Observational constraints of a new unified dark fluid and the $H_0$ tension

TL;DR

The paper investigates a new one-parameter unified dark fluid (UM) as a minimal extension of $\Lambda$CDM to address the dark sector and the $H_0$ tension. It derives a tachyon-field–based equation of state $p_u = -\rho_u + \rho_u \, {\rm sinc}\left(\frac{\mu \pi \rho_{u,0}}{\rho_u}\right)$ and develops linear perturbation theory with entropy perturbations to ensure a stable effective sound speed, implementing a modified CAMB for the calculations. Using Planck 2015 CMB, Pantheon, CC, and Riess2018 data, the authors constrain $\mu \approx 0.9$ and find $H_0$ values that can be higher than LCDM depending on dataset, thereby alleviating the tension but not fully resolving it. Bayesian evidence analyses consistently favor $\Lambda$CDM over UM across dataset combinations, though the UM remains viable for exploring large-scale structure and future data; the authors suggest future tests with upcoming surveys and neutrino physics.

Abstract

Unified cosmological models have received a lot of attention in astrophysics community for explaining both the dark matter and dark energy evolution. The Chaplygin cosmologies, a well known name in this group have been investigated matched with observations from different sources. Obviously, Chaplygin cosmologies have to obey restrictions in order to be consistent with the observational data. As a consequence, alternative unified models, differing from Chaplygin model, are of special interest. In the present work we consider a specific example of such a unified cosmological model, that is quantified by only a single parameter $μ$, that can be considered as a minimal extension of the $Λ$-cold dark matter cosmology. We investigate its observational boundaries together with an analysis of the universe at large scale. Our study shows that at early time the model behaves like a dust, and as time evolves, it mimics a dark energy fluid depicting a clear transition from the early decelerating phase to the late cosmic accelerating phase. Finally, the model approaches the cosmological constant boundary in an asymptotic manner. We remark that for the present unified model, the estimations of $H_0$ are slightly higher than its local estimation and thus alleviating the $H_0$ tension.

Figures (15)

• Figure 1: The evolution of the equation of state for the unified cosmic fluid (\ref{['eos-explicit']}) [left graph] and of the GCG model [right graph] have been shown using different values of the key parameters, namely, $\mu$ of the present model and $\alpha$ of the GCG model. From both the graphs, one can see that the evolution of these fluids seem to be similar in which the equation of state has a smooth transition from the dust fluid to the cosmological constant dominated universe ($w =-1$).
• Figure 2: The evolution of the deceleration parameter for the unified cosmic fluid (\ref{['eos-explicit']}) [left graph] and of the GCG model [right graph] have been shown using different values of the key parameters, namely, $\mu$ of the present model and $\alpha$ of the GCG model.
• Figure 3: The evolution of the density parameters for the unified cosmic scenario (\ref{['eos-explicit']}) [left graph] and of the GCG scenario [right graph] have been shown using different values of the key parameters, namely, $\mu$ of the present model and $\alpha$ of the GCG model.
• Figure 4: We compare the observational constraints through the one-dimensional posterior distrbibutions of some parameters of the unified model before and after the inclusion of CMB data with the background datasets, namely CC, Pantheon and Pantheon+CC.
• Figure 5: 68% and 95% confidence-level contour plots between the free parameters of the unified model as well as the one dimensional marginalized posterior distributions for all the model parameters using several combinations of the datasets. From this figure one can clearly see that $\mu$ has a very strong positive correlation with $H_0$ irrespective of the observational datasets.