scholarly journals VISCOUS CARDASSIAN UNIVERSE

2009 ◽  
Vol 18 (08) ◽  
pp. 1303-1318 ◽  
Author(s):  
CHANG-BO SUN ◽  
JIA-LING WANG ◽  
XIN-ZHOU LI
Keyword(s):  
Bulk Viscosity ◽  
State Parameter ◽  
Viscosity Coefficient ◽  
Dynamical Analysis ◽  
Model Parameters ◽  
Singular Curve ◽  
Type Ia ◽  
Cardassian Universe ◽  
Bulk Viscosity Coefficient ◽  
Best Fit

The viscous Cardassian cosmology is discussed, assuming that there is a bulk viscosity in the cosmic fluid. The dynamical analysis indicates that there exists a singular curve in the phase diagram of the viscous Cardassian model. In the viscous PL model, the equation-of-state parameter wk is no longer a constant and it can cross the cosmological constant divide wΛ = -1, in contrast with the same problem of the ordinary PL model. Other models possess similar characteristics. For MP and exp models, wk evolves more near -1 than the case without viscosity. The bulk viscosity also affects the virialization process of a collapse system in the universe: R vir /R ta is increasingly large when the bulk viscosity is increasing. In other words, the bulk viscosity retards the progress of the collapse system. In addition, we fit the viscous Cardassian models to current type Ia supernova data and give the best fit value of the model parameters, including the bulk viscosity coefficient τ.

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 Variable Chaplygin gas in Kaluza–Klein framework

2019 ◽  
Vol 97 (2) ◽  
pp. 117-124 ◽  
Author(s):  
M. Salti ◽  
O. Aydogdu ◽  
A. Tas ◽  
K. Sogut ◽  
E.E. Kangal
Keyword(s):  
Dark Matter ◽  
Dark Energy ◽  
Chaplygin Gas ◽  
Model Parameters ◽  
Energy Scenario ◽  
Type Ia ◽  
The Stability ◽  
Best Fit ◽  
Matter Energy ◽  
Kaluza Klein

We investigate cosmological features of the variable Chaplygin gas (VCG) describing a unified dark matter–energy scenario in a universe governed by the five dimensional (5D) Kaluza–Klein (KK) gravity. In such a proposal, the VCG evolves from the dust-like phase to the phantom or the quintessence phases. It is concluded that the background evolution for the KK-type VCG definition is equivalent to that for the dark energy interacting with the dark matter. Next, after performing neo-classical tests, we calculated the proper, luminosity, and angular diameter distances. Additionally, we construct a connection between the VCG in the KK universe and a homogenous minimally coupled scalar field by introducing its self-interacting potential and also we confirm the stability of the KK-type VCG model by making use of thermodynamics. Moreover, we use data from type Ia supernova, observational H(z) dataset and Planck-2015 results to place constraints on the model parameters. Subsequently, according to the best-fit values of the model parameters we analyze our results numerically.

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.

scholarly journals THE EFFECT OF DIFFERENT OBSERVATIONAL DATA IN CONSTRAINING COSMOLOGICAL PARAMETERS

2012 ◽  
Vol 10 ◽  
pp. 85-94 ◽  
Author(s):  
YUNGUI GONG ◽  
QING GAO ◽  
ZONG-HONG ZHU
Keyword(s):  
Dark Energy ◽  
Equation Of State ◽  
Observational Data ◽  
Cosmological Parameters ◽  
State Parameter ◽  
Wilkinson Microwave Anisotropy Probe ◽  
Model Parameters ◽  
Type Ia Supernova ◽  
Equation Of State Parameter ◽  
Type Ia

We use the SNLS3 compilation of 472 type Ia supernova data, the baryon acoustic oscillation measurement of distance, and the cosmic microwave background radiation data from the seven year Wilkinson Microwave Anisotropy Probe to study the effect of their different combinations on the fittings of cosmological parameters. Neither BAO nor WMAP7 data alone gives good constraint on the equation of state parameter of dark energy, but both WMAP7 data and BAO data help type Ia supernova data break the degeneracies among the model parameters, hence tighten the constraint on the variation of equation of state parameter wa, and WMAP7 data does the job a little better. Although BAO and WMAP7 data provide reasonably good constraints on Ωm and Ωk, it is not able to constrain the dynamics of dark energy, we need SNe Ia data to probe the property of dark energy, especially the variation of the equation of state parameter of dark energy. For the SNLS SNe Ia data, the nuisance parameters α and β are consistent for all different combinations of the above data. Their impacts on the fittings of cosmological parameters are minimal. ΛCDM model is consistent with current observational data.

scholarly journals Redshift parametrizations of dark energy and observational constraint on their parameters: Galileon gravity as background

2015 ◽  
Vol 30 (31) ◽  
pp. 1550151 ◽  
Author(s):  
Prabir Rudra ◽  
Chayan Ranjit ◽  
Sujata Kundu
Keyword(s):  
Dark Energy ◽  
Data Analysis ◽  
Theoretical Models ◽  
Acoustic Oscillation ◽  
Model Parameters ◽  
Distance Modulus ◽  
Microwave Background ◽  
Frw Universe ◽  
Type Ia ◽  
Best Fit

In this work, Friedmann–Robertson–Walker (FRW) universe filled with dark matter (DM) (perfect fluid with negligible pressure) along with dark energy (DE) in the background of Galileon gravity is considered. Four DE models with different equation of state (EoS) parametrizations have been employed namely, linear, Chevallier–Polarski–Lindler (CPL), Jassal–Bagla–Padmanabhan (JBP) and logarithmic parametrizations. From Stern, Stern+Baryonic Acoustic Oscillation (BAO) and Stern+BAO+Cosmic Microwave Background (CMB) joint data analysis, we have obtained the bounds of the arbitrary parameters [Formula: see text] and [Formula: see text] by minimizing the [Formula: see text] test. The best fit values and bounds of the parameters are obtained at 66%, 90% and 99% confidence levels which are shown by closed confidence contours in the figures. For the logarithmic model unbounded confidence contours are obtained and hence the model parameters could not be finitely constrained. The distance modulus [Formula: see text](z) against redshift [Formula: see text] has also been plotted for our predicted theoretical models for the best fit values of the parameters and compared with the observed Union2 data sample and SNe Type Ia 292 data and we have shown that our predicted theoretical models permits the observational datasets. From the data fitting it is seen that at lower redshifts [Formula: see text] the SNe Type Ia 292 data gives a better fit with our theoretical models compared to the Union2 data sample. So, from the data analysis, SNe Type Ia 292 data is the more favored data sample over its counterpart given the present choice of free parameters. From the study, it is also seen that the logarithmic parametrization model is less supported by the observational data. Finally, we have generated the plot for the deceleration parameter against the redshift parameter for all the theoretical models and compared the results with the work of Farooq et al., (2013).

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.

Observational data analysis for generalized cosmic Chaplygin gas in the background of Brans–Dicke theory

2021 ◽  
pp. 2150157
Author(s):  
Ujjal Debnath
Keyword(s):  
Data Analysis ◽  
State Parameter ◽  
Chaplygin Gas ◽  
Information Criteria ◽  
Model Parameters ◽  
Confidence Levels ◽  
Expansion Of The Universe ◽  
Generalized Cosmic Chaplygin Gas ◽  
Best Fit ◽  
The Universe

In this paper, we have considered the generalized cosmic Chaplygin gas (GCCG) in the background of Brans–Dicke (BD) theory and also assumed that the Universe is filled in GCCG, dark matter and radiation. To investigate the data fitting of model parameters, we have constrained the model using recent observations. Using [Formula: see text] minimum test, the best-fit values of the model parameters are determined by OHD+CMB+BAO+SNIa joint data analysis. We have drawn the contour figures for different confidence levels [Formula: see text], [Formula: see text] and [Formula: see text]. To examine the viability of the GCCG model in BD theory, we have also determined △AIC and △BIC using the information criteria (AIC and BIC). Graphically, we have analyzed the natures of the equation of state parameter and deceleration parameter for our best-fit values of model parameters. Also, we have studied the square speed of sound [Formula: see text] which lies in the interval [Formula: see text] for expansion of the Universe. So, our considered model is classically stable by considering the best-fit values of the model parameters due to the data analysis.

A study on the bouncing behavior of modified Chaplygin gas in presence of bulk viscosity and its consequences in the modified gravity framework

2017 ◽  
Vol 14 (12) ◽  
pp. 1750181 ◽  
Author(s):  
Surajit Chattopadhyay
Keyword(s):  
Bulk Viscosity ◽  
Modified Gravity ◽  
State Parameter ◽  
Chaplygin Gas ◽  
Scale Factor ◽  
Accelerated Expansion ◽  
Model Parameters ◽  
Modified Chaplygin Gas ◽  
Frw Universe

The present paper reports a study on the bouncing behavior of the viscous modified Chaplygin gas (MCG) in Einstein as well as modified gravity framework. For a bouncing scale factor proposed by Cai et al., Class. Quantum Grav. 28 (2011) 215011, we have studied the cosmology of MCG in presence of bulk viscosity. In Einstein gravity framework, we have studied the equation of state parameter and it has been found to cross [Formula: see text] indicating the end of the early accelerated expansion and it has also been observed that for flat FRW universe the presence of bulk viscosity induces the crossing of phantom boundary. Role of the model parameters of the MCG has also been investigated before and after the bounce. A Hubble flow dynamics has been carried out and, it was revealed that MCG is capable of realizing inflationary phase as well as an exit from inflation. A [Formula: see text] gravitational paradigm has also been considered, where the MCG density has been reconstructed in presence of bulk viscosity. Role of [Formula: see text] of the bouncing scale factor, describing how fast the bounce takes place, has also been studied in this framework.

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