A New Unified Dark Fluid Model and Its Cosmic Constraint

TL;DR

This paper introduces a new unified dark fluid model with a flexible equation of state, tests its compatibility with current cosmic observations, and finds it fits data as well as the standard model, with potential differences in structure formation.

Contribution

The paper proposes a novel unified dark fluid model that generalizes existing models and demonstrates its viability using observational data, highlighting potential deviations from the $\Lambda$CDM model.

Findings

Model fits current cosmic data as well as $\Lambda$CDM.

Deviations from $\Lambda$CDM$ are within 3σ confidence level.

Model predicts a non-zero sound speed, unlike $\Lambda$CDM$.

Abstract

In this paper, we propose a new unified dark fluid (UDF) model with equation of state (EoS) $w(a)=-\alpha/(\beta a^{-n}+1)$, which includes the generalized Chaplygin gas model (gGg) as its special case, where $\alpha$, $\beta$ and $n$ are three positive numbers. It is clear that this model reduces to the gCg model with EoS $w(a)=-B_s/(B_s+(1-B_s)a^{-3(1+\alpha)})$, when $\alpha=1$, $\beta=(1-B_s)/B_s$ and $n=3(1+\alpha)$. By combination the cold dark matter and the cosmological constant, one can coin a EoS of unified dark fluid in the form of $w(a)=-1/(1+(1-\Omega_{\Lambda})a^{-3}/\Omega_{\Lambda})$. With this observations, our proposed EoS provides a possible deviation from $\Lambda$CDM model when the model parameters $\alpha$ and $n$ deviate from 1 and 3 respectively. By using the currently available cosmic observations from type Ia supernovae (SN Ia) Union2.1, baryon acoustic oscillation (BAO) and cosmic microwave background radiation (CMB), we test the viability of this model and detect the possible devotion from the $\Lambda$CDM model. The results show that the new UDF model fits the cosmic observation as well as that of the $\Lambda$CDM model and no deviation is found from the $\Lambda$CDM model in $3\sigma$ confidence level. However, our new UDF model can give a non-zero sound speed, as a contrast, which is zero for the $\Lambda$CDM model. We expect the large structure formation information can distinct the new UDF model from the $\Lambda$CDM model.