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We consider a unified barotropic dark fluid model with dissipation. Our fluid asymptotes between two power laws and so can interpolate between the dust and dark energy (DE) equations-of-state at early and late times. The dissipative part is a bulk viscous part with constant viscosity coefficient. The model is analyzed using the phase-space methodology which helps to understand the dynamical behavior of the model in a robust manner without reference to the system solution. The parameters of the model are constrained through many observational constraints. The model is tested through many physical and observational tests. We first considered the model independent O m (z) test. Results for O m (z) are plotted against the BAO data for this quantity from different authors, which shows that the model is consistent with the data points for the full redshift range. The χ 2 statistics results in the value of χ 2 red = 0. 3 0 with a p -value of 0. 9 5. The Hubble parameter equation is solved numerically and results are plotted against the recent set of Hubble data. The χ 2 test with the Hubble data resulted in the χ 2 red value of 0. 6 7 2 with a p -value of 0. 9 4. The distance modulus at different values of redshift is calculated numerically and results are compared to the newest set of SNe Ia data, the Pantheon Sample. We obtained a χ 2 red value of 1. 0 4 with a p -value of 0. 1 7. These results show that our model is efficiently consistent with observations. The model expectations for the evolution of the universe are also studied by testing the evolution of the deceleration parameter, the density of the universe, and the effective equation-of-state parameter of the model and of its underlying dark energy candidate. The value of the present day viscosity coefficient of the cosmic fluid, ζ 0 , is estimated. It is found to be 8 × 1 0 6 Pa s. We argue that this model is able to explain the behavior of the universe evolution. [ABSTRACT FROM AUTHOR]

The formalism of dark energy based on modeling speed of sound as a function of equation-of-state parameter is elaborated. A specific model which allows detailed study of cosmological constant boundary crossing is introduced and analytical solutions for the model dynamics are obtained. It is shown how in specific parameter regimes dark energy can be a transient phenomenon. It is further demonstrated how the model reproduces specific features of recent fits of dark energy w (z) to observational data. [ABSTRACT FROM AUTHOR]

We explore the possibility of formation of Δ baryons (1232 MeV) in neutron star matter in an effective chiral model within the relativistic mean-field framework. With variation in delta-meson couplings, consistent with the constraints imposed on them, the resulting equation-of-state (EoS) is obtained and the neutron star properties are calculated for static and spherical configuration. Within the framework of our model, the critical densities of formation of Δ s and the properties of neutron stars (NS) are found to be very sensitive to the iso-vector coupling compared to the scalar or vector couplings. We revisit the Δ puzzle and look for the possibility of phase transition from nonstrange hadronic matter (including nucleons and Δ s) to deconfined quark matter, based on QCD theories. The resultant hybrid star configurations satisfy the observational constraints on mass from the most massive pulsars PSR J1614-2230 and PSR J0348+0432 in static condition obtained with the general hydrostatic equilibrium based on GTR. Our radius estimates are well within the limits imposed from observational analysis of QLMBXs. The obtained values of R 1. 4 are in agreement with the recent bounds specified from the observation of gravitational wave (GW170817) from binary neutron star merger. The constraint on baryonic mass from the study of binary system PSR J0737-3039 is also satisfied with our hybrid EoS. [ABSTRACT FROM AUTHOR]

We study for the first time how a new class of stars could impact an ensemble of pulsars with known masses and spin-periods. These new compact objects are strange stars admixed with condensed dark matter (DM). In this exploratory theoretical work, our goal is to determine how the basic parameters of pulsars are modified for such a new class of compact objects. In particular, we consider three different scenarios that correspond to a DM mass fraction of , and . Within each scenario with fixed parameters, we predict theoretically other properties of the pulsars, such as the radius, the compactness, the moment of inertia, as well as the angular momentum. Our numerical results are summarized in tables and also shown graphically for better visualization, where a comparison between the different scenarios can be made. [ABSTRACT FROM AUTHOR]

This paper is devoted to study the compact objects whose pressure and density are related through polytropic equation-of-state (EoS) and MIT bag model (for quark stars) in the background of gravity. We solve the field equations together with the hydrostatic equilibrium equation numerically for the model and discuss physical properties of the resulting solution. It is observed that for both types of stars (polytropic and quark stars), the effects of model parameters and remain the same. We also obtain that the energy conditions are satisfied and stellar configurations are stable for both EoS. [ABSTRACT FROM AUTHOR]

We study the Starobinsky or model of for neutron stars with the structure equations represented by the coupled differential equations and the polytropic type of the matter equation-of-state (EoS). The junction conditions of gravity are used as the boundary conditions to match the Schwarzchild solution at the surface of the star. Based on these the conditions, we demonstrate that the coupled differential equations can be solved directly. In particular, from the dimensionless EoS with and and the constraint of , we obtain the minimal mass of the NS to be around 1.44 . In addition, if is larger than 5.0, the mass and radius of the NS would be smaller. [ABSTRACT FROM AUTHOR]

We study the role played by an equation-of-state during gravitational collapse of a radiating star. Starting from an initially static matter configuration obeying a linear equation-of-state, the star loses hydrostatic equilibrium and undergoes dissipative collapse in the form of a radial heat flux. We show that the equation-of-state parameter plays an important role in determining the temperature profiles of the collapsing body. [ABSTRACT FROM AUTHOR]

We analyze the properties of a generic cosmological fluid described by the van der Waals equation-of-state. Exact solutions for the energy-density evolution are found as implicit functions of the scale factor for a flat Friedmann-Robertson-Walker (FRW) spacetime. The possible values of the free parameter in the van der Waals equation are selected a posteriori, in accordance with asymptotic behaviors that are physically relevant. The stability of the model against small perturbations is studied through the hydrodynamic perturbations of the fluid for the relevant cases. It is found that a van der Waals fluid seems appropriate to describe noneternal inflationary scenarios. [ABSTRACT FROM AUTHOR]