standard cosmological paradigm, introducing the concept of a matter-like field with non-zero pressure.