Observations and nonstandard FRW models

1987 ◽  
Vol 318 ◽  
pp. 487 ◽  
Author(s):  
A. A. Coley
Keyword(s):  
Frw Models

Flat FRW models with variableG and ?

1992 ◽  
Vol 24 (4) ◽  
pp. 351-357 ◽  
Author(s):  
D. Kalligas ◽  
P. Wesson ◽  
C. W. F. Everitt
Keyword(s):  
Frw Models

Some FRW models with variable G and

1997 ◽  
Vol 14 (4) ◽  
pp. 945-953 ◽  
Author(s):  
Abdussattar ◽  
R G Vishwakarma
Keyword(s):  
Frw Models ◽  
Variable G

The quasi-steady-state cosmology in a radiation-dominated phase

2019 ◽  
Vol 34 (32) ◽  
pp. 1950262
Author(s):  
Raj Bali
Keyword(s):  
Steady State ◽  
Present Model ◽  
Accelerating Universe ◽  
Type I ◽  
Quasi Steady State ◽  
Proper Distance ◽  
Frame Work ◽  
Frw Models ◽  
Friedmann Robertson Walker ◽  
Flatness Problem

Analytical solutions for radiation-dominated phase of Quasi-Steady-State Cosmology (QSSC) in Friedmann–Robertson–Walker models are obtained. We find that matter density is positive in all the cases [Formula: see text]. The nature of Hubble parameter (H) in [Formula: see text] is discussed. The deceleration parameter [Formula: see text] is marginally less than zero indicating accelerating universe. The scale factor [Formula: see text] is graphically shown with time. The model represents oscillating universe between the above-mentioned limits. Because of the bounce in QSSC, the maximum density phase is still matter-dominated. The models represent singularity-free model. We also find that the models have event horizon i.e. no observer beyond the proper distance [Formula: see text] can communicate each other in FRW models for radiation-dominated phase in the frame work of QSSC. The FRW models are special classes of Bianchi type I, V, IX spacetimes with zero, negative and positive curvatures, respectively. Initially i.e. at [Formula: see text], the models represent steady model. We have tried to show how a good fit can be obtained to the observations in the framework of QSSC during radiation-dominated phase. The present model is free from singularity, particle horizon and provides a natural explanation for the flatness problem. Therefore, our model is superior to other models.

scholarly journals USING (4+1) SPLIT AND ENERGY CONDITIONS TO STUDY THE INDUCED MATTER IN 5D RICCI-FLAT COSMOLOGY

2007 ◽  
Vol 22 (05) ◽  
pp. 985-994 ◽  
Author(s):  
YONGLI PING ◽  
HONGYA LIU ◽  
LIXIN XU
Keyword(s):  
Energy Condition ◽  
Energy Conditions ◽  
Dominant Energy Condition ◽  
Strong Energy Condition ◽  
Ricci Flat ◽  
Strong Energy ◽  
Expansion Of The Universe ◽  
Frw Models ◽  
Induced Matter ◽  
The Universe

We use (4+1) split to derive the 4D induced energy density ρ and pressure p of the 5D Ricci-flat cosmological solutions which are characterized by having a bounce instead of a bang. The solutions contain two arbitrary functions of time t and, therefore, are mathematically rich in giving various cosmological models. By using four known energy conditions (null, weak, strong, and dominant) to pick out and study physically meaningful solutions, we find that the 4D part of the 5D solutions asymptotically approaches to the standard 4D FRW models and the expansion of the universe is decelerating for normal induced matter for which all the four energy conditions are satisfied. We also find that quintessence might be normal or abnormal, depending on the parameter w of the equation of state. If -1 ≤ w < -1/3, the expansion of the universe is accelerating and the quintessence is abnormal because the strong energy condition is violated while other three are satisfied. For phantom, all the four energy conditions are violated. Before the bounce, all the four energy conditions are violated, implying that the cosmic matter before the bounce could be explained as a phantom that has a large negative pressure and makes the universe bouncing. In the early times after the bounce, the dominant energy condition is violated, while the other three are satisfied, and so the cosmic matter could be explained as a super-luminal acoustic matter.

scholarly journals Some Robertson-Walker Models with Variable G and λ

1997 ◽  
Vol 50 (5) ◽  
pp. 893 ◽  
Author(s):  
Abdussattar ◽  
R. G. Vishwakarma
Keyword(s):  
Fluid Flow ◽  
Cosmological Parameters ◽  
Time Varying ◽  
Variable G And Λ ◽  
Flow Vector ◽  
Ricci Collineation ◽  
Frw Models ◽  
Variable G

Some Robertson-Walker (RW) models admitting a contracted Ricci collineation along the fluid flow vector and having time-varying G and Λ are investigated. The nature of the expansion of the models obtained in the cases k = ±l is found to be interchanged from the corresponding standard FRW models. Estimates of the present values of various cosmological parameters are obtained and found to be well within the observational limits.

scholarly journals REVISITING ROTATIONAL PERTURBATIONS AND THE MICROWAVE BACKGROUND

2007 ◽  
Vol 16 (11) ◽  
pp. 1715-1723 ◽  
Author(s):  
VIKTOR G. CZINNER ◽  
MÁTYÁS VASÚTH
Keyword(s):  
Background Radiation ◽  
Field Equations ◽  
Microwave Background ◽  
Rotation Tensor ◽  
Microwave Background Radiation ◽  
Order Of Magnitude ◽  
Frw Model ◽  
General Relativistic ◽  
Frw Models ◽  
The Given

We consider general-relativistic rotational perturbations in homogeneous and isotropic Friedmann–Robertson–Walker (FRW) cosmologies. Taking linear perturbations of FRW models, the general solution to the field equations contains tensorial, vectorial and scalar functions. The vectorial terms are in connection with rotations in the given model and due to the Sachs–Wolfe effect they produce contributions to the temperature fluctuations of the cosmic microwave background radiation (CMBR). In this paper we obtain the analytic time dependence of these contributions in a spatially flat FRW model with pressureless ideal fluid, in the presence and the absence of a cosmological constant Λ. We find that the solution can be separated into an integrable and a nonintegrable part, as in the case of scalar perturbations. Analyzing the solutions and using the results of recent observations, we estimate the order of magnitude of the angular velocity corresponding to the rotation tensor at the time of decoupling and today.

Thermodynamical stability of FRW models with quintessence

2018 ◽  
Vol 33 (07n08) ◽  
pp. 1850045
Author(s):  
M. Sharif ◽  
Sara Ashraf
Keyword(s):  
Thermodynamic Stability ◽  
Physical Parameters ◽  
Model Parameters ◽  
Suitable Model ◽  
Universe Model ◽  
Isotropic Universe ◽  
Frw Models ◽  
Quintessence Model

In this paper, we study the thermodynamic stability of quintessence in the background of homogeneous and isotropic universe model. For the evolutionary picture, we consider two different forms of potentials and investigate the behavior of different physical parameters. We conclude that the quintessence model expands adiabatically and this expansion is thermodynamically stable for both potentials with suitable model parameters.

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