scholarly journals Dynamics in Interacting Scalar-Torsion Cosmology

Universe ◽  
2021 ◽  
Vol 7 (7) ◽  
pp. 244
Author(s):  
Andronikos Paliathanasis
Keyword(s):  
Special Interest ◽  
System Analysis ◽  
Stable Solution ◽  
Gravitational Theory ◽  
De Sitter ◽  
Dark Sector ◽  
Background Space ◽  
Qualitative Evolution ◽  
De Sitter Universe ◽  
The Universe

In a spatially flat Friedmann–Lemaître–Robertson–Walker background space, we consider a scalar-torsion gravitational model which has similar properties to the dilaton theory. This teleparallel model is invariant under a discrete transformation similar to the Gasperini–Veneziano duality transformation. Moreover, in the gravitational action integral, we introduce the Lagrangian function of a pressureless fluid source which is coupled to the teleparallel dilaton field. This specific gravitational theory with interaction in the dark sector of the universe was investigated by using methods of the dynamical system analysis. We calculate that the theory provides various areas of special interest for the evolution of the cosmological history. Inflationary scaling solutions and the de Sitter universe are recovered. Furthermore, we calculate that there exist an attractor which provides a stable solution where the two fluid components, the scalar field and the pressureless matter, contribute in the cosmological fluid. This solution is of special interest because it can describe the present epoch. Finally, the qualitative evolution of the cosmographic parameters is discussed.

scholarly journals Emergent classical universes from initial quantum states in a tomographical description

2020 ◽  
Vol 17 (11) ◽  
pp. 2050167
Author(s):  
Cosimo Stornaiolo
Keyword(s):  
Necessary Conditions ◽  
Phenomenological Approach ◽  
De Sitter ◽  
Present Value ◽  
Asymptotic Expressions ◽  
Classical Transition ◽  
Quantum Tomograms ◽  
De Sitter Universe ◽  
The Universe ◽  
Extract Information

Quantum and classical physical states are represented in a unified way when they aredescribed by symplectic tomography. Therefore this representation allows us to study directly the necessary conditions for a classical universe to emerge from a quantum state. In a previous work on the de Sitter universe this was done by comparing the classical limit of the quantum tomograms with those resulting from the classical cosmological equations. In this paper, we first review these results and extend them to all the de Sitter models. We show further that these tomograms can be obtained directly from transposing the Wheeler–De Witt equation to the tomographic variables. Subsequently, because the classic limits of the quantum tomograms are identified with their asymptotic expressions, we find the necessary conditions to extend the previous results by taking the tomograms of the WKB solutions of the Wheeler–DeWitt equation with an any potential. Furthermore, in the previous works, we found that the de Sitter models undergo the quantum-to-classical transition when the cosmological constant decays to its present value, we discuss at the end how far we can extend this result to more general models. In the conclusions, after discussing any improvements and developments of the results of this work, we sketch a phenomenological approach from which to extract information about the initial states of the universe.

scholarly journals WE PROBABLY LIVE ON AN INFLATING BRANE-WORLD

2011 ◽  
Vol 20 (14) ◽  
pp. 2809-2816
Author(s):  
ISHWAREE P. NEUPANE
Keyword(s):  
Zero Mode ◽  
De Sitter Space ◽  
Brane World ◽  
Conformal Field Theories ◽  
De Sitter ◽  
Conformal Field ◽  
Sitter Spacetime ◽  
Acceleration Of The Universe ◽  
De Sitter Universe ◽  
The Universe

Brane-world models, where observers are trapped within the thickness of a 3-brane, offer novel perspectives on gravitation and cosmology. In this essay, it is argued that the problem of a late epoch acceleration of the universe is well explained in the framework of a 4-dimensional de Sitter universe embedded in a 5-dimensional de Sitter spacetime. While a 5-dimensional Anti-de Sitter space background is important for studying conformal field theories for its role in the AdS/CFT correspondence, the existence of a 5-dimensional de Sitter space is crucial for finding an effective 4-dimensional Newton constant that remains finite and a normalizable zero-mode graviton wave function.

scholarly journals Interacting holographic Ricci dark energy as running vacuum

2019 ◽  
Vol 28 (04) ◽  
pp. 1950060 ◽  
Author(s):  
Paxy George ◽  
V. Mohammed Shareef ◽  
Titus K. Mathew
Keyword(s):  
System Analysis ◽  
Component Model ◽  
Coupling Constant ◽  
De Sitter ◽  
Ricci Dark Energy ◽  
Macroscopic System ◽  
Two Component ◽  
Cosmological Data ◽  
End Stage ◽  
The Universe

Earlier studies have shown that in a two component model of the universe with dark matter and the running vacuum energy, which is phenomenologically a combination of [Formula: see text] and [Formula: see text] either eternal deceleration or acceleration is produced in the absence of a bare constant in the density of the running vacuum. In this paper, we have shown that in the interaction scenario, where the interaction between matter and vacuum is introduced through a phenomenological term, the two component model is capable of causing a transition from a prior decelerated to a later accelerated epoch without a bare constant in the running vacuum density. On contrasting the model with the cosmological data, we have found that the interaction coupling constant is small enough for a slow decay of the running vacuum. The model is subjected to dynamical system analysis, which revealed that the end de Sitter phase of the model is a stable one. We did an analysis on the thermal behavior of the system, which shows that the entropy is bounded at the end stage so that the system behaves like an ordinary macroscopic system. Apart from these, we have also performed the state finder diagnostic analysis which implies the quintessence nature of running vacuum and confirms that the model will approach the standard [Formula: see text]CDM in the future.

scholarly journals THE RETURN OF A STATIC UNIVERSE AND THE END OF COSMOLOGY

2008 ◽  
Vol 17 (03n04) ◽  
pp. 685-690 ◽  
Author(s):  
LAWRENCE M. KRAUSS ◽  
ROBERT J. SCHERRER
Keyword(s):  
Coordinate System ◽  
Vacuum Energy ◽  
Big Bang ◽  
Light Elements ◽  
De Sitter ◽  
True Nature ◽  
Static Universe ◽  
Hubble Expansion ◽  
De Sitter Universe ◽  
The Universe

We demonstrate that as we extrapolate the current ΛCDM universe forward in time, all evidence of the Hubble expansion will disappear, so that observers in our "island universe" will be fundamentally incapable of determining the true nature of the universe, including the existence of the highly dominant vacuum energy, the existence of the CMB, and the primordial origin of light elements. With these pillars of the modern Big Bang gone, this epoch will mark the end of cosmology and the return of a static universe. In this sense, the coordinate system appropriate for future observers will perhaps fittingly resemble the static coordinate system in which the de Sitter universe was first presented.

scholarly journals Cosmic voids in modified gravity scenarios

2019 ◽  
Vol 632 ◽  
pp. A52 ◽  
Author(s):  
Eder L. D. Perico ◽  
Rodrigo Voivodic ◽  
Marcos Lima ◽  
David F. Mota
Keyword(s):  
General Relativity ◽  
Modified Gravity ◽  
Gravitational Theory ◽  
Gravity Models ◽  
Consistency Check ◽  
Dark Sector ◽  
Modified Gravity Theories ◽  
Acceleration Of The Universe ◽  
Best Fit ◽  
The Universe

Modified gravity (MG) theories aim to reproduce the observed acceleration of the Universe by reducing the dark sector while simultaneously recovering General Relativity (GR) within dense environments. Void studies appear to be a suitable scenario to search for imprints of alternative gravity models on cosmological scales. Voids cover an interesting range of density scales where screening mechanisms fade out, which reaches from a density contrast δ ≈ −1 close to their centers to δ ≈ 0 close to their boundaries. We present an analysis of the level of distinction between GR and two modified gravity theories, the Hu–Sawicki f(R) and the symmetron theory. This study relies on the abundance, linear bias, and density profile of voids detected in N-body cosmological simulations. We define voids as connected regions made up of the union of spheres with a mean density given by ρ̅v = 0.2 ρ̅m, but disconnected from any other voids. We find that the height of void walls is considerably affected by the gravitational theory, such that it increases for stronger gravity modifications. Finally, we show that at the level of dark matter N-body simulations, our constraints allow us to distinguish between GR and MG models with |fR0| > 10−6 and zSSB >  1. Differences of best-fit values for MG parameters that are derived independently from multiple void probes may indicate an incorrect MG model. This serves as an important consistency check.

scholarly journals Does holographic equipartition demand a pure cosmological constant?

2020 ◽  
pp. 2050334
Author(s):  
P. B. Krishna ◽  
Titus K. Mathew
Keyword(s):  
Dark Energy ◽  
Cosmological Constant ◽  
Dark Energy Model ◽  
State Parameter ◽  
De Sitter ◽  
Entropy Maximization ◽  
Expansion Of The Universe ◽  
The Law ◽  
De Sitter Universe ◽  
The Universe

The spacial expansion of the universe could be described as a tendency for satisfying holographic equipartition which inevitably demands the presence of dark energy. We explore whether this novel idea proposed by Padmanabhan gives any additional insights into the nature of dark energy. In particular, we obtain the constraints imposed by the law of emergence on the equation of state parameter, [Formula: see text]. We also present a thermodynamic motivation for the obtained constraints on [Formula: see text]. Further, we explicitly prove the feasibility of describing a dynamic dark energy model through the law of emergence. Interestingly, both holographic equipartition and the entropy maximization demand an asymptotically de Sitter universe with [Formula: see text], rather than a pure cosmological constant.

The Large Numbers hypothesis and the Einstein theory of gravitation

1979 ◽  
Vol 365 (1720) ◽  
pp. 19-30 ◽  
Keyword(s):  
Gravitational Theory ◽  
Conclusive Evidence ◽  
De Sitter ◽  
Dimensionless Numbers ◽  
Einstein Theory ◽  
Observable Universe ◽  
Slowing Down ◽  
Large Numbers ◽  
Theory Of Gravitation ◽  
The Universe

A study of the relations between large dimensionless numbers leads one to believe that G , expressed in atomic units, varies with the epoch. The Einstein theory requires G to be constant. One can reconcile these two requirements by supposing that the Einstein theory applies with a metric that differs from the atomic metric. The theory can be developed with conservation of mass by supposing that the continual increase in the mass of the observable universe arises from a continual slowing down of the velocity of recession of the galaxies. One is led to a model of the Universe that was first proposed jointly by Einstein & de Sitter (the E.S. model). The observations of the microwave radiation fit in with this model. The static Schwarzchild metric has to be modified to fit in with the E.S. model for large r . The modification is worked out, and also the motion of planets with the new metric. It is found that there is a difference between ephemeris time and atomic time, and also that there should be an inward spiralling of the planets, referred to atomic units, superposed on the motion given by ordinary gravitational theory. These are effects that can be checked by observation, but there is no conclusive evidence up to the present.

scholarly journals f(R) TERM AND GEOMETRIC ORIGIN OF THE DARK SECTOR IN HOŘAVA–LIFSHITZ GRAVITY

2011 ◽  
Vol 26 (06) ◽  
pp. 387-398 ◽  
Author(s):  
ANZHONG WANG
Keyword(s):  
Detailed Balance ◽  
De Sitter ◽  
Dark Sector ◽  
Detailed Balance Condition ◽  
Balance Condition ◽  
Nonzero Curvature ◽  
Geometric Origin ◽  
The Universe ◽  
Scalar Perturbations ◽  
Scalar Mode

Inclusion of f(R) term in the action of Hořava–Lifshitz quantum gravity with projectability but without detailed balance condition is investigated, where R denotes the 3-spatial dimensional Ricci scalar. Conditions for the spin-0 graviton to be free of ghosts and instability are studied. The requirement that the theory reduces to general relativity in the IR makes the scalar mode unstable in the Minkowski background but stable in the de Sitter. It is remarkable that the dark sector, dark matter and dark energy, of the universe has a naturally geometric origin in such a setup. Bouncing universes can also be constructed. Scalar perturbations in the FRW backgrounds with nonzero curvature are also studied.

scholarly journals Diagnostics for generalized power-law torsion–matter coupling f(T) model

2019 ◽  
Vol 28 (02) ◽  
pp. 1950031
Author(s):  
Rui-Hui Lin ◽  
Qiang Wen ◽  
Xiang-Hua Zhai ◽  
Xin-Zhou Li
Keyword(s):  
Dark Energy ◽  
Power Law ◽  
System Analysis ◽  
Coupling Model ◽  
Chaplygin Gas ◽  
Accelerated Expansion ◽  
De Sitter ◽  
Matter Coupling ◽  
Energy Models ◽  
The Universe

The currently accelerated expansion of our universe is unarguably one of the most intriguing problems in today’s physics research. Two realistic nonminimal torsion–matter coupling [Formula: see text] models have been established and studied in our previous papers [C. J. Feng, F. F. Ge, X. Z. Li, R. H. Lin and X. H. Zhai, Phys. Rev. D 92 (2015) 104038; R. H. Lin, X. H. Zhai and X. Z. Li, Eur. Phys. J. C 77 (2017) 504] aiming to explain this “dark energy” problem. In this paper, we study the generalized power-law torsion–matter coupling [Formula: see text] model. Dynamical system analysis shows that the three expansion phases of the universe, i.e. the radiation-dominated era, the matter-dominated era and the dark energy-dominated era, can all be reproduced in this generalized model. By using the statefinder and [Formula: see text] diagnostics, we find that the different cases of the model can be distinguished from each other and from other dark energy models such as the two models in our previous papers, [Formula: see text]CDM, quintessence and Chaplygin gas. Furthermore, the analyses also show that all kinds of generalized power-law torsion–matter coupling model are able to cross the [Formula: see text] divide from below to above, which is a realization of quintom scenario. The decrease of the energy density resulting from the crossing of [Formula: see text] will make the catastrophic fate of the universe avoided and a de Sitter expansion fate in the future will be approached.

scholarly journals Evolution of Inhomogeneities in the Inflationary Universe -No Hair Theorem or Multi-Production of Universes?-

1988 ◽  
Vol 130 ◽  
pp. 67-75
Author(s):  
Katsuhiko Sato
Keyword(s):  
Critical Length ◽  
High Density ◽  
Density Contrast ◽  
Inflationary Universe ◽  
De Sitter ◽  
Inhomogeneous Universe ◽  
Sitter Universe ◽  
De Sitter Universe ◽  
The Universe

Recent investigations on the evolution of the inhomogeneities in the inflationary universe are reviewed. 1) Strict cosmological no hair theorem does not hold, but the class of inhomogeneous universe which evolve to homogeneous de Sitter universe is finite, i.e, “weak cosmic no hair theorem” holds. 2) High density regions in the inhomogeneous universe evolve to wormholes provided that i) the size of the regions is greater than the horizon length, but smaller than a critical length which is the function of the density contrast, and ii) the density is three times higher than that of surrounding region. 3) If wormholes are formed copiously in the period of inflation, they evolve to causally disconnected “child- universes”. In this scenario, the universe we are now observing is one of the locally flat regions.

Sign in / Sign up

Export Citation Format

Share Document