scholarly journals Thermodynamics in the viscous early universe

2010 ◽  
Vol 88 (11) ◽  
pp. 825-831 ◽  
Author(s):  
A. Tawfik
Keyword(s):  
Early Universe ◽  
Bulk Viscosity ◽  
Viscosity Coefficient ◽  
Matter Content ◽  
Einstein Field Equations ◽  
Field Equations ◽  
Curvature Parameter ◽  
Radical Revision ◽  
Bulk Viscosity Coefficient ◽  
The Universe

Assuming that the matter in the background geometry is a free gas and that no phase transitions were occurring in the early Universe, we discuss the thermodynamics of this closed system using classical approaches. We find that essential cosmological quantities, such as the Hubble parameter H, the scaling factor a, and the curvature parameter k, can be derived from this simple model, which on one hand fulfills and entirely obeys the laws of thermodynamics, and on the other hand, its results are compatible with the Friedmann–Robertson–Walker model and the Einstein field equations. Including a finite bulk viscosity coefficient leads to important changes in all these cosmological quantities. Accordingly, our picture about the evolution of the Universe and its astrophysical consequences seems to undergoing a radical revision. We find that k strongly depends on the thermodynamics of background matter. The time scale at which negative curvature might take place depends on the relation between the matter content and the total energy. Using quantum and statistical approaches, we introduce expressions for H and the bulk viscosity coefficient ξ.

scholarly journals Thermodynamics of viscous matter and radiation in the early universe

2012 ◽  
Vol 90 (5) ◽  
pp. 433-440 ◽  
Author(s):  
A. Tawfik ◽  
H. Magdy
Keyword(s):  
Early Universe ◽  
Viscosity Coefficient ◽  
Matter Content ◽  
Field Equations ◽  
Expansion Of The Universe ◽  
Curvature Parameter ◽  
Radical Revision ◽  
Quantum Treatment ◽  
Bulk Viscosity Coefficient ◽  
The Universe

Assuming that the background geometry is filled with a free gas consisting of matter and radiation and that no phase transitions are occurring in the early universe, we discuss the thermodynamics of this closed system using classical approaches. We find that essential cosmological quantities, such as the Hubble parameter H, scale factor a, and curvature parameter k, can be derived from this simple model. On one hand, it obeys the laws of thermodynamics entirely. On the other hand, the results are compatible with the Friedmann–Lemaitre–Robertson–Walker model and the Einstein field equations. The inclusion of a finite bulk viscosity coefficient derives important changes in all of these cosmological quantities. The thermodynamics of the viscous universe is studied and a conservation law is found. Accordingly, our picture of the evolution of the early universe and its astrophysical consequences seems to be the subject of radical revision. We find that the parameter k, for instance, strongly depends on the thermodynamics of the background matter. The time scale, at which a negative curvature might take place, depends on the relation between the matter content and the total energy. Using quantum and statistical approaches, we assume that the size of the universe is given by the volume occupied by one particle and one photon. Different types of interactions between matter and photon are taken into account. In this quantum treatment, expressions for H and a are also introduced. Therefore, the expansion of the universe turns out to be accessible.

scholarly journals Viscous cosmology in the Weyl-type f(Q, T) gravity

2021 ◽  
Vol 81 (12) ◽  
Author(s):  
Gaurav N. Gadbail ◽  
Simran Arora ◽  
P. K. Sahoo
Keyword(s):  
Bulk Viscosity ◽  
Viscosity Coefficient ◽  
Momentum Tensor ◽  
Model Parameters ◽  
Field Equations ◽  
Eos Parameter ◽  
Bulk Viscous ◽  
The Impact ◽  
Bulk Viscosity Coefficient ◽  
The Universe

AbstractBulk viscosity is the only viscous influence that can change the background dynamics in a homogeneous and isotropic universe. In the present work, we analyze the bulk viscous cosmological model with the bulk viscosity coefficient of the form $$\zeta =\zeta _0+\zeta _1H+\zeta _2\left( \frac{\dot{H}}{H}+H\right) $$ ζ = ζ 0 + ζ 1 H + ζ 2 H ˙ H + H where, $$\zeta _0$$ ζ 0 , $$\zeta _1$$ ζ 1 and $$\zeta _2$$ ζ 2 are bulk viscous parameters, and H is the Hubble parameter. We investigate the impact of the bulk viscous parameter on dynamics of the universe in the recently proposed Weyl-type f(Q, T) gravity, where Q is the non-metricity, and T is the trace of the matter energy–momentum tensor. The exact solutions to the corresponding field equations are obtained with the viscous fluid and the linear model of the form $$f(Q, T)=\alpha Q+\frac{\beta }{6\kappa ^2}T$$ f ( Q , T ) = α Q + β 6 κ 2 T , where $$\alpha $$ α and $$\beta $$ β are model parameters. Further, we constrain the model parameters using the 57 points Hubble dataset the recently released 1048 points Pantheon sample and the combination Hz + BAO + Pantheon, which shows our model is good congeniality with observations. We study the possible scenarios and the evolution of the universe through the deceleration parameter, the equation of state (EoS) parameter, the statefinder diagnostics, and the Om diagnostics. It is observed that the universe exhibits a transition from a decelerated to an accelerated phase of the universe under certain constraints of model parameters.

scholarly journals Anisotropic cosmological models in f(R,T) gravity with variable deceleration parameter

2017 ◽  
Vol 14 (06) ◽  
pp. 1750097 ◽  
Author(s):  
Pradyumn Kumar Sahoo ◽  
Parbati Sahoo ◽  
Binaya Kumar Bishi
Keyword(s):  
Bulk Viscosity ◽  
Deceleration Parameter ◽  
Momentum Tensor ◽  
Matter Content ◽  
Accelerating Universe ◽  
Data Sets ◽  
Type I ◽  
Field Equations ◽  
Future Evolution ◽  
The Universe

The objective of this work enclosed with the study of spatially homogeneous anisotropic Bianchi type-I universe in [Formula: see text] gravity (where [Formula: see text] is the Ricci scalar and [Formula: see text] is the trace of stress energy momentum tensor) in two different cases viz. [Formula: see text] and [Formula: see text] with bulk viscosity matter content. In this study, we consider a time varying deceleration parameter (DP), which generates an accelerating universe to obtain the exact solution of the field equations. The physical and kinematical properties of both the models are discussed in detail for the future evolution of the universe. We have explored the nature of WEC, DEC, SEC and energy density for both the cases. We have found that both the models, with bulk viscosity matter component, show an acceleration of the universe. We have also shown that the cosmic jerk parameter is compatible with the three kinematical data sets.

DECELERATING CAUSAL BULK VISCOUS COSMOLOGICAL MODELS

2000 ◽  
Vol 09 (02) ◽  
pp. 97-110 ◽  
Author(s):  
T. HARKO ◽  
M. K. MAK
Keyword(s):  
Relaxation Time ◽  
Bulk Viscosity ◽  
Viscosity Coefficient ◽  
State Equation ◽  
Cosmological Models ◽  
Boltzmann Gas ◽  
Bulk Viscous ◽  
Barotropic Equation ◽  
Bulk Viscosity Coefficient ◽  
The Universe

The dynamics of a causal bulk viscous cosmological fluid filled flat constantly decelerating noninflationary Robertson–Walker spacetime is considered. The matter component of the Universe is assumed to satisfy a linear barotropic equation of state and the state equation of the small temperature Boltzmann gas. The resulting cosmological models satisfy the condition of smallness of the viscous stress. The evolution of the relaxation time, temperature, bulk viscosity coefficient and comoving entropy of the dissipative cosmological fluid are obtained by assuming several bulk viscosity coefficient-relaxation time relations.

scholarly journals BULK VISCOSITY EFFECTS ON THE EARLY UNIVERSE STABILITY

2005 ◽  
Vol 20 (23) ◽  
pp. 1729-1739 ◽  
Author(s):  
NAKIA CARLEVARO ◽  
GIOVANNI MONTANI
Keyword(s):  
Early Universe ◽  
Bulk Viscosity ◽  
Viscosity Coefficient ◽  
Density Contrast ◽  
Thermal Bath ◽  
Viscosity Effects ◽  
Homogeneous Universe ◽  
Homogeneous Background ◽  
The Stability ◽  
The Universe

We present a discussion of the effects induced by the bulk viscosity on the very early Universe stability. The matter filling the cosmological (isotropic and homogeneous) background is described by a viscous fluid having an ultrarelativistic equation of state and whose viscosity coefficient is related to the energy density via a power-law of the form ζ = ζ0 ρν. The analytic expression of the density contrast (obtained for ν = 1/2) shows that, for small values of the constant ζ0, its behavior is not significantly different from the non-viscous one derived by Lifshitz. But as soon as ζ0 overcomes a critical value, the growth of the density contrast is suppressed forward in time by the viscosity and the stability of the Universe is favored in the expanding picture. On the other hand, in such a regime, the asymptotic approach to the initial singularity (taken at t = 0) is deeply modified by the apparency of significant viscosity in the primordial thermal bath, i.e. the isotropic and homogeneous Universe admits an unstable collapsing picture. In our model this feature also regards scalar perturbations while in the non-viscous case it appears only for tensor modes.

scholarly journals CAUSAL BULK VISCOUS DISSIPATIVE ISOTROPIC COSMOLOGIES WITH VARIABLE GRAVITATIONAL AND COSMOLOGICAL CONSTANTS

2002 ◽  
Vol 11 (08) ◽  
pp. 1265-1283 ◽  
Author(s):  
M. K. MAK ◽  
T. HARKO ◽  
J. A. BELINCHÓN
Keyword(s):  
Cosmological Constant ◽  
Hubble Parameter ◽  
Early Period ◽  
Viscosity Coefficient ◽  
Field Equations ◽  
Gravitational And Cosmological Constants ◽  
Bulk Viscous ◽  
Cosmological Constants ◽  
Bulk Viscosity Coefficient ◽  
The Universe

We consider the evolution of a flat Friedmann–Robertson–Walker Universe, filled with a causal bulk viscous cosmological fluid, in the presence of variable gravitational and cosmological constants. The basic equation for the Hubble parameter, generalizing the evolution equation in the case of constant gravitational coupling and cosmological term is derived, under the supplementary assumption that the total energy of the Universe is conserved. By assuming that the cosmological constant is proportional to the square of the Hubble parameter and a power law dependence of the bulk viscosity coefficient, temperature and relaxation time on the energy density of the cosmological fluid, two classes of exact solutions of the field equations are obtained. In the first class of solutions the Universe ends in an inflationary era, while in the second class of solutions the expan-sion of the Universe is noninflationary for all times. In both models the cosmological "constant" is a decreasing function of time, while the gravitational "constant" increases in the early period of evolution of the Universe and tends in the large time limit to a constant value.

scholarly journals Generalized Phenomenological Models of Dark Energy

2020 ◽  
Vol 2020 ◽  
pp. 1-8
Author(s):  
Prasenjit Paul ◽  
Rikpratik Sengupta
Keyword(s):  
General Relativity ◽  
Dark Energy ◽  
Energy Density ◽  
Cosmological Constant ◽  
Phenomenological Models ◽  
Einstein Field Equations ◽  
Field Equations ◽  
Free Parameters ◽  
The Universe ◽  
Matter Energy

It was first observed at the end of the last century that the universe is presently accelerating. Ever since, there have been several attempts to explain this observation theoretically. There are two possible approaches. The more conventional one is to modify the matter part of the Einstein field equations, and the second one is to modify the geometry part. We shall consider two phenomenological models based on the former, more conventional approach within the context of general relativity. The phenomenological models in this paper consider a Λ term firstly a function of a¨/a and secondly a function of ρ, where a and ρ are the scale factor and matter energy density, respectively. Constraining the free parameters of the models with the latest observational data gives satisfactory values of parameters as considered by us initially. Without any field theoretic interpretation, we explain the recent observations with a dynamical cosmological constant.

scholarly journals Spinning black holes with a separable Hamilton–Jacobi equation from a modified Newman–Janis algorithm

2020 ◽  
Vol 80 (11) ◽  
Author(s):  
Haroldo C. D. Lima Junior ◽  
Luís C. B. Crispino ◽  
Pedro V. P. Cunha ◽  
Carlos A. R. Herdeiro
Keyword(s):  
Black Holes ◽  
Jacobi Equation ◽  
Plasma Frequency ◽  
Conformal Factor ◽  
Matter Content ◽  
Spherically Symmetric ◽  
Einstein Field Equations ◽  
Field Equations ◽  
Original Procedure ◽  
Hamilton Jacobi Equation

AbstractObtaining solutions of the Einstein field equations describing spinning compact bodies is typically challenging. The Newman–Janis algorithm provides a procedure to obtain rotating spacetimes from a static, spherically symmetric, seed metric. It is not guaranteed, however, that the resulting rotating spacetime solves the same field equations as the seed. Moreover, the former may not be circular, and thus expressible in Boyer–Lindquist-like coordinates. Amongst the variations of the original procedure, a modified Newman–Janis algorithm (MNJA) has been proposed that, by construction, originates a circular, spinning spacetime, expressible in Boyer–Lindquist-like coordinates. As a down side, the procedure introduces an ambiguity, that requires extra assumptions on the matter content of the model. In this paper we observe that the rotating spacetimes obtained through the MNJA always admit separability of the Hamilton–Jacobi equation for the case of null geodesics, in which case, moreover, the aforementioned ambiguity has no impact, since it amounts to an overall metric conformal factor. We also show that the Hamilton–Jacobi equation for light rays propagating in a plasma admits separability if the plasma frequency obeys a certain constraint. As an illustration, we compute the shadow and lensing of some spinning black holes obtained by the MNJA.

Vorticity generated by sound

1954 ◽  
Vol 226 (1164) ◽  
pp. 7-16 ◽  
Keyword(s):  
Wind Speed ◽  
Bulk Viscosity ◽  
Viscosity Coefficient ◽  
Order Effects ◽  
Relaxation Processes ◽  
Acoustic Disturbances ◽  
Specific Heats ◽  
Coefficient Of Viscosity ◽  
Bulk Viscosity Coefficient ◽  
Intermolecular Relaxation

A revision is given of the basic theory of second-order effects caused by acoustic disturbances in a fluid, especially the vorticity giving rise to the ultrasonic wind, which was first explained by Eckart. The ultrasonic wind is produced by the interaction of the radiative and the non-radiative components of the acoustic motion. The wind speed is for the most part proportional to the acoustic attenuation coefficient. Wind-speed measurements thus usually furnish no more information about the second coefficient of viscosity, or the bulk viscosity, than do other attenuation measurements. It appears reasonable to regard the Stokesian bulk viscosity coefficient as a parameter of intramolecular and intermolecular relaxation processes. It does not have a unique value for all frequencies. Provided other parameters such as the coefficients of shear viscosity and heat conduction, and the specific heats are known independently, this effective bulk viscosity can be evaluated from any type of attenuation measurement. Measurements over large enough frequency ranges can distinguish among the contributions of different relaxation processes to the effective bulk viscosity coefficient.

Sign in / Sign up

Export Citation Format

Share Document