scholarly journals Phenomenological models of Universe with varying G and Λ

Open Physics ◽  
2014 ◽  
Vol 12 (5) ◽  
Author(s):  
Martiros Khurshudyan
Keyword(s):  
Cosmological Constant ◽  
Phenomenological Models ◽  
Gravitational Constant ◽  
Fluid Model ◽  
Cosmological Parameters ◽  
The Other ◽  
Barotropic Fluid ◽  
Two Component ◽  
And Behavior ◽  
The Universe

AbstractIn this article we will consider several phenomenological models for the Universe with varying G and Λ(t), where G is the gravitational ”constant” and Λ(t) is a varying cosmological ”constant”. Two-component fluid model are taken into account. An interaction of the phenomenological form between a barotropic fluid and a quintessence DE is supposed. Three different forms of Λ(t) will be considered. The problem is analysed numerically and behavior of different cosmological parameters investigated graphically. Conclusion and discussions are given at the end of the work. In an Appendix information concerning to the other cosmological parameters is presented.

scholarly journals Generalized Phenomenological Models of Dark Energy

2020 ◽  
Vol 2020 ◽  
pp. 1-8
Author(s):  
Prasenjit Paul ◽  
Rikpratik Sengupta
Keyword(s):  
General Relativity ◽  
Dark Energy ◽  
Energy Density ◽  
Cosmological Constant ◽  
Phenomenological Models ◽  
Einstein Field Equations ◽  
Field Equations ◽  
Free Parameters ◽  
The Universe ◽  
Matter Energy

It was first observed at the end of the last century that the universe is presently accelerating. Ever since, there have been several attempts to explain this observation theoretically. There are two possible approaches. The more conventional one is to modify the matter part of the Einstein field equations, and the second one is to modify the geometry part. We shall consider two phenomenological models based on the former, more conventional approach within the context of general relativity. The phenomenological models in this paper consider a Λ term firstly a function of a¨/a and secondly a function of ρ, where a and ρ are the scale factor and matter energy density, respectively. Constraining the free parameters of the models with the latest observational data gives satisfactory values of parameters as considered by us initially. Without any field theoretic interpretation, we explain the recent observations with a dynamical cosmological constant.

scholarly journals GRAVITATIONAL LENSING BOUND ON THE AVERAGE REDSHIFT OF GAMMA RAY BURSTS IN MODELS WITH EVOLVING LENSES

1999 ◽  
Vol 08 (04) ◽  
pp. 507-517 ◽  
Author(s):  
DEEPAK JAIN ◽  
N. PANCHAPAKESAN ◽  
S. MAHAJAN ◽  
V. B. BHATIA
Keyword(s):  
Gravitational Lensing ◽  
Gamma Ray ◽  
Cosmological Parameters ◽  
Evolutionary Model ◽  
Gamma Ray Bursts ◽  
The Other ◽  
Upper Limit ◽  
Evolving Model ◽  
Evolving Models ◽  
The Universe

Identification of gravitationally lensed Gamma Ray Bursts (GRBs) in the BATSE 4B catalog can be used to constrain the average redshift <z> of the GRBs. In this paper we investigate the effect of evolving lenses on the <z> of GRBs in different cosmological models of the universe. The cosmological parameters Ω and Λ have an effect on the <z> of GRBs. The other factor which can change the <z> is the evolution of galaxies. We consider three evolutionary model of galaxies. In particular, we find that the upper limit on <z> of GRBs is higher in evolving model of galaxies as compared to non-evolving models of galaxies.

Observational and astrophysical cosmology: 1980–2018

2019 ◽  
pp. 375-423
Author(s):  
Malcolm S. Longair
Keyword(s):  
Dark Matter ◽  
Dark Energy ◽  
Cosmological Constant ◽  
Large Scale ◽  
Cosmological Parameters ◽  
Semiconductor Detectors ◽  
Empirical Knowledge ◽  
Global Geometry ◽  
The Universe ◽  
Large Scale Structure Formation

Since 1980, our empirical knowledge of the universe has advanced tremendously and precision cosmology has become a reality. These developments have been largely technology-driven, the result of increased computer power, new generations of telescopes for all wavebands, new types of semiconductor detectors, such as CCDs, and major investments by many nations in superb observing facilities. The discipline also benefitted from the influx of experimental and theoretical physicists into the cosmological arena. The accuracy and reliability of the values of the cosmological parameters has improved dramatically, many of them now being known to about 1%. The ΛCDM model provides a remarkable fit to all the observational data, demonstrating that the cosmological constant is non-zero and that the global geometry of the universe is flat. The underlying physics of galaxy and large-scale structure formation has advanced dramatically and demonstrated the key roles played by dark matter and dark energy.

COSMOLOGICAL TERM AS A CORRECTION OF METRIC TENSOR FIELD

2001 ◽  
Vol 10 (06) ◽  
pp. 893-904 ◽  
Author(s):  
TAKAO FUKUI
Keyword(s):  
Cosmological Constant ◽  
Tensor Field ◽  
Cosmological Term ◽  
The Other ◽  
Metric Tensor ◽  
Linear Correction ◽  
The Universe

Models of the universe with a cosmological term which is introduced as a correction of the metric tensor field are studied. By revisiting with these models some of the conventional success, we infer that a model with a linear correction is a favourable candidate for a model of the universe. The cosmological constant and the flatness problems are examined in the model. There might be a possibility to solve the other cosmological problems only with the metric tensor field.

The “still point” cosmology

2005 ◽  
Vol 201 ◽  
pp. 514-515
Author(s):  
Ivan I. Shevchenko
Keyword(s):  
Scalar Field ◽  
Cosmological Constant ◽  
Hubble Parameter ◽  
Gravitational Constant ◽  
Critical Density ◽  
Matter Density ◽  
Future Evolution ◽  
The Future ◽  
The Universe

Recent results on supernovae as standard candles (Riess et al. 1998; Perlmutter et al. 1999) and on CMB anisotropy (Lineweaver 1998) indicate that ΩM ≍ 0.3-0.4, Ωv ≍ 0.6-0.7, ΩM + Ωv ≍ 1. By definition, ΩM = ρM/ρcr, ΩV = ρv/ρcr, where ρM is the matter density, ρv is the vacuum density; the critical density ρcr = 3H2/8πG; H is the Hubble parameter, G is the gravitational constant. In the standard Friedmann-Lemaître cosmologies, these results seriously constrain the non-dimensional cosmological constant (as defined below): Δ ≫ 1, meaning that the Universe expands forever. If a scalar field is present, the future evolution may be different.

scholarly journals CONSTRAINING THE RUNAWAY DILATON AND QUINTESSENTIAL DARK ENERGY

2010 ◽  
Vol 19 (03) ◽  
pp. 367-394 ◽  
Author(s):  
ISHWAREE P. NEUPANE ◽  
HOLLY TROWLAND
Keyword(s):  
Dark Energy ◽  
Energy Density ◽  
Cosmological Constant ◽  
Cosmological Parameters ◽  
Scalar Fields ◽  
Current Data ◽  
Gravity Models ◽  
Model Parameters ◽  
Dilaton Field ◽  
The Universe

Dark energy is some of the weirdest and most mysterious stuff in the universe that tends to increase the rate of expansion of the universe. Two commonly known forms of dark energy are the cosmological constant, a constant energy density filling space homogeneously, and scalar fields such as quintessence or moduli whose energy density can vary with time. We explore one particular model for dynamic dark energy: quintessence driven by a scalar dilaton field. We propose an ansatz for the form of the dilaton field, |ϕ(a)|mP ≡ α1 ln t + α2tn = α ln a + βa2ζ, where a is the scale factor and α and ζ are parameters of the model. This phenomenological ansatz for ϕ can be motivated by generic solutions of a scalar dilaton field in many effective string theory and string-inspired gravity models in four dimensions. Most of the earlier discussions in the literature correspond to the choice that ζ = 0 so that ϕ(t) ∝ ln t or ϕ(t) ∝ ln a(t). Using a compilation of current data including type Ia supernovae, we impose observational constraints on the slope parameters like α and ζ and then discuss the relation of our results to analytical constraints on various cosmological parameters, including the dark energy equation of state. Some useful constraints are imposed on model parameters like α and ζ as well as on the dark energy/dark matter couplings using results from structure formation. The constraints of this model are shown to encompass the cosmological constant limit within 1σ error bars.

scholarly journals Cosmological consequences of a variable cosmological constant model

2016 ◽  
Vol 26 (07) ◽  
pp. 1750060 ◽  
Author(s):  
Hemza Azri ◽  
A. Bounames
Keyword(s):  
Cosmological Constant ◽  
Vacuum Energy ◽  
Gravitational Constant ◽  
Scale Factor ◽  
Accelerated Expansion ◽  
Variable Cosmological Constant ◽  
Age Of The Universe ◽  
Two Phases ◽  
Limiting Case ◽  
The Universe

We derive a model of dark energy which evolves with time via the scale factor. The equation-of-state is studied as a function of a parameter [Formula: see text] introduced in this model as [Formula: see text]. In addition to the recent accelerated expansion, the model predicts another decelerated phase. These two phases are studied via the parameter [Formula: see text]. The age of the universe is found to be almost consistent with the observation. In the limiting case, the cosmological constant model, we find that vacuum energy gravitates with a tiny gravitational constant which evolves with the scale factor, rather than with Newton’s constant. This enables degravitation of the vacuum energy which in turn produces the tiny observed curvature, rather than a 120 orders of magnitude larger value.

scholarly journals BEKENSTEIN BOUND OF INFORMATION NUMBER N AND ITS RELATION TO COSMOLOGICAL PARAMETERS IN A UNIVERSE WITH AND WITHOUT COSMOLOGICAL CONSTANT

2013 ◽  
Vol 28 (19) ◽  
pp. 1350077 ◽  
Author(s):  
IOANNIS HARANAS ◽  
IOANNIS GKIGKITZIS
Keyword(s):  
Cosmological Constant ◽  
Physical System ◽  
Cosmological Parameters ◽  
Hubble Constant ◽  
Short Paper ◽  
Information Number ◽  
Gravitating Systems ◽  
Two Universes ◽  
Cosmological Constants ◽  
The Universe

Bekenstein has obtained an upper limit on the entropy S, and from that, an information number bound N is deduced. In other words, this is the information contained within a given finite region of space that includes a finite amount of energy. Similarly, this can be thought as the maximum amount of information required to perfectly describe a given physical system down to its quantum level. If the energy and the region of space are finite then the number of information N required in describing the physical system is also finite. In this short paper, two information number bounds are derived and compared for two types of universe. First, a universe without a cosmological constant Λ and second a universe with a cosmological constant Λ are investigated. This is achieved with the derivation of two different relations that connect the Hubble constant and cosmological constants to the number of information N. We find that the number of information N involved in the two universes are identical or N2 = N2Λ, and that the total mass of the universe scales as the square root of the information number N, containing. an information number N of the order of 10122. Finally, we expressed Calogero's quantization action as a function of the number of information N. We also have found that in self-gravitating systems the number of information N in nats is the ratio of the total kinetic to total thermal energy of the system.

Tachyonic braneworld mimetic cosmology

2019 ◽  
Vol 34 (21) ◽  
pp. 1950162 ◽  
Author(s):  
S. Davood Sadatian ◽  
Alireza Sepehri
Keyword(s):  
Cosmological Model ◽  
Separation Distance ◽  
The Other ◽  
Late Time ◽  
Moduli Stabilization ◽  
Inflation Model ◽  
Braneworld Model ◽  
And Behavior ◽  
The Universe ◽  
Mimetic Gravity

Recently, some authors have generalized the idea of mimetic gravity to a Randall–Sundrum II braneworld model [L. Randall and R. Sundrum, Phys. Rev. Lett. 83, 3370 (1999); L. Randall and R. Sundrum, Phys. Rev. Lett. 83, 4690 (1999)] and introduced Braneworld Mimetic Cosmology. In this paper, we extend their new cosmological model to brane–anti-brane systems and obtain the explicit form of potential which appeared in their action. This potential depends on the tachyonic fields, the separation distance between two branes and time. On the other hand, our universe is located on one of the branes and its evolution is controlled by the potential between two branes. By passing time and decreasing the separation distance between branes, more energy dissolves into branes and the universe expands. In the following, we presented the physical applications such as late time accelerating phase, inflation model and behavior of perturbations, with respect to brane–anti-brane system. Finally, we briefly discussed the moduli stabilization of the model.

scholarly journals Nuclear Constraints on the Age of the Universe

1983 ◽  
Vol 6 ◽  
pp. 241-253 ◽  
Author(s):  
David N. Schramm
Keyword(s):  
Cosmological Constant ◽  
Nuclear Physics ◽  
Big Bang ◽  
Distance Scale ◽  
The Other ◽  
Single Measurement ◽  
Age Of The Universe ◽  
The Big Bang ◽  
The Universe ◽  
Fair Degree

In this paper a review will be made of how one can use nuclear physics to put rather stringent limits on the age of the universe and thus the cosmic distance scale. As the other papers in this session have demonstrated there is some disagreement on the distance scale and thus the limits on the age of the universe (if the cosmological constant Λ = 0.) However, the disagreement is only over the last factor of 2, the basic timescale seems to really be remarkably well agreed upon. The universe is billions of years old - not thousands, not quintillions but billions of years. That our universe has a finite age is philosophically intriguing. That we can estimate that age to a fair degree of accuracy is truly impressive.No single measurement of the time since the Big Bang gives a specific, unambiguous age. Fortunately, we have at our disposal several methods that together fix the age with surprising precision.

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