scholarly journals A cyclic universe with varying cosmological constant in f(R, T) gravity

2019 ◽  
Vol 97 (10) ◽  
pp. 1075-1082 ◽  
Author(s):  
Nasr Ahmed ◽  
Sultan Z. Alamri
Keyword(s):  
Cosmological Constant ◽  
Early Time ◽  
State Parameter ◽  
Singular Values ◽  
Energy Conditions ◽  
Late Time ◽  
Time Varying ◽  
Causality Condition ◽  
Cyclic Universe ◽  
Cyclic Models

A new kind of evolution for cyclic models in which the Hubble parameter oscillates and remains positive has been explored in a specific f(R, T) gravity reconstruction. A singularity-free cyclic universe with negative varying cosmological constant has been obtained, which supports the role suggested for negative Λ in stopping the eternal acceleration. The cosmological solutions have been obtained for the case of a flat universe, supported by observations. The cosmic pressure grows without singular values; it is positive during the early-time decelerated expansion and negative during the late-time accelerating epoch. The time-varying equation of state parameter ω(t) shows quintom behavior and is restricted to the range –2.25 ≤ ω(t) ≲ 1/3. The validity of the classical linear energy conditions and the sound speed causality condition has been studied. The non-conventional mechanism of negative cosmological constant that are expected to address the late-time acceleration has been discussed.

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

2020 ◽  
Vol 17 (05) ◽  
pp. 2050075
Author(s):  
Nasr Ahmed ◽  
Kazuharu Bamba ◽  
F. Salama
Keyword(s):  
Dark Energy ◽  
Cosmological Constant ◽  
Cosmological Models ◽  
Brane World ◽  
Fine Tuning ◽  
Energy Conditions ◽  
Late Time ◽  
Swiss Cheese ◽  
Black Strings ◽  
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 fine–tuned values to calculate and plot all parameters and energy conditions. The deceleration–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 deceleration–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.

scholarly journals Unrivaled quantum vacuum in the primordial universe

2021 ◽  
Vol 36 (01) ◽  
pp. 2150002
Author(s):  
Davood Momeni
Keyword(s):  
Scalar Field ◽  
Cosmological Constant ◽  
Effective Mass ◽  
Conformal Symmetry ◽  
Early Time ◽  
Late Time ◽  
Quantization Scheme ◽  
Quantum Vacuum ◽  
Vacuum States ◽  
Dominant Part

In a primordial universe pre- (post-)inflationary era, there could be phases of early universe made of cold gas baryons, radiation and early post-inflationary cosmological constant. I showed that in the baryonic epoch, the quantum vacuum is unique. By using the standard quantization scheme for a massive minimally coupled scalar field with maximal conformal symmetry in the classical space–time, I demonstrated that the scalar modes had an effective mass [Formula: see text] (or [Formula: see text]). This argument was validated when the conformal time [Formula: see text] kept so close to the inflation ending time [Formula: see text]. The energy density of the baryonic matter diverged at the inflation border and vanishes at the late time future. Furthermore, I argued that at very early accelerating epoch when the radiation was the dominant part in the close competition with the early time cosmological constant, fine-tuned mass of the scalar field [Formula: see text] also provided a unique quantum vacuum. The reason is that the effective mass eventually is vanished. A remarkable observation was that all the other possible vacuum states “squeezed” eternally.

scholarly journals ACCELERATING UNIVERSE IN A BIG BOUNCE MODEL

2004 ◽  
Vol 19 (06) ◽  
pp. 449-456 ◽  
Author(s):  
BEILI WANG ◽  
HONGYA LIU ◽  
LIXIN XU
Keyword(s):  
Dark Energy ◽  
Cosmological Constant ◽  
Early Time ◽  
Accelerating Universe ◽  
Late Time ◽  
Total Energy Density ◽  
Variable Cosmological Constant ◽  
Type Ia ◽  
The Universe ◽  
Big Bounce

Recent observations of Type Ia supernovae provide evidence for the acceleration of our universe, which leads to the possibility that the universe is entering an inflationary epoch. We simulate it under a "big bounce" model, which contains a time variable cosmological "constant" that is derived from a higher dimension and manifests itself in 4D spacetime as dark energy. By properly choosing the two arbitrary functions contained in the model, we obtain a simple exact solution in which the evolution of the universe is divided into several stages. Before the big bounce, the universe contracts from a Λ-dominated vacuum, and after the bounce, the universe expands. In the early time after the bounce, the expansion of the universe is decelerating. In the late time after the bounce, dark energy (i.e. the variable cosmological "constant") overtakes dark matter and baryons, and the expansion enters an accelerating stage. When time tends to infinity, the contribution of dark energy tends to two thirds of the total energy density of the universe, qualitatively in agreement with observations.

scholarly journals Ricci–Gauss–Bonnet holographic dark energy in Chern–Simons modified gravity: A flat FLRW quintessence-dominated universe

2019 ◽  
Vol 35 (05) ◽  
pp. 2050007 ◽  
Author(s):  
Nasr Ahmed
Keyword(s):  
Dark Energy ◽  
Modified Gravity ◽  
Holographic Dark Energy ◽  
Cosmic Time ◽  
Early Time ◽  
State Parameter ◽  
Late Time ◽  
Equation Of State Parameter ◽  
Chern Simons ◽  
Far Future

We discuss the recently suggested Ricci–Gauss–Bonnet holographic dark energy in Chern–Simons modified gravity. We have tested some general forms of the scale factor [Formula: see text], and used two physically reasonable forms which have been proved to be consistent with observations. Both solutions predict a sign flipping in the evolution of cosmic pressure which is positive during the early-time deceleration and negative during the late-time acceleration. This sign flipping in the evolution of cosmic pressure helps in explaining the cosmic deceleration–acceleration transition, and it has appeared in other cosmological models in different contexts. However, this work shows a pressure singularity which needs to be explained. The evolution of the equation of state parameter [Formula: see text] shows the same asymptotic behavior for both solutions indicating a quintessence-dominated universe in the far future. We also note that [Formula: see text] goes to negative values (leaving the decelerating dust-dominated era at [Formula: see text]) at exactly the same time the pressure becomes negative. Again, there is another singularity in the behavior of [Formula: see text] which happens at the same cosmic time of the pressure singularity.

scholarly journals REQUIRED CONDITIONS FOR LATE TIME ACCELERATION AND EARLY TIME DECELERATION IN GENERALIZED SCALAR–TENSOR THEORIES

2003 ◽  
Vol 18 (04) ◽  
pp. 651-671 ◽  
Author(s):  
L. M. DIAZ-RIVERA ◽  
LUIS O. PIMENTEL
Keyword(s):  
Scalar Field ◽  
Cosmological Constant ◽  
Cosmological Parameters ◽  
Early Time ◽  
Weak Field ◽  
Scale Factor ◽  
Coupling Function ◽  
Accelerated Expansion ◽  
Late Time ◽  
Cosmological Constants

We consider a generalized scalar–tensor theory, where we let the coupling function ω(ϕ) and the effective cosmological constants Λ(ϕ) be undetermined. We obtain general expressions for ω(ϕ) and Λ(ϕ) in terms of the scalar field and the scale factor, and show that ω(ϕ) depends on the scalar field and the scale factor in a complicated way. In order to study the conditions for an accelerated expansion at the present time and a decelerated expansion in the past, we assume a power law evolution for the scalar field and the scale factor. We analyze the required conditions that allow our model to satisfy the weak field limits on ω(ϕ), and at the same time, to obtain the correct values of cosmological parameters, as the energy density Ωm0 and cosmological constant Λ(t0). We also study the conditions for a decelerated expansion at an early time dominated by radiation. We find values for ω(ϕ) and Λ(ϕ) consistent with the expectations of a where the cosmological constant decreases with the time and the coupling function increases until the values are accepted today.

scholarly journals A stable flat entropy-corrected FRW universe

2019 ◽  
Vol 16 (10) ◽  
pp. 1950159 ◽  
Author(s):  
Nasr Ahmed ◽  
Sultan Z. Alamri
Keyword(s):  
Dark Energy ◽  
Early Time ◽  
Energy Conditions ◽  
Positive Pressure ◽  
Late Time ◽  
Frw Universe ◽  
Ricci Dark Energy ◽  
Flat Universe ◽  
Corrected Entropy ◽  
Good Agreement

In this paper, a general entropy-corrected FRW cosmological model has been presented in which a deceleration-to-acceleration transition occurs according to recent observations. We found that the case for the flat universe ([Formula: see text]), supported by observations, is the most stable one where it successfully passes all stability tests. The stability of the model has been studied through testing the sound speed, the classical and the new nonlinear energy conditions. The model predicts a positive pressure during the early-time decelerating epoch, and a negative pressure during the late-time accelerating epoch in a good agreement with cosmic history and dark energy assumption. We have investigated all possible values of the prefactors [Formula: see text] and [Formula: see text] in the corrected entropy-area relation to find the best values required for a stable flat universe. We have also made use of the evolution of the equation of state parameters [Formula: see text] in predicting the correct values of [Formula: see text] and [Formula: see text]. The jerk and density parameters have been calculated where a good agreement with observations and [Formula: see text]CDM model has been obtained. Two dark energy proposals have been investigated in this model, the entropy-corrected holographic dark energy and the modified holographic Ricci dark energy.

scholarly journals Late-Time Mild Inflation: A Possible Solution of a Dilemma: The Cosmic Age and the Hubble Parameter

1997 ◽  
Vol 06 (01) ◽  
pp. 69-90 ◽  
Author(s):  
Takeshi Fukuyama ◽  
Masae Miyoshi ◽  
Mikiko Hatakeyama ◽  
Masahiro Morikawa ◽  
Akika Nakamichi
Keyword(s):  
Cosmological Constant ◽  
Hubble Parameter ◽  
Gravitational Constant ◽  
High Redshift ◽  
Fine Tuning ◽  
Energy Conditions ◽  
Late Time ◽  
Distant Galaxy ◽  
Cepheid Variables ◽  
Varying Gravitational Constant

We explore the cosmological model in which a late-time mild inflation (LMI) is realized after the star formation epoch. Nonvanishing curvature coupling of a classical boson field yields this mild inflation without a cosmological constant. Accordingly the lifetime of the present Universe is remarkably increased in this LMI model. Thus we show that the present observed high value of the Hubble parameter H0 ≈ 70–80 km/sec/Mpc is compatible with the age of the oldest stars 14 Gyr without introducing the cosmological constant or the dilute Universe model. Moreover in this LMI model, the local Hubble parameter becomes larger than the global one. Thus we show that the present observed local Hubble parameter measured by using the Cepheid variables is compatible with the global Hubble parameter measured by using the Sunyaev–Zeldovich effect. Furthermore we examine several aspects of the LMI model: (a) The energy conditions are violated. We examine the consequences of these violations. (b) There is a natural evolution of the effective gravitational "constant" in high redshift region. This yields a drastic change of the stellar luminosity through the constructive equations of a star. We point out that a distant galaxy becomes much dimmer by this effect. (c) This varying gravitational "constant" affects the cosmic expansion speed and the nucleosynthesis process in the early Universe. We point out that this effect constrains the parameters of the LMI model though fine tuning is always possible.

Radar target identification using a combined early-time/late-time E-pulse technique

1998 ◽  
Vol 46 (9) ◽  
pp. 1272-1278 ◽  
Author(s):  
Q. Li ◽  
P. Ilavarasan ◽  
J.E. Ross ◽  
E.J. Rothwell ◽  
Kun-Mu Chen ◽  
...  
Keyword(s):  
Target Identification ◽  
Early Time ◽  
Radar Target ◽  
Late Time ◽  
Pulse Technique

scholarly journals Almost-Pseudo-Ricci Symmetric FRW Universe with a Dynamic Cosmological Term and Equation of State

Universe ◽  
2021 ◽  
Vol 7 (7) ◽  
pp. 205
Author(s):  
Sanjay Mandal ◽  
Avik De ◽  
Tee-How Loo ◽  
Pradyumn Kumar Sahoo
Keyword(s):  
Equation Of State ◽  
Cosmological Parameters ◽  
Cosmological Term ◽  
Ricci Scalar ◽  
Energy Conditions ◽  
Late Time ◽  
Frw Universe ◽  
Accelerating Expansion ◽  
Expansion Of The Universe ◽  
The Universe

The objective of the present paper is to investigate an almost-pseudo-Ricci symmetric FRW spacetime with a constant Ricci scalar in a dynamic cosmological term Λ(t) and equation of state (EoS) ω(t) scenario. Several cosmological parameters are calculated in this setting and thoroughly studied, which shows that the model satisfies the late-time accelerating expansion of the universe. We also examine all of the energy conditions to check our model’s self-stability.

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