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Arbab I. Arbab

Publications and source records attributed to Arbab I. Arbab.

33 records · Page 2Linked to original sources
arXiv2004

Quantization of Gravitational System and its Cosmological Consequences

We have found that the hierarchial problems appearing in cosmology is a manifestation of the quantum nature of the universe. The universe is still described by the same formulae that once hold at Planck's time. The universe is found to be governed by the Machian equation, $G M=Rc^2$, where $M$ and $R$ are mass and radius of the universe. A Planck's constant for different cosmic scales is provided. The status of the universe at different stages is shown to be described in terms of the fundamental constants (eg. \textcolor{blue}{$c, \hbar, G, Λ, H$}) only. The concept of maximal (minimal) acceleration, power, temperature, etc., is introduced and justified. The electromagnetic interactions are shown to be active at a cosmic level. Their contribution would exclude the inclusion of dark energy in cosmology.

gr-qc↗
arXiv2003

Conspirative cosmology with variable constants

We have investigated a cosmological model with variable speed of light (c), gravitational constant (G) and cosmological constant (Lambda). The model is shown to solve the horizon, flatness and monopole problems of the early universe. We have found that with a certain variation of these parameters the model predicts a cosmic acceleration. The model also predicts that for a flat universe Lambda vanishes in both radiation and matter phases. If the gravitational constant is allowed to increase then we might not need the existence of dark matter.

astro-ph↗
arXiv2003

The Universe With Bulk Viscosity

Exact solutions for a model with variable $G$, $Λ$ and bulk viscosity are obtained. Inflationary solutions with constant (de Sitter-type) and variable energy density are found. An expanding anisotropic universe is found to isotropize during its course of expansion but a static universe is not. The gravitational constant is found to increase with time and the cosmological constant decreases with time as $Λ\propto t^{-2}$.

gr-qc↗
arXiv2002

Cosmological consequences of a built-in cosmological constant model

We have analyzed the implications of a phenomenological model of a built-in cosmological constant of Rastall and Al-Rawaf -Taha type. We have produced the presently observed cosmic acceleration with the aid of introducing a positive cosmological constant. We have shown that the cosmological models of the type $Λ\propto\frac{\ddot R}{R}, Λ\propto H^2$ or $Λ\propto 8πGρ$ are equivalent to a built-in cosmological constant model of Al-Rawaf and Taha type. Such models are compatible with the recent observational data and do not suffer from fine tuning problems. We predict a higher lensing probability than in Einstein de Sitter and Abdel Rahman models. We also predict a higher value for the luminosity and angular diameter distances than that of Abdel Rahman model. The lens distribution peaks at relatively smaller values of the lens red-shift and occurs at higher red-shift.

astro-ph↗
arXiv2002

Cosmic Acceleration With A Positive Cosmological Constant

We have considered a cosmological model with a phenomenological model for the cosmological constant of the form $Λ=\bt\fr{\ddot R}{R}$, $ \bt$ is a constant. For age parameter consistent with observational data the Universe must be accelerating in the presence of a positive cosmological constant. The minimum age of the Universe is $H_0^{-1}$, where $H_0$ is the present Hubble constant. The cosmological constant is found to decrease as $t^{-2}$. Allowing the gravitational constant to change with time leads to an ever increasing gravitational constant at the present epoch. In the presence of a viscous fluid this decay law for $Λ$ is equivalent to the one with $Λ=3αH^2$ ($α=\rm const.$) provided $α=\fr{\bt}{3(\bt-2)}$. The inflationary solution obtained from this model is that of the de-Sitter type.

gr-qc↗
arXiv2001

FRW-Type Universe With Vacuum Energy Density

We have considered a cosmological model with a cosmological constant of the form $Λ=3α\frac{\dot R^2}{R^2}+\bt\frac{\ddot R}{R} α, \bt=\rm const.$ The cosmological constant is found to decrease as $t^{-2}$ and the rate of particle creation is smaller than the Steady State value. We have found that this behavior gives $\frac{Λ_{Pl}}{Λ_p}=10^{120}$ where $Λ_{Pl}$ is the value of $Λ$ at Planck time. Solutions with $\bt=3α$ in the radiation dominated era and $\bt=6α$ in the matter dominated era are equivalent to the FRW results. We have found an inflationary solution of the de-Sitter type with $\bt=3-3α$. Some problems of the Standard Model may be resolved with the presence of the above cosmological constant in the Einstein's equation. Since observations suggest a contribution of the vacuum energy density in the range $0.40<Ω^Λ<0.76$, one gets $4<β<12$. If $α=0$ the minimum age of the universe is found to be $H_p^{-1}$ ($H_p$ is the present Hubble constant) with $β=\infty$.

gr-qc↗
arXiv2001

The Evolving Universe and the Puzzling Cosmological Parametrs

The universe is found to have undergone several phases in which the gravitational constant had different behaviors. During some epochs the energy density of the universe remained constant and the universe remained static. In the radiation dominated epoch the radiation field satisfies Stefan's formula while the scale factor varies linearly with time. The model enhances the formation of the structure in the universe as observed today.

astro-ph↗
arXiv2001

Cosmological Models With Variable G an Lambda and Bulk viscosity

We have analyzed a nonsingular model with a variable cosmological term following the Carvalho {\it et al}. ansatz. The model was shown to approximate to the model of Freese {\it et al}. in one direction and to the Özer-Taha in the other. We have then included the effect of viscosity in this cosmology, as this effect has not been considered before. The analysis showed that this viscous effect could be important with a present contribution to the cosmic pressure, at most, of order of that of radiation. The model puts a stronger upper bound on the baryonic matter than that required by the standard model. A variable gravitational and cosmological constant were then introduced in a scenario which conserves the energy and momentum in the presence of bulk viscosity. The result of the analysis reveals that various models could be viscous. A noteworthy result is that some nonsingular closed models evolve asymptotically into a singular viscous one. The considered models solve for many of the standard model problems. Though the introduction of bulk viscosity results in the creation of particles, this scenario conserves energy and momentum. As in the standard model the entropy remains constant. We have not explained the generation of bulk viscosity but some workers attributes this to neutrinos. Though the role of viscosity today is minute it could, nevertheless, have had an important contribution at early times. We have shown that these models encompass many of the old and recently proposed models, in particular, Brans-Dicke, Dirac, Freese {\it et al}., Berman, Abdel Rahman and Kalligas {\it et al}. models. Hence we claim that the introduction of bulk viscosity enriches the adopted cosmology.

gr-qc↗
arXiv1999

A linearly Expanding Universe With Variable G and Lambda

We have studied a cosmological model with a cosmological term of the form $Λ=3α\fr{\dot R^2}{R^2}+\bt\fr{\ddot R}{R}+\fr{3γ}{R^2} α, \ \bt γ$ are constants. The scale factor (R) is found to vary linearly with time for both radiation and matter dominated epochs. The cosmological constant is found to decrease as $t^{-2}$ and the rate of particle creation is smaller than the Steady State value. The model gives $Ω^Λ=\fr{1}{3}$ and $Ω^m=\fr{2}{3}$ in the present era, $Ω^Λ=Ω^m=\fr{1}{2}$ in the radiation era. The present age of the universe $(\rm t_p$) is found to be $\rm t_p=H_p^{-1}$ , where $\rm H_p$ is the Hubble constant. The model is free from the main problems of the Standard Model. Since the scale factor $\rm R\propto t$ during the entire evolution of the universe the ratio of the cosmological constant at the Planck and present time is $\rm\fr{Λ_{Pl}}{Λ_p}=10^{120}$. This decay law justifies why, today, the cosmological constant is exceedingly small.

gr-qc↗
arXiv1999

Primordial Black Holes With Variable Gravity

We have studied the evolution of primordial black holes (PBHs) in a universe with a variable gravitational constant and bulk viscosity. We have found that the strength of gravity has changed appreciably in the early universe. The gravitational constant attained its greatest value at t=10^{-23} sec after the Big Bang. PBHs formed at the GUT and electroweak epochs would have masses about $1.85\times 10^9 g$ and $1.8\times 10^{-7} g$ respectively. Their temperatures when they explode are $6\times 10^8K$ and 6 K respectively. PBHs formed during nuclear epoch are hard to detect at the present time. The gravitational constant ($G$) is found to increase as $G\propto t^2$ in the radiation epoch. The gamma rays bursts (GRBs) may have their origin in the evaporation of the PBHs formed during GUT time.

astro-ph↗
arXiv1998

Causal Dissipative Cosmology With Variable G and Lambda

A cosmological model with variable G and Lambda is considered in the framework of Israel-Stewart-Hiscock (ISH) causal theory. Power law as well as inflationary solutions are obtained. The gravitational constant is found to increase with time.

gr-qc↗
arXiv1998

A brief History of the Earth

The possibility of an increasing gravitational constant $G$ and its implication on the Earth's history are discussed. The model is consistent with geophysical and astronomical data. The number of days in early epochs predicted by the model is in agreement with those obtained from fossil corals. The model predicts that the Earth was initially cold and is gradually heating up during its course of evolution. The exact law for the variation of $G$ is still to be obtained from the variation of the Earth's parameters with cosmic expansion.

physics.geo-ph↗
arXiv1998

Cosmological implications of the bulk viscous model with variable G and $Λ$

We have investigated the cosmological implications of the bulk viscous cosmological model with variable G and $Λ$. These results are found to be compatible with the present observations. The classical cosmological tests for this model encompass the Freese et al ones. The model has some sprits of the Standard Model. The inflationary solution which solves the Standard Model problems is obtained as a special solution. The influence of viscosity is shown to affect the past and the future of the Universe.

astro-ph↗
arXiv1998

Viscous big bang cosmology with variable G and $Λ$

We have examined the cosmological solutions with variable G and $λ$ and bulk viscosity. It is found that these solutions are free from the cosmological problems of the Standard Model. The proposed cosmology consists of two arbitrary parameters which are well constrained. The inflationary solution does not require $Ω=1$ and arises with or without the cosmological constant.

astro-ph↗