scholarly journals The asymptotic behavior of bouncing cosmological models in F(š’¢) gravity theory

2017 ā—½ Ā 
Vol 14 (10) ā—½ Ā 
pp. 1750148 ā—½ Ā 
Author(s): Ā 
Andrey N. Makarenko ā—½ Ā 
Alexander N. Myagky
Keyword(s): Ā 
Asymptotic Behavior ā—½ Ā 
Early Universe ā—½ Ā 
Asymptotic Expansions ā—½ Ā 
Scale Factor ā—½ Ā 
Cosmological Models ā—½ Ā 
Gravity Theory ā—½ Ā 
Late Time ā—½ Ā 
Puiseux Series ā—½ Ā 
Friedmann Equations

We reconstruct [Formula: see text] gravity theory with an exponential scale factor to realize the bouncing behavior in the early universe and examine the asymptotic behavior of late-time solutions in this model. We propose an approach for the construction of asymptotic expansions of solutions of the Friedmann equations on the basis of Puiseux series.

scholarly journals Anisotropic cosmological reconstruction in f(R, T) gravity

2018 ā—½ Ā 
Vol 33 (29) ā—½ Ā 
pp. 1850170 ā—½ Ā 
Author(s): Ā 
B. Mishra ā—½ Ā 
Sankarsan Tarai ā—½ Ā 
S. K. Tripathy
Keyword(s): Ā 
Simple Approach ā—½ Ā 
Cosmological Models ā—½ Ā 
Gravity Theory ā—½ Ā 
Cosmic Acceleration ā—½ Ā 
Late Time ā—½ Ā 
Coupling Constant ā—½ Ā 
Anisotropic Cosmological Models

Anisotropic cosmological models are constructed in f(R, T) gravity theory to investigate the dynamics of universe concerning the late time cosmic acceleration. Using a more general and simple approach, the effect of the coupling constant and anisotropy on the cosmic dynamics have been investigated. In this study, it is found that cosmic anisotropy substantially affects cosmic dynamics.

scholarly journals Effect of Fast Scale Factor Fluctuations on Cosmological Evolution

Universe ā—½ Ā 
2021 ā—½ Ā 
Vol 7 (6) ā—½ Ā 
pp. 164
Author(s): Ā 
Igor I. Smolyaninov
Keyword(s): Ā 
Quantum Fluctuations ā—½ Ā 
Scale Factor ā—½ Ā 
Cosmological Evolution ā—½ Ā 
Accelerated Expansion ā—½ Ā 
Late Time ā—½ Ā 
Cosmological Constant Problem ā—½ Ā 
Friedmann Equations ā—½ Ā 
Newtonian Dynamics ā—½ Ā 
Expansion Of The Universe ā—½ Ā 
The Universe

In this paper, we study the corrections to the Friedmann equations due to fast fluctuations in the universe scale factor. Such fast quantum fluctuations were recently proposed as a potential solution to the cosmological constant problem. They also induce strong changes to the current sign and magnitude of the average cosmological force, thus making them one of the potential probable causes of the modification of Newtonian dynamics in galaxy-scale systems. It appears that quantum fluctuations in the scale factor also modify the Friedmann equations, leading to a considerable modification of cosmological evolution. In particular, they give rise to the late-time accelerated expansion of the universe, and they may also considerably modify the effective universe potential.

The spatially flat Friedmann equation for early rainbow universe

2015 ā—½ Ā 
Vol 30 (31) ā—½ Ā 
pp. 1550165
Author(s): Ā 
Han Siong Chā€™ng ā—½ Ā 
Geri Gopir ā—½ Ā 
Shahidan Radiman
Keyword(s): Ā 
Early Universe ā—½ Ā 
Quantum Cosmology ā—½ Ā 
Gravitational Constant ā—½ Ā 
Friedmann Equation ā—½ Ā 
Late Time ā—½ Ā 
Friedmann Equations ā—½ Ā 
Bohm Interpretation ā—½ Ā 
The One ā—½ Ā 
Canonical Quantum ā—½ Ā 
De Broglie Bohm

We derive the spatially flat rainbow-Friedmann equation from de Broglieā€“Bohm interpretation in canonical quantum cosmology. Our result shows that the spatially flat rainbow-Friedmann equations of early and late-time universe are having different forms. The spatially flat rainbow-Friedmann equation of early universe which is obtained in this paper is quite different from the one which was initially derived by Magueijo and Smolin [Class. Quantum Grav. 21, 1725 (2004)]. However, the spatially flat rainbow-Friedmann equation for late-time universe obtained in this paper is found to be the same as the one derived by Magueijo and Smolin (for the case [Formula: see text] and Newtonā€™s gravitational constant [Formula: see text]. The new spatially flat rainbow-Friedmann equation obtained in this paper could provide an alternative way in understanding the evolution of the early rainbow universe.

scholarly journals Cosmological models with a hybrid scale factor

2021 ā—½ Ā 
pp. 2140005
Author(s): Ā 
S. K. Tripathy ā—½ Ā 
B. Mishra ā—½ Ā 
Maxim Khlopov ā—½ Ā 
Saibal Ray
Keyword(s): Ā 
General Relativity ā—½ Ā 
Scale Factor ā—½ Ā 
Cosmological Models ā—½ Ā 
Late Time ā—½ Ā 
Late Time Acceleration ā—½ Ā 
Present Universe ā—½ Ā 
Matter Fields

In this paper, we present some cosmological models with a hybrid scale factor (HSF) in the framework of general relativity (GR). The HSF fosters an early deceleration as well as a late-time acceleration and mimics the present Universe. The dynamical aspects of different cosmological models with HSF in the presence of different matter fields have been discussed.

Some late-time cosmological aspects of a Gaussā€“Bonnet gravity with nonminimal coupling Ć  la Bransā€“Dicke: solutions and perspectives

10.1139/cjp-2012-0366 ā—½ Ā 
2013 ā—½ Ā 
Vol 91 (4) ā—½ Ā 
pp. 300-321 ā—½ Ā 
Author(s): Ā 
Rami Ahmad El-Nabulsi
Keyword(s): Ā 
Scalar Field ā—½ Ā 
String Theory ā—½ Ā 
Modified Gravity ā—½ Ā 
Cosmological Models ā—½ Ā 
Gravity Theory ā—½ Ā 
Accelerating Universe ā—½ Ā 
Late Time ā—½ Ā 
Nonminimal Coupling ā—½ Ā 
Bonnet Gravity ā—½ Ā 
Time Dynamics

In this paper, we study modified homogeneous and isotropic cosmological models based on the Gaussā€“Bonnet invariant term as models of an accelerating universe. We discuss and criticize the late-time dynamics of six independent cosmological models: in the first model, we discuss the case of the modified gravity f(R) āˆ R1+Ī“ for Ī“ = āˆ’1/2 and 1 augmented by the Gaussā€“Bonnet invariant term; in the second model, we discuss the general case of f(R) āˆ R1+Ī“ accompanied by a nonminimal coupling between the scalar field and the Ricci curvature as well as the Gaussā€“Bonnet invariant; in the third model, we discuss a generalized modified gravity model that includes the Einsteinā€“Hilbert action, a dynamical cosmological constant, and an effective gravitational coupling constant; in the fourth model, we discuss a more generalized modified scalarā€“tensor cosmology that includes in addition to the Gaussā€“Bonnet invariant term, stringy corrections motivated from string and heterotic superstring arguments; in the fifth model, we discuss the cosmological dynamics of a nonminimal scalar Gaussā€“Bonnet gravity theory motivated from string theory; and finally in the sixth model, we discuss the possibility of having an extension of the generalized modified gravity theory, free from nonminimal coupling with Ī“ = 0, with a Hubble expansion rate and an equation of state parameter that depend on the Gaussā€“Bonnet invariant term. In the first five models, we conjecture that the Hubble parameter is related to the scalar field by the relation [Formula: see text], which is applied merely to the late time epoch. This ansatz is in fact motivated by some recent advances in scalarā€“tensor theory and string theory. All of the six models reveal interesting consequences, which are discussed in some detail. Our main objective in this work is to analyze, criticize, and differentiate between viable realistic models and those that are not. Many critical points are discussed in some detail.

Cosmological models for an homogeneous, isotropic universe

2018 ā—½ Ā 
Author(s): Ā 
Michael Kachelriess
Keyword(s): Ā 
Time Evolution ā—½ Ā 
Space Time ā—½ Ā 
Scale Factor ā—½ Ā 
Cosmological Models ā—½ Ā 
Isotropic Universe ā—½ Ā 
Friedmann Equations ā—½ Ā 
Homogeneous Isotropic Universe ā—½ Ā 
The Universe

The universe is homogeneous and isotropic on sufficiently large scales. The metric which describes such a space-time is derived and it is shown that it is determined by the scale factor a(t) and the parameter k distinguishing between hyperbolic, flat, or spherical 3-spaces. The Friedmann equations, which describe the time evolution of these models, are derived and specific models are discussed.

scholarly journals IMPLICATIONS OF A QUANTUM MECHANICAL TREATMENT OF THE UNIVERSE

1998 ā—½ Ā 
Vol 13 (05) ā—½ Ā 
pp. 347-351 ā—½ Ā 
Author(s): Ā 
MURAT ƖZER
Keyword(s): Ā 
Quantum Mechanics ā—½ Ā 
Wave Packet ā—½ Ā 
Early Universe ā—½ Ā 
Mechanical Treatment ā—½ Ā 
Correspondence Principle ā—½ Ā 
Scale Factor ā—½ Ā 
Quantum Mechanical ā—½ Ā 
Quantum Mechanical Treatment ā—½ Ā 
The Standard Model ā—½ Ā 
The Universe

We attempt to treat the very early Universe according to quantum mechanics. Identifying the scale factor of the Universe with the width of the wave packet associated with it, we show that there cannot be an initial singularity and that the Universe expands. Invoking the correspondence principle, we obtain the scale factor of the Universe and demonstrate that the causality problem of the standard model is solved.

scholarly journals Late Time Attractors of Some Varying Chaplygin Gas Cosmological Models

Symmetry ā—½ Ā 
10.3390/sym13050769 ā—½ Ā 
2021 ā—½ Ā 
Vol 13 (5) ā—½ Ā 
pp. 769
Author(s): Ā 
Martiros Khurshudyan ā—½ Ā 
Ratbay Myrzakulov
Keyword(s): Ā 
Dark Energy ā—½ Ā 
Gravitational Interaction ā—½ Ā 
Chaplygin Gas ā—½ Ā 
Cosmological Models ā—½ Ā 
Late Time ā—½ Ā 
Time Behavior ā—½ Ā 
Coincidence Problem ā—½ Ā 
Energy Models ā—½ Ā 
Non Linear ā—½ Ā 
Bayesian Machine Learning

The goal of this paper is to study new cosmological models where the dark energy is a varying Chaplygin gas. This specific dark energy model with non-linear EoS had been often discussed in modern cosmology. Contrary to previous studies, we consider new forms of non-linear non-gravitational interaction between dark matter and assumed dark energy models. We applied the phase space analysis allowing understanding the late time behavior of the models. It allows demonstrating that considered non-gravitational interactions can solve the cosmological coincidence problem. On the other hand, we applied Bayesian Machine Learning technique to learn the constraints on the free parameters. In this way, we gained a better understanding of the models providing a hint which of them can be ruled out. Moreover, the learning based on the simulated expansion rate data shows that the models cannot solve the H0 tension problem.

The possibility of a stable flat dark energy-dominated Swiss-cheese brane-world universe

2020 ā—½ Ā 
Vol 17 (05) ā—½ Ā 
pp. 2050075
Author(s): Ā 
Nasr Ahmed ā—½ Ā 
Kazuharu Bamba ā—½ Ā 
F. Salama
Keyword(s): Ā 
Dark Energy ā—½ Ā 
Cosmological Constant ā—½ Ā 
Cosmological Models ā—½ Ā 
Brane World ā—½ Ā 
Fine Tuning ā—½ Ā 
Energy Conditions ā—½ Ā 
Late Time ā—½ Ā 
Swiss Cheese ā—½ Ā 
Black Strings ā—½ Ā 
The Stability

In this paper, we study the possibility of obtaining a stable flat dark energy-dominated universe in a good agreement with observations in the framework of Swiss-cheese brane-world cosmology. Two different brane-world cosmologies with black strings have been introduced for any cosmological constant [Formula: see text] using two empirical forms of the scale factor. In both models, we have performed a fine-tuning between the brane tension and the cosmological constant so that the Equation of state (EoS) parameter [Formula: see text] for the current epoch, where the redshift [Formula: see text]. We then used these fineā€“tuned values to calculate and plot all parameters and energy conditions. The decelerationā€“acceleration cosmic transition is allowed in both models, and the jerk parameter [Formula: see text] at late-times. Both solutions predict a future dark energy-dominated universe in which [Formula: see text] with no crossing to the phantom divide line. While the pressure in the first solution is always negative, the second solution predicts a better behavior of cosmic pressure where the pressure is negative only in the late-time accelerating era but positive in the early-time decelerating era. Such a positive-to-negative transition in the evolution of pressure helps to explain the cosmic decelerationā€“acceleration transition. Since black strings have been proved to be unstable by some authors, this instability can actually reflect doubts on the stability of cosmological models with black strings (Swiss-cheese type brane-worlds cosmological models). For this reason, we have carefully investigated the stability through energy conditions and sound speed. Because of the presence of quadratic energy terms in Swiss-cheese type brane-world cosmology, we have tested the new nonlinear energy conditions in addition to the classical energy conditions. We have also found that a negative tension brane is not allowed in both models of the current work as the energy density will no longer be well defined.

Hypersurface-homogeneous cosmological models with dynamical equation of state parameter in Lyra geometry

10.1139/cjp-2015-0040 ā—½ Ā 
2015 ā—½ Ā 
Vol 93 (10) ā—½ Ā 
pp. 1100-1105 ā—½ Ā 
Author(s): Ā 
Shri Ram ā—½ Ā 
S. Chandel ā—½ Ā 
M.K. Verma
Keyword(s): Ā 
Dynamical Equation ā—½ Ā 
State Parameter ā—½ Ā 
Lyra Geometry ā—½ Ā 
Cosmological Models ā—½ Ā 
Anisotropic Fluid ā—½ Ā 
Late Time ā—½ Ā 
Field Equations ā—½ Ā 
Homogeneous Cosmological Models ā—½ Ā 
Negative Constant ā—½ Ā 
Equation Of State Parameter

The hypersurface homogeneous cosmological models are investigated in the presence of an anisotropic fluid in the framework of Lyra geometry. Exact solutions of field equations are obtained by applying a special law of variation for mean Hubble parameter that gives a negative constant value of the deceleration parameter. These solutions correspond to anisotropic accelerated expanding cosmological models that isotropize for late time even in the presence of anisotropic fluid. The anisotropy of the fluid also isotropizes at late time. Some physical and kinematical properties of the model are also discussed.

Sign in / Sign up

Export Citation Format

Share Document