Cosmological test on viscous bulk models using Hubble Parameter measurements and type Ia Supernovae data

From a phenomenological point of view, we analyze the dynamics of the Universe at late times by introducing a polynomial and hyperbolic bulk viscosity into the Einstein field equations respectively. We constrain their free parameters using the observational Hubble parameter data and the Type Ia Supernovae dataset to reconstruct the deceleration $q$ and the jerk $j$ parameters within the redshift region $0<z<2.5$. At current epochs, we obtain $q_0 = -0.680^{+0.085}_{-0.102}$ and $j_0 = 2.782^{+1.198}_{-0.741}$ for the polynomial model and $q_0 = -0.539^{+0.040}_{-0.038}$ ($-0.594^{+0.056}_{-0.056}$) and $j_0 = 0.297^{+0.051}_{-0.050}$ ($1.124^{+0.196}_{-0.178}$) for the tanh (cosh) model. Furthermore, we explore the statefinder diagnostic that gives us evident differences with respect to the concordance model (LCDM). According to our results this kind of models is not supported by the data over LCDM.Comment: Accepted for publication in EPJ

A hybrid model of viscous and Chaplygin gas to tackle the Universe acceleration

Motivated by two seminal models proposed to explain the Universe acceleration, this paper is devoted to study a hybrid model which is constructed through a generalized Chaplygin gas with the addition of a bulk viscosity. We call the model a Viscous Generalized Chaplygin Gas (VGCG) and its free parameters are constrained through several cosmological data like the Observational Hubble Parameter, Type Ia Supernovae, Baryon Acoustic Oscillations, Strong Lensing Systems, HII Galaxies and using Joint Bayesian analysis. In addition, we implement a Om-diagnostic to analyze the VGCC dynamics and its difference with the standard cosmological model. The hybrid model shows important differences when compared with the standard cosmological model. Finally, based on our Joint analysis we find that the VGCG could be an interesting candidate to alleviate the well-known Hubble constant tension.Comment: Accepted for publication in EPJ

An exponential equation of state of dark energy in the light of 2018 CMB Planck data

The dynamics of the Universe is analyzed using an exponential function for the dark energy equation of state, known as Gong-Zhang parameterization. The phase space of the free parameters presented in the model is constrained using Cosmic Microwave Background radiation, Cosmic Chronometers, modulus distance from Hydrogen II Galaxies, Type Ia Supernovae and measurements from Baryon Acoustic Oscillations, together with a stronger bound from a Joint analysis. The cosmological model is confronted with $\Lambda$CDM, observing there is a strong evidence for $\Lambda$CDM in the Joint analysis although the exponential model is preferred when the data are separated. Based on the Joint analysis, a value of $\omega_0 = -1.202^{+0.027}_{-0.026}$ is found for the characteristic parameter presented in the equation of state. Additionally, the cosmographic parameters at current times are reported, having $q_0 = -0.789^{+0.034}_{-0.036}$, $j_0=1.779^{+0.130}_{-0.119}$, and a transition deceleration-acceleration redshift $z_T = 0.644^{+0.011}_{-0.012}$. Furthermore, the age of the Universe is estimated as $t_U = 13.788^{+0.019}_{-0.019}$ Gyrs. Finally, we open a discussion if this model could alleviate the $H_0$ and $S_8$ tensions.Comment: Accepted in Physics of Dark Univers