Viscous Dark Energy Model Research Articles

Global 21-cm brightness temperature in viscous dark energy models

We investigate the global 21-cm brightness temperature in the context of viscous dark energy (VDE) models. The bulk viscosity of dark energy perturbs the Hubble evolution of the Universe which could cool baryons faster, and hence, alter the 21-cm brightness temperature. An additional amount of entropy is also produced as an outcome of the viscous flow. We study the combined contribution of Hawking radiation from primordial black holes, decay and annihilation of particle dark matter and baryon-dark matter scattering in the backdrop of VDE models towards modification of the 21-cm temperature. We obtain bounds on the VDE model parameters which can account for the observational excess of the EDGES experiment (-500+200 -500 mK at redshift 14 < z < 20) due to the interplay of the above effects. Moreover, our analysis yields modified constraints on the dark matter mass and scattering cross-section compared to the case of the ΛCDM model.

Growth of matter perturbations in the extended viscous dark energy models

In this work, we study the extended viscous dark energy models in the context of matter perturbations. To do this, we assume an alternative interpretation of the flat Friedmann–Lemaître–Robertson–Walker Universe, through the nonadditive entropy and the viscous dark energy. We implement the relativistic equations to obtain the growth of matter fluctuations for a smooth version of dark energy. As result, we show that the matter density contrast evolves similarly to the Lambda CDM model in high redshift; however, in late time, it is slightly different from the standard model. Using the latest geometrical and growth rate observational data, we carry out a Bayesian analysis to constrain parameters and compare models. We see that our viscous models are compatible with cosmological probes, and the Lambda CDM recovered with a 1sigma confidence level. The viscous dark energy models relieve the tension of H_0 in 2 sim 3 sigma . Yet, by involving the sigma _8 tension, some models can alleviate it. In the model selection framework, the data discards the extended viscous dark energy models.

Exploring new physics beyond the standard cosmology with Dark Energy Survey Year 1 Data

Abstract With the recent data from the first year of the Dark Energy Survey (DES Y1), we attempt to probe whether there is new physics beyond the standard cosmology and to reconcile the σ 8 tension between the DES Y1 and Planck datasets in three alternative cosmological models. Combining the galaxy clustering and weak gravitational lensing data from the DES Y1 with other cosmological observations including cosmic microwave background radiation, baryon acoustic oscillations, Type Ia supernovae (SNe Ia) and Planck lensing, we find that: (i) For the interacting dark energy model, we have an updated constraint on the modified matter expansion rate ϵ = − 0 . 00004 ± 0 . 00031 , which indicates that there is no hint of interaction between dark matter and dark energy in the dark sector of the universe; (ii) For the viscous dark energy model, up to now, the strictest constraint on the bulk viscosity coefficient η 0 . 00152 at the 2 σ confidence level is obtained; (iii) For the dynamical dark energy model, our constraint on the typical parameter β = − 0 . 04 ± 0 . 12 implies that there is still no evidence of dynamical dark energy. Meanwhile, we find the σ 8 tension can be effectively alleviated in the above three alternatives by constraining them with the DES Y1 and Planck datasets separately.

The integrated Sachs–Wolfe effect in the bulk viscous dark energy model

We examine linear perturbation theory to evaluate the contribution of viscosity coefficient in the growing of dark matter perturbations in the context of the bulk viscous dark energy model inspired by thermodynamical dissipative phenomena proposed by [B. Mostaghel et al., 2017, The European Physical Journal C, 77, 541]. As the cosmological implementations, we investigate the {\it Integrated Sachs-Wolfe} (ISW) auto-power spectrum, the ISW-galaxy cross-power spectrum and derive limits on $f\sigma_8$. The dimensionless bulk viscosity coefficient ($\gamma$) in the absence of interaction between dark sectors, modifies the Hubble parameter and the growth function, while the Poisson equation remains unchanged. Increasing $\gamma$ reduces the dark matter growing mode at the early epoch while a considerable enhancement will be achieved at the late time. This behavior imposes non-monotonic variation in the time evolution of gravitational potential generating a unique signature on the CMB photons. The bulk viscous dark energy model leads to almost a decreasing in ISW source function at the late time. Implementation of the Redshift Space Distortion (RSD) observations based on "Gold-2017" catalogue, shows $\Omega^0_{\rm m} =0.303^{+0.044}_{-0.038}$, $\gamma=0.033^{+0.098}_{-0.033}$ and $\sigma_8= 0.769^{+0.080}_{-0.089}$ at $1\sigma$ level of confidence. Finally, tension in the $\sigma_8$ is alleviated in our viscous dark energy model.

Constraining viscous dark energy models with the latest cosmological data

Based on the assumption that the dark energy possessing bulk viscosity is homogeneously and isotropically permeated in the universe, we propose three new viscous dark energy (VDE) models to characterize the accelerating universe. By constraining these three models with the latest cosmological observations, we find that they just deviate very slightly from the standard cosmological model and can alleviate effectively the current H_0 tension between the local observation by the Hubble Space Telescope and the global measurement by the Planck Satellite. Interestingly, we conclude that a spatially flat universe in our VDE model with cosmic curvature is still supported by current data, and the scale invariant primordial power spectrum is strongly excluded at least at the 5.5sigma confidence level in the three VDE models as the Planck result. We also give the 95% upper limits of the typical bulk viscosity parameter eta in the three VDE scenarios.

Non-minimal derivative coupling scalar field and bulk viscous dark energy

Inspired by thermodynamical dissipative phenomena, we consider bulk viscosity for dark fluid in a spatially flat two-component Universe. Our viscous dark energy model represents phantom-crossing which avoids big-rip singularity. We propose a non-minimal derivative coupling scalar field with zero potential leading to accelerated expansion of the Universe in the framework of bulk viscous dark energy model. In this approach, the coupling constant, kappa , is related to viscosity coefficient, gamma , and the present dark energy density, varOmega _mathrm{DE}^0. This coupling is bounded as kappa in [-1/9H_0^2(1-varOmega _mathrm{DE}^0), 0]. We implement recent observational data sets including a joint light-curve analysis (JLA) for SNIa, gamma ray bursts (GRBs) for most luminous astrophysical objects at high redshifts, baryon acoustic oscillations (BAO) from different surveys, Hubble parameter from HST project, Planck CMB power spectrum and lensing to constrain model free parameters. The joint analysis of JLA + GRBs + BAO + HST shows that varOmega _mathrm{DE}^0=0.696pm 0.010, gamma =0.1404pm 0.0014 and H_0=68.1pm 1.3. Planck TT observation provides gamma =0.32^{+0.31}_{-0.26} in the 68% confidence limit for the viscosity coefficient. The cosmographic distance ratio indicates that current observed data prefer to increase bulk viscosity. The competition between phantom and quintessence behavior of the viscous dark energy model can accommodate cosmological old objects reported as a sign of age crisis in the varLambda CDM model. Finally, tension in the Hubble parameter is alleviated in this model.

Dynamical Study of a Constant Viscous Dark Energy Model in Classical and Loop Quantum Cosmology**Supported by the Algerian Ministry of Education and Research, and DGRSDT.

Dynamical behaviors and stability properties of a flat space Friedmann–Robertson–Walker universe filled with pressureless dark matter and viscous dark energy are studied in the context of standard classical and loop quantum cosmology. Assuming that the dark energy has a constant bulk viscosity, it is found that the bulk viscosity effects influence only the quintessence model case leading to the existence of a viscous late time attractor solution of de-Sitter type, whereas the quantum geometry effects influence the phantom model case where the big rip singularity is removed. Moreover, our results of the Hubble parameter as a function of the redshift are in good agreement with the more recent data.

Interacting two-fluid viscous dark energy models in a non-flat universe

We study the evolution of the dark energy parameter within the scope of a spatially non-flat and isotropic Friedmann-Robertson-Walker model filled with barotropic fluid and bulk viscous stresses. We have obtained cosmological solutions that do not have a Big Rip singularity, and concluded that in both non-interacting and interacting cases the non-flat open Universe crosses the phantom region. We find that during the evolution of the Universe, the equation of state for dark energy ωD changes from ωeffD > −1 to ωeffD < −1, which is consistent with recent observations.

Evolution of Interacting Viscous Dark Energy Model in Einstein Cosmology

We investigate the evolution of the viscous dark energy (DE) interacting with the dark matter (DM) in the Einstein cosmology model. By using the linearizing theory of the dynamical system, we find that, in our model, there exists a stable late time scaling solution which corresponds to the accelerating universe. We also find the unstable solution under some appropriate parameters. In order to alleviate the coincidence problem, some authors considered the effect of quantum correction due to the conform anomaly and the interacting dark energy with the dark matter. However, if we take into account the bulk viscosity of the cosmic fluid, the coincidence problem will be softened just like the interacting dark energy cosmology model. That is to say, both the non-perfect fluid model and the interacting the dark energy cosmic model can alleviate or soften the singularity of the universe.

Viscous Dark Energy Models with Variable G and A

We consider a cosmological model with bulk viscosity η and variable cosmological A α ρ -α, alpha = const and gravitational G constants. The model exhibits many interesting cosmological features. Inflation proceeds du to the presence of bulk viscosity and dark energy without requiring the equation of state p = —ρ. During the inflationary era the energy density ρ does not remain constant, as in the de-Sitter type. Moreover, the cosmological and gravitational constants increase exponentially with time, whereas the energy density and viscosity decrease exponentially with time. The rate of mass creation during inflation is found to be very huge suggesting that all matter in the universe is created during inflation.