Vacuum expansion in arbitrary–gauge Lyra geometry

2014 ◽  
Vol 92 (4) ◽  
pp. 311-315 ◽  
Author(s):  
Hoavo Hova
Keyword(s):  
Lyra Geometry ◽  
Cosmological Term ◽  
Accelerated Expansion ◽  
Time Varying ◽  
De Sitter ◽  
Lyra’S Geometry ◽  
De Sitter Universe ◽  
The Universe ◽  
Matter Fields ◽  
Arbitrary Gauge

We propose a cosmological model containing a cosmological term in arbitrary gauge in Lyra’s geometry. In the absence of matter fields (such as radiation or baryonic), a constant cosmological term does not lead to the de Sitter universe as it is seen in Riemannian geometry, while a time-varying cosmological term can drive the universe into an accelerated expansion.

scholarly journals Gauss–Bonnet modified gravity models with bouncing behavior

2016 ◽  
Vol 31 (17) ◽  
pp. 1650108 ◽  
Author(s):  
Anna Escofet ◽  
Emilio Elizalde
Keyword(s):  
Modified Gravity ◽  
Physical Nature ◽  
Gravity Models ◽  
Accelerated Expansion ◽  
De Sitter ◽  
Eos Parameter ◽  
Energy Models ◽  
De Sitter Universe ◽  
The Universe ◽  
Modified Gravity Models

The following issue is addressed: How the addition of a Gauss–Bonnet term (generically coming from most fundamental theories, as string and M theories), to a viable model, can change the specific properties, and even the physical nature, of the corresponding cosmological solutions? Specifically, brand new original dark energy models are obtained in this way with quite interesting properties, which exhibit, in a unified fashion, the three distinguished possible cosmological phases corresponding to phantom matter, quintessence and ordinary matter, respectively. A model, in which the equation of state (EoS) parameter, w, is a function of time, is seen to lead either to a singularity of the Big Rip kind or to a bouncing solution which evolves into a de Sitter universe with w = −1. Moreover, new Gauss–Bonnet modified gravity models with bouncing behavior in the early stages of the universe evolution are obtained and tested for the validity and stability of the corresponding solutions. They allow for a remarkably natural, unified description of a bouncing behavior at early times and accelerated expansion at present.

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 COSMOLOGY IN SCALAR–TENSOR THEORY AND ASYMPTOTICALLY de SITTER UNIVERSE

2001 ◽  
Vol 16 (20) ◽  
pp. 1303-1313 ◽  
Author(s):  
A. A. SEN ◽  
S. SEN
Keyword(s):  
Accelerated Expansion ◽  
Late Time ◽  
Scalar Tensor Theory ◽  
Dilaton Field ◽  
De Sitter ◽  
Positive Cosmological Constant ◽  
Expansion Phase ◽  
Tensor Theory ◽  
Sitter Universe ◽  
De Sitter Universe

We have investigated the cosmological scenarios with a four-dimensional effective action which is connected with multidimensional, supergravity and string theories. The solution for the scale factor is such that initially universe undergoes a decelerated expansion but in late times it enters into the accelerated expansion phase. In fact, it asymptotically becomes a de Sitter universe. The dilaton field in our model is a decreasing function of time and it becomes a constant in late time resulting the exit from the scalar–tensor theory to the standard Einstein's gravity. Also the dilaton field results in the existence of a positive cosmological constant in late times.

scholarly journals TACHYON FIELDS WITH EFFECTS OF QUANTUM MATTER IN AN ANTI-DE SITTER UNIVERSE

2005 ◽  
Vol 14 (08) ◽  
pp. 1439-1449 ◽  
Author(s):  
EMILIO ELIZALDE ◽  
JOHN QUIROGA HURTADO
Keyword(s):  
Perfect Fluid ◽  
Quantum Effects ◽  
Accelerated Expansion ◽  
Energy Conditions ◽  
De Sitter ◽  
Quantum Matter ◽  
Trace Anomaly ◽  
Sitter Universe ◽  
Proposed Model ◽  
De Sitter Universe

We consider an Anti-de Sitter universe filled by quantum conformal matter with the contribution from the usual tachyon and a perfect fluid. The model represents the combination of a trace-anomaly annihilated and a tachyon driven Anti-de Sitter universe. The influence exerted by the quantum effects and by the tachyon on the AdS space is studied. The radius corresponding to this universe is calculated and the effect of the tachyon potential is discussed, in particular, concerning the possibility to get an accelerated scale factor for the proposed model (which yields an accelerated expansion of the AdS type of universe). Fulfillment of the cosmological energy conditions in the model is also investigated.

scholarly journals A novel teleparallel dark energy model

2016 ◽  
Vol 25 (02) ◽  
pp. 1650025 ◽  
Author(s):  
Giovanni Otalora
Keyword(s):  
Dark Energy ◽  
Scalar Field ◽  
Teleparallel Gravity ◽  
Accelerated Expansion ◽  
Late Time ◽  
De Sitter ◽  
Sitter Group ◽  
Current State ◽  
Expansion Of The Universe ◽  
The Universe

Although equivalent to general relativity, teleparallel gravity (TG) is conceptually speaking a completely different theory. In this theory, the gravitational field is described by torsion, not by curvature. By working in this context, a new model is proposed in which the four-derivative of a canonical scalar field representing dark energy is nonminimally coupled to the “vector torsion”. This type of coupling is motivated by the fact that a scalar field couples to torsion through its four-derivative, which is consistent with local spacetime kinematics regulated by the de Sitter group [Formula: see text]. It is found that the current state of accelerated expansion of the universe corresponds to a late-time attractor that can be (i) a dark energy-dominated de Sitter solution ([Formula: see text]), (ii) a quintessence-type solution with [Formula: see text], or (iii) a phantom-type [Formula: see text] dark energy.

scholarly journals The Asymmetric Cosmic Time – The Key to a New Cosmological Model

2020 ◽  
Vol 2 (1) ◽  
pp. 97-111
Author(s):  
Horst Fritsch ◽  
Eberhard Schluecker
Keyword(s):  
Cosmological Model ◽  
Initial Conditions ◽  
Logical Consequence ◽  
Cosmic Time ◽  
Big Bang ◽  
Relativity Theory ◽  
Accelerated Expansion ◽  
Theory Of Relativity ◽  
De Sitter ◽  
The Universe

The asymmetric cosmic time is a logical consequence of the General Theory of Relativity (GR), if one demands that it should apply to the entire cosmos. From the simplest cosmological model that is consistent with the ART (Einstein-de Sitter model) thus follows the < Cosmic Time Hypothesis > (CTH), which offers solutions for many unsolved problems of cosmology that the current standard model of cosmology (ɅCDM model) cannot explain. According to the CTH, space, time and matter form a unit and develop evolutionarily according to identical, time-dependent laws. According to the CTH time has neither beginning nor end. The "big bang" disappears into the infinite past, which is why the universe manages without inflation. The accelerated expansion of the universe is also unlikely to occur if the SN-Ia measurement results are interpreted using the CTH. The cosmological constant Ʌ can then be omitted (Ʌ=0) and consequently no "dark energy" is needed. In addition, the CTH also provides interesting results on the topics: Initial conditions for hypotheses, stability of the expanding, flat universe (Ω=1), cosmic energy balance (is there negative energy ?), theory of earth expansion, unification of natural forces, Mach's principle. Should the CTH receive broad experimental confirmation, the GR could be extended to the "Universal Relativity Theory" (UR).

scholarly journals Origin of Universe accelerated expansion and prediction of the right deceleration parameter

2019 ◽  
Author(s):  
Paolo Di Sia
Keyword(s):  
Dark Energy ◽  
Deceleration Parameter ◽  
Ad Hoc ◽  
Accelerated Expansion ◽  
Dense Layer ◽  
De Sitter ◽  
Dark Energy Density ◽  
Particle Velocities ◽  
De Sitter Universe ◽  
The Right

The filament (f) theory implies initial isotropic particle-velocities with uniform value-distribution between zero and the speed of light c. That leads to a universe boundary coinciding with the events horizon of its centre. The very dense layer of particles and antiparticles expanding with almost c observed at retarded times attracts the internal particles thus implying for them an accelerated expansion. There is no need for an "ad hoc" dark energy implying repulsion. The predicted negative deceleration parameter q corresponds to a dark energy density 95.5 % of the critical value. If the red-shift of far galaxies was due to the only Doppler-Fizeau effect, the standard value 73 % is obtained. The past and future q is predicted. The q value was a negative maximum just after the primordial annihilation of the particles with the antiparticles and will vanish at about 3TH (where TH denotes the Hubble time) after the present time t. For t larger than 3TH, it will be positive, tending to 1/2, typical of the Einstein-De Sitter universe.

scholarly journals Time-Varying Fine-Structure Constant Requires Cosmological Constant

2019 ◽  
Author(s):  
Rainer Kühne
Keyword(s):  
Fine Structure ◽  
Cosmological Constant ◽  
Structure Constant ◽  
Preliminary Evidence ◽  
Fine Structure Constant ◽  
Time Varying ◽  
De Sitter ◽  
Sitter Universe ◽  
De Sitter Universe

Webb et al. presented preliminary evidence for a time-varying fine-structure constant. We show Teller's formula for this variation to be ruled out within the Einstein-de Sitter universe, however, it is compatible with cosmologies which require a large cosmological constant.

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 Generalized second law of thermodynamics on the apparent horizon in modified Gauss–Bonnet gravity

2016 ◽  
Vol 25 (04) ◽  
pp. 1650040 ◽  
Author(s):  
A. Abdolmaleki ◽  
T. Najafi
Keyword(s):  
Deceleration Parameter ◽  
Apparent Horizon ◽  
Accelerated Expansion ◽  
Energy Conditions ◽  
Second Law ◽  
De Sitter ◽  
Frw Universe ◽  
Curvature Invariant ◽  
Generalized Second Law ◽  
The Universe

Modified gravity (MG) and generalized second law (GSL) of thermodynamics are interesting topics in the modern cosmology. In this regard, we investigate the GSL of gravitational thermodynamics in the framework of modified Gauss–Bonnet (GB) gravity or [Formula: see text]-gravity. We consider a spatially FRW universe filled with the pressureless matter and radiation enclosed by the dynamical apparent horizon with the Hawking temperature. For two viable [Formula: see text] models, we first numerically solve the set of differential equations governing the dynamics of [Formula: see text]-gravity. Then, we obtain the evolutions of the Hubble parameter, the GB curvature invariant term, the density and equation of state (EoS) parameters as well as the deceleration parameter. In addition, we check the energy conditions for both models and finally examine the validity of the GSL. For the selected [Formula: see text] models, we conclude that both models have a stable de Sitter attractor. The EoS parameters behave quite similar to those of the [Formula: see text]CDM model in the radiation/matter dominated epochs, then they enter the phantom region before reaching the de Sitter attractor with [Formula: see text]. The deceleration parameter starts from the radiation/matter dominated eras, then transits from a cosmic deceleration to acceleration and finally approaches a de Sitter regime at late times, as expected. Furthermore, the GSL is respected for both models during the standard radiation/matter dominated epochs. Thereafter when the universe becomes accelerating, the GSL is violated in some ranges of scale factor. At late times, the evolution of the GSL predicts an adiabatic behavior for the accelerated expansion of the universe.

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