Dimensionless Equations Of State Research Articles

On the anisotropic bouncing universe with viscosity

We investigate the role of bulk and shear viscosity in the spatially homogeneous anisotropic spacetime, in particular, the Kantowski–Sachs (KS) spacetime. General conditions for the bouncing evolution of universe in anisotropic background have been obtained by using the derived propagation equations of expansion scalar, shear scalar and spatial 3-curvature. We show that the presence of shear viscosity in the model prohibits the energy density to attain its extremum in the bouncing model. We explore the qualitative behavior of KS cosmologies by formulating the Einstein’s field equations into a plane-autonomous system of equations by taking dimensionless equation of state. The stability of the system has been investigated by evaluating and analyzing the eigenvalues at the critical points. The stable solutions exist for the system composed of bulk and shear viscosity. The present analysis through dynamical system method illustrates that the universe does not exhibit synchronous bounce with perfect fluid and/or viscous fluids in the KS spacetime.

A Better Reconciliation of Hubble Tension in the Dark Energy Scalar Field

Hubble tension between the local measurement and global observation has been a key problem in cosmology. In this paper, we consider the quintessence scalar field, phantom field and quintom field as the dark energy to reconcile this problem. Different from most previous work, we start from the dimensionless equation of state (w) of dark energy, not a parameterization of potential. The combined analysis shows that observational data sets favor Hubble constant , which can reconcile Hubble tension within 1.20σ. We also perform a Bayes factor analysis using the MCEvidence code, and confirm that the phantom scalar field is still the most effective. To investigate the reason of Hubble tension, we analyze the density parameter. The comparison shows that the scalar fields provide a slightly larger Ω b h 2 and smaller Ω c h 2 than the standard ΛCDM model. We finally analyze a possible reason of Hubble tension from the kinematic acceleration . We find an interesting physical phenomenon. The acceleration in these scalar fields are similar as the ΛCDM model at about redshift z > 0.5. However, they increase and deviate from each other at low redshift, especially in the near future. Only the in phantom scalar field will decrease in the future.

Equation-of-state of neutron stars with junction conditions in the Starobinsky model

We study the Starobinsky or [Formula: see text] model of [Formula: see text] 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 [Formula: see text] 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 [Formula: see text] with [Formula: see text] and [Formula: see text] and the constraint of [Formula: see text], we obtain the minimal mass of the NS to be around 1.44 [Formula: see text]. In addition, if [Formula: see text] is larger than 5.0, the mass and radius of the NS would be smaller.

Qualitative viscous cosmology.

The Full (non--truncated) Israel--Stewart theory of bulk viscosity is applied to dissipative FRW spacetimes. Dimensionless variables and dimensionless equations of state are used to write the Einstein--thermodynamic equations as a plane autonomous system and the qualitative behaviour of this system is determined. Entropy production in these models is also discussed.

Viscous fluid cosmology

In this paper we shall examine the effects of both bulk and shear viscosities upon a variety of cosmological models. We assume that the viscous terms can be modelled by dimensionless equations of state, and this allows us to write the Einstein equations as a system of autonomous differential equations. After briefly discussing the Friedmann--Robertson--Walker and Bianchi I models, we discuss in some detail the Bianchi V and Kantowski--Sachs models. In all cases we find the critical points of the flow and elucidate their nature, making use of the energy conditions. Because of our choice of dimensionless variables, (almost) all critial points represent self-similar solutions to the field equations. We also study the case of a Bianchi V perfect fluid model with a non-linear equation of state. We find that the models are structurally stable under the addition of shear viscosity, whereas they are structurally unstable under the introduction of bulk viscosity. Almost all of the Bianchi V models examined have initial singularities where the matter is dynamically unimportant; the Kantowski--Sachs models have final singularities where the matter is dynamically important.

Experimental determination of the hydrodynamic moment associated with the nonsymmetric penetration of a disk into a compressible fluid

The nonsymmetric penetration of a disk (circular cylinder) into a compressible fluid is investigated. The results are obtained by physical modeling. A fluid with a low speed of sound (finely dispersed medium with gas bubbles, whose dimensionless equation of state coincides with the dimensionless equation of state of water [5]) was used as the working medium. The experiments were carried out at entry angles on the interval 54‡ < θ < 88‡, angles of attack on the interval −15‡ < α < +15‡ and Mach numbers on the interval 0.002 ≤ M ≤ 0.2.

Dimensionless equations of state and attenuation of shock waves

Dimensionless equations of state and attenuation of shock waves