scholarly journals A cosmology with variable c

2006 ◽  
Vol 84 (10) ◽  
pp. 933-944 ◽  
Author(s):  
H Shojaie ◽  
M Farhoudi
Keyword(s):  
Cosmological Model ◽  
Dynamic Equation ◽  
Deceleration Parameter ◽  
Energy Momentum Tensor ◽  
Energy Content ◽  
Big Bang ◽  
Momentum Tensor ◽  
Constant Deceleration Parameter ◽  
Extra Energy ◽  
The Universe

A new varying-c cosmological model, constructed using two additional assumptions, which was introduced in our previous work, is briefly reviewed and the dynamic equation of the model is derived distinctly from a semi-Newtonian approach. The results of this model, using a [Formula: see text] term and an extra energy-momentum tensor, are considered separately. It is shown that the Universe began from a hot Big Bang and expands forever with a constant deceleration parameter regardless of its curvature. Finally, the age, the radius, and the energy content of the Universe are estimated and some discussion about the type of the geometry of the Universe is provided. PACS Nos.: 98.80.Bp, 98.80.Jk

scholarly journals Energy-Momentum Tensor and Parameters in Cosmological Model

2020 ◽  
Author(s):  
Ying-Qiu Gu
Keyword(s):  
Cosmological Model ◽  
Cosmological Constant ◽  
Energy Momentum Tensor ◽  
Equations Of State ◽  
Mass Density ◽  
Big Bang ◽  
Momentum Tensor ◽  
Ideal Gas ◽  
Dimensional Sphere ◽  
The Universe

In cosmology, the cosmic curvature $K$ and the cosmological constant $\Lambda$ are two most important parameters, whose values have strong influence on the behavior of the universe. By analyzing the energy-momentum tensor and equations of state of ideal gas, scalar, spinor and vector potential in detail, we find that the total mass density of all matter is always positive, and the initial total pressure is negative. Under these conditions, by qualitatively analyzing the global behavior of the dynamical equation of cosmological model, we get the following results: (i) $K= 1$, namely, the global spatial structure of the universe should be a 3-dimensional sphere $S^3$. (ii) $0\le\Lambda < 10 ^ {-24} {\rm ly} ^ {-2}$, the cosmological constant should be zero or an infinitesimal. (iii) $a(t)>0$, the initial singularity of the universe is unreachable, and the evolution of universe should be cyclic in time. This means that the initial Big Bang is impossible at all. Since the matter components considered are quite complete and the proof is very elementary and strict, these logical conclusions should be quite reliable. Obviously, these conclusions will be much helpful to correct some popular misconceptions and bring great convenience to further research other problems in cosmology such as property of dark matter and dark energy.

Gravity, stability and cosmological models

2017 ◽  
Vol 26 (12) ◽  
pp. 1743026
Author(s):  
Asher Yahalom
Keyword(s):  
Stability Analysis ◽  
Cosmological Model ◽  
Cosmological Constant ◽  
Energy Momentum Tensor ◽  
Momentum Tensor ◽  
Lorentzian Metric ◽  
Standard Cosmological Model ◽  
Flat Spacetimes ◽  
Stable Solutions ◽  
The Universe

Stability analysis plays a major rule in our understanding of nature. For example it was shown that among empty flat spacetimes only those with a Lorentzian metric are stable [A. Yahalom, Found Phys. 38 (2008) 489–497; Int. J. Mod. Phys. D 18(4) (2009) 2155–2158]. However, the universe is not empty and the energy momentum tensor is metric dependent an thus effects stability. In this essay we concentrate on simple perturbations of the standard cosmological model with and without cosmological constant which is based on a uniform mass distribution. The results suggest that while Euclidean, open or closed section models are valid solutions, the choice of stable solutions is limited. In particular, the popular Lambda-CDM model is unstable.

scholarly journals STRING COSMOLOGY IN ANISOTROPIC BIANCHI-II SPACETIME

2011 ◽  
Vol 26 (11) ◽  
pp. 779-793 ◽  
Author(s):  
SURESH KUMAR
Keyword(s):  
Cosmological Model ◽  
Energy Momentum Tensor ◽  
Momentum Tensor ◽  
Hubble’S Parameter ◽  
Massive String ◽  
Field Equations ◽  
Expansion Scalar ◽  
Spatially Homogeneous ◽  
The Universe ◽  
Shear Tensor

The present study deals with a spatially homogeneous and anisotropic Bianchi-II cosmological model representing massive strings. The energy–momentum tensor, as formulated by Letelier,10 has been used to construct a massive string cosmological model for which the expansion scalar is proportional to one of the components of shear tensor. The Einstein's field equations have been solved by applying a variation law for generalized Hubble's parameter that yields a constant value of deceleration parameter in Bianchi-II spacetime. A comparative study of accelerating and decelerating modes of the evolution of universe has been carried out in the presence of string scenario. The study reveals that massive strings dominate the early Universe. The strings eventually disappear from the Universe for sufficiently large times, which is in agreement with the current astronomical observations.

scholarly journals O UNIVERSO VIVO

2012 ◽  
Vol 19 (1.2) ◽  
Author(s):  
Francisco César de Sá Barreto ◽  
Luiz Paulo Ribeiro Vaz ◽  
Gabriel Armando Pellegatti Franco
Keyword(s):  
Cosmological Model ◽  
Chemical Elements ◽  
Big Bang ◽  
Living System ◽  
Irreversible Processes ◽  
Thermodynamics Of Irreversible Processes ◽  
Protons And Neutrons ◽  
Standard Cosmological Model ◽  
The Big Bang ◽  
The Universe

The standard cosmological model suggests that after the “Big Bang”, 14 billion of years ago, the universe entered a period of expansion and cooling. In the first one millionth of a second appear quarks, glúons, electrons and neutrinos, followed by the appearance of protons and neutrons. In this paper, we describe the “cosmic battle” between gravitation and energy, responsible for the lighter chemical elements and the formation of the stars. We describe the thermodynamics of irreversible processes of systems which are far away from equilibrium, a route that is followed by the universe, seen as a living system.

UNUSUAL HYDRODYNAMICS

1987 ◽  
Vol 02 (05) ◽  
pp. 1591-1615 ◽  
Author(s):  
V.A. BEREZIN
Keyword(s):  
Black Hole ◽  
Cosmological Model ◽  
Production Process ◽  
Particle Production ◽  
Energy Momentum Tensor ◽  
Equations Of Motion ◽  
Momentum Tensor ◽  
Point Of View ◽  
Modified Equations ◽  
Nonsingular Cosmological Model

A method for the phenomenological description of particle production is proposed. Correspondingly modified equations of motion and energy-momentum tensor are obtained. In order to illustrate this method we reconsider from the new point of view of (i) the C-field Hoyle-Narlikar cosmology, (ii) the influence of the particle production process on metric inside the event horizon of a charged black hole and (iii) a nonsingular cosmological model.

scholarly journals Augustine of Hippo’s Philosophy of Time Meets General Relativity

KronoScope ◽  
2014 ◽  
Vol 14 (1) ◽  
pp. 71-89 ◽  
Author(s):  
Ettore Minguzzi
Keyword(s):  
General Relativity ◽  
Cosmological Model ◽  
Degrees Of Freedom ◽  
Big Bang ◽  
Null Hypersurface ◽  
Philosophy Of Time ◽  
The Big Bang ◽  
The Universe

Abstract This paper proposes a cosmological model that uses a causality argument to solve the homogeneity and entropy problems of cosmology. In this model, a chronology violating region of spacetime causally precedes the remainder of the Universe, and a theorem establishes the existence of time functions precisely outside the chronology violating region. This model is shown to nicely reproduce Augustine of Hippo’s thought on time and the beginning of the Universe. In the model, the spacelike boundary representing the Big Bang is replaced by a null hypersurface at which the gravitational degrees of freedom are almost frozen while the matter and radiation content is highly homogeneous and thermalized.

Cosmic acceleration with tachyon field

2018 ◽  
Vol 33 (34) ◽  
pp. 1850199 ◽  
Author(s):  
A. I. Keskin
Keyword(s):  
Scalar Field ◽  
Energy Momentum Tensor ◽  
Momentum Tensor ◽  
Cosmic Acceleration ◽  
Late Time ◽  
De Sitter ◽  
Tachyon Field ◽  
Minimal Coupling ◽  
Acceleration Of The Universe ◽  
The Universe

In this study, we examine two models of the scalar field, that is, a normal scalar field and a tachyon scalar field in [Formula: see text] gravity to describe cosmic acceleration of the universe, where [Formula: see text], [Formula: see text] and [Formula: see text] are Ricci curvature scalar, trace of energy–momentum tensor and kinetic energy of scalar field [Formula: see text], respectively. Using the minimal-coupling Lagrangian [Formula: see text], for both the scalar models we obtain a viable cosmological system, where [Formula: see text] and [Formula: see text] are real constants. While a normal scalar field gives a system describing expansion from the deceleration to the late-time acceleration, tachyon field together with [Formula: see text] in the system produces a quintessential expansion which is very close to de Sitter point, where we find a new condition [Formula: see text] for inflation.

scholarly journals Dark Matter in the Universe

1986 ◽  
Vol 7 ◽  
pp. 27-38 ◽  
Author(s):  
Vera C. Rubin
Keyword(s):  
Dark Matter ◽  
Deceleration Parameter ◽  
Big Bang ◽  
Theoretical Cosmology ◽  
Observational Cosmology ◽  
The Past ◽  
The Galaxy ◽  
Expansion Of The Universe ◽  
The Big Bang ◽  
The Universe

Thirty years ago, observational cosmology consisted of the search for two numbers: Ho, the rate of expansion of the universe at the position of the Galaxy; and qo, the deceleration parameter. Twenty years ago, the discovery of the relic radiation from the Big Bang produced another number, 3oK. But it is the past decade which has seen the enormous development in both observational and theoretical cosmology. The universe is known to be immeasurably richer and more varied than we had thought. There is growing acceptance of a universe in which most of the matter is not luminous. Nature has played a trick on astronomers, for we thought we were studying the universe. We now know that we were studying only the small fraction of it that is luminous. I suspect that this talk this evening is the first IAU Discourse devoted to something that astronomers cannot see at any wavelength: Dark Matter in the Universe.

scholarly journals LRS Bianchi type-I cosmological model with constant deceleration parameter in f(R,T) gravity

2017 ◽  
Vol 14 (11) ◽  
pp. 1750158 ◽  
Author(s):  
Binaya K. Bishi ◽  
S. K. J. Pacif ◽  
P. K. Sahoo ◽  
G. P. Singh
Keyword(s):  
Cosmological Model ◽  
Deceleration Parameter ◽  
Hubble Parameter ◽  
Bianchi Type ◽  
Specific Model ◽  
Type I ◽  
Hubble’S Parameter ◽  
Constant Deceleration Parameter ◽  
Bianchi Type I ◽  
Spatially Homogeneous

A spatially homogeneous anisotropic LRS Bianchi type-I cosmological model is studied in [Formula: see text] gravity with a special form of Hubble's parameter, which leads to constant deceleration parameter. The parameters involved in the considered form of Hubble parameter can be tuned to match, our models with the [Formula: see text]CDM model. With the present observed value of the deceleration parameter, we have discussed physical and kinematical properties of a specific model. Moreover, we have discussed the cosmological distances for our model.

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