SOME COSMOLOGICAL CONSEQUENCES OF A Λ-TERM VARYING AS βH2+αR−n

1993 ◽  
Vol 08 (07) ◽  
pp. 591-599 ◽  
Author(s):  
J.A.S. LIMA ◽  
J.M.F. MAIA
Keyword(s):  
Analytic Solutions ◽  
Angular Diameter ◽  
Density Parameter ◽  
Luminosity Distance ◽  
Closed Model ◽  
Frw Model ◽  
The Universe ◽  
Universe Evolution

A new phenomenological decay law for the cosmological Λ-term is proposed and its influence on the universe evolution is investigated. Unlike the standard FRW model are the possibility of nonsingular solutions, recollapsing open universes and an everexpanding closed model. Explicit analytic solutions are given for the flat case both for parametric and cosmological times. In this case, models with the present density parameter Ω0 smaller than 2/3 and age bigger than [Formula: see text] can always be obtained. It is also shown that kinematic expressions as the luminosity distance and angular diameter versus redshift relation are significantly modified.

scholarly journals The failure of testing for cosmic opacity via the distance-duality relation

2020 ◽  
Vol 497 (1) ◽  
pp. 378-388
Author(s):  
Václav Vavryčuk ◽  
Pavel Kroupa
Keyword(s):  
Cosmological Model ◽  
Convincing Evidence ◽  
Duality Relation ◽  
Angular Diameter ◽  
Luminosity Distance ◽  
Angular Diameter Distance ◽  
Frequency Independent ◽  
Unique Interpretation ◽  
The Universe

ABSTRACT The distance-duality relation (DDR) between the luminosity distance DL and the angular diameter distance DA is viewed as a powerful tool for testing for the opacity of the Universe, being independent of any cosmological model. It was applied by many authors, who mostly confirm its validity and report a negligible opacity of the Universe. Nevertheless, a thorough analysis reveals that applying the DDR in cosmic opacity tests is tricky. Its applicability is strongly limited because of a non-unique interpretation of the DL data in terms of cosmic opacity and a rather low accuracy and deficient extent of currently available DA data. Moreover, authors usually assume that cosmic opacity is frequency independent and parametrize it in their tests by a prescribed phenomenological function. In this way, they only prove that cosmic opacity does not follow their assumptions. As a consequence, no convincing evidence of transparency of the universe using the DDR has so far been presented.

scholarly journals EVOLUTION OF THE DENSITY PARAMETER IN THE ANISOTROPIC DGP COSMOLOGY

2011 ◽  
Vol 20 (13) ◽  
pp. 2599-2611
Author(s):  
RIZWAN UL HAQ ANSARI ◽  
P. K. SURESH
Keyword(s):  
Density Parameter ◽  
Shear Parameter ◽  
Frw Model ◽  
Braneworld Model ◽  
Using Data ◽  
The Universe ◽  
Dgp Braneworld

Evolution of the density parameter in the anisotropic DGP braneworld model is studied. The role of shear and cross-over scale in the evolution of Ωρ is examined for both the branches of solution in the DGP model. The evolution is modified significantly compared to the FRW model and further it does not depend on the value of γ alone. Behavior of the cosmological density parameter Ωρ is unaltered in the late universe. The study of decceleration parameter shows that the entry of the universe into self-accelerating phase is determined by the value of shear. We also obtain an estimate of the shear parameter [Formula: see text], which is in agreement with the constraints obtained in the literature using data.

scholarly journals Background Light from Population III Stars

1987 ◽  
Vol 117 ◽  
pp. 414-414
Author(s):  
Jonathan C. McDowell
Keyword(s):  
Dark Matter ◽  
Background Radiation ◽  
Large Fraction ◽  
Rest Mass ◽  
Critical Density ◽  
Density Parameter ◽  
Extragalactic Background Light ◽  
Background Light ◽  
Far Ultraviolet ◽  
The Universe

It has been proposed (e.g. Carr, Bond and Arnett 1984) that the first generation of stars may have been Very Massive Objects (VMOs, of mass above 200 M⊙) which existed at large redshifts and left a large fraction of the mass of the universe in black hole remnants which now provide the dynamical ‘dark matter’. The radiation from these stars would be present today as extragalactic background light. For stars with density parameter Ω* which convert a fraction ϵ of their rest-mass to radiation at a redshift of z, the energy density of background radiation in units of the critical density is ΩR = εΩ* / (1+z). The VMOs would be far-ultraviolet sources with effective temperatures of 105 K. If the radiation is not absorbed, the constraints provided by measurements of background radiation imply (for H =50 km/s/Mpc) that the stars cannot close the universe unless they formed at a redshift of 40 or more. To provide the dark matter (of one-tenth closure density) the optical limits imply that they must have existed at redshifts above 25.

scholarly journals De Sitter and power-law solutions in non-local Gauss–Bonnet gravity

2018 ◽  
Vol 15 (11) ◽  
pp. 1850188 ◽  
Author(s):  
E. Elizalde ◽  
S. D. Odintsov ◽  
E. O. Pozdeeva ◽  
S. Yu. Vernov
Keyword(s):  
Power Law ◽  
Sufficient Conditions ◽  
Model Parameters ◽  
Local Form ◽  
De Sitter ◽  
Dynamical Equations ◽  
Non Local ◽  
Necessary And Sufficient ◽  
The Universe ◽  
Universe Evolution

The cosmological dynamics of a non-locally corrected gravity theory, involving a power of the inverse d’Alembertian, is investigated. Casting the dynamical equations into local form, the fixed points of the models are derived, as well as corresponding de Sitter and power-law solutions. Necessary and sufficient conditions on the model parameters for the existence of de Sitter solutions are obtained. The possible existence of power-law solutions is investigated, and it is proven that models with de Sitter solutions have no power-law solutions. A model is found, which allows to describe the matter-dominated phase of the Universe evolution.

scholarly journals Dynamic of the Accelerated Expansion of the Universe in the DGP Model

2011 ◽  
Vol 21 (3) ◽  
pp. 253 ◽  
Author(s):  
Vo Quoc Phong
Keyword(s):  
Experimental Data ◽  
Growth Rate ◽  
Cold Dark Matter ◽  
Growth Index ◽  
Cosmic Acceleration ◽  
Accelerated Expansion ◽  
Density Parameter ◽  
Type Ia ◽  
Age Of The Universe ◽  
The Universe

According to experimental data of SNe Ia (Supernovae type Ia), we will discuss in detial dynamics of the DGP model and introduce a simple parametrization of matter $\omega$, in order to analyze scenarios of the expanding universe and the evolution of the scale factor. We find that the dimensionless matter density parameter at the present epoch $\Omega^0_m=0.3$, the age of the universe $t_0= 12.48$ Gyr, $\frac{a}{a_0}=-2.4e^{\frac{-t}{25.56}}+2.45$. The next we study the linear growth of matter perturbations, and we assume a definition of the growth rate, $f \equiv \frac{dln\delta}{dlna}$. As many authors for many years, we have been using a good approximation to the growth rate $f \approx \Omega^{\gamma(z)}_m$, we also find that the best fit of the growth index, $\gamma(z)\approx 0.687 - \frac{40.67}{1 + e^{1.7. (4.48 + z)}}$, or $\gamma(z)= 0.667 + 0.033z$ when $z\ll1$. We also compare the age of the universe and the growth index with other models and experimental data. We can see that the DGP model describes the cosmic acceleration as well as other models that usually refers to dark energy and Cold Dark Matter (CDM).

Phase space analysis of the Umami Chaplygin model

2021 ◽  
pp. 2150052
Author(s):  
Qihong Huang ◽  
Ruanjing Zhang ◽  
Jun Chen ◽  
He Huang ◽  
Feiquan Tu
Keyword(s):  
Phase Space ◽  
Cosmic Acceleration ◽  
Accelerated Expansion ◽  
Late Time ◽  
Phase Space Analysis ◽  
Space Analysis ◽  
History Of ◽  
Evolutionary Trajectories ◽  
The Universe ◽  
Universe Evolution

In this paper, we analyze the universe evolution and phase space behavior of the Umami Chaplygin model, where the Umami Chaplygin fluid replaces both a dark energy and a dark and baryonic matter. We find the Umami Chaplygin model can be stable against perturbations under some conditions and can be used to explain the late-time cosmic acceleration. The results of phase space analysis show that there exists a late-time accelerated expansion attractor with [Formula: see text], which indicates the Umami Chaplygin fluid can behave as a cosmological constant. Moreover, the Umami Chaplygin model can describe the expansion history of the universe. The evolutionary trajectories of the statefinder diagnostic pairs and the finite time future singularities are also discussed.

Some Basic Statements of the General Theory of the Universe Evolution at the First Stages of Life

2021 ◽  
Vol 59 (11) ◽  
pp. 1106-1112
Author(s):  
L. A. Gribov ◽  
V. I. Baranov ◽  
I. V. Mikhailov
Keyword(s):  
General Theory ◽  
The Universe ◽  
Universe Evolution

scholarly journals Exploring the deviation of cosmological constant by a generalized pressure parameterization

2019 ◽  
Vol 79 (10) ◽  
Author(s):  
Jun-Chao Wang ◽  
Xin-He Meng
Keyword(s):  
Hubble Constant ◽  
Scalar Fields ◽  
Parametric Model ◽  
Matter Density ◽  
Density Parameter ◽  
Present Value ◽  
Big Rip ◽  
Phantom Scalar ◽  
Ultimate Fate ◽  
The Universe

Abstract We bring forward a generalized pressure (GP) parameterization for dark energy to explore the evolution of the universe. This parametric model has covered three common pressure parameterization types and can be reconstructed as quintessence and phantom scalar fields, respectively. We adopt the cosmic chronometer (CC) datasets to constrain the parameters. The results show that the inferred late-universe parameters of the GP parameterization are (within $$1\sigma $$1σ): the present value of Hubble constant $$H_{0}=(72.30^{+1.26}_{-1.37}) \ \hbox {km s}^{-1}\hbox { Mpc}^{-1}$$H0=(72.30-1.37+1.26)kms-1Mpc-1; the matter density parameter $$\Omega _{\text {m0}}=0.302^{+0.046}_{-0.047}$$Ωm0=0.302-0.047+0.046, and the bias of the universe towards quintessence. Then we perform a dynamic analysis on the GP parameterization and find that there is an attractor or a saddle point in the system corresponding to the different values of the parameters. Finally, we discuss the ultimate fate of the universe under the phantom scenario in the GP parameterization. It is demonstrated that the three cases of pseudo rip, little rip, and big rip are all possible.

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