scholarly journals DOES QUANTUM COSMOLOGY PREDICT A CONSTANT DILATONIC FIELD?

2005 ◽  
Vol 14 (02) ◽  
pp. 291-307 ◽  
Author(s):  
F. G. ALVARENGA ◽  
A. B. BATISTA ◽  
J. C. FABRIS
Keyword(s):  
Cosmological Model ◽  
Quantum Cosmology ◽  
Initial Conditions ◽  
Specific Model ◽  
Scale Factor ◽  
Gravitational Coupling ◽  
Expectation Values ◽  
Time Coordinate ◽  
Classical Models ◽  
The Universe

Quantum cosmology may permit to determine the initial conditions of the Universe. In particular, it may select a specific model between many possible classical models. In this work, we study a quantum cosmological model based on the string effective action coupled to matter. The Schutz's formalism is employed in the description of the fluid. A radiation fluid is considered. In this way, a time coordinate may be identified and the Wheeler–DeWitt equation reduces in the minisuperspace to a Schrödinger-like equation. It is shown that, under some quite natural assumptions, the expectation values indicate a null axionic field and a constant dilatonic field. At the same time the scale factor exhibits a bounce revealing a singularity-free cosmological model. In some cases, the mininum value of the scale factor can be related to the value of gravitational coupling.

scholarly journals Generalized F(R, T) cosmological models with fermionic fields

2021 ◽  
Vol 2090 (1) ◽  
pp. 012063
Author(s):  
Koblandy Yerzhanov ◽  
Gulnur Bauyrzhan ◽  
Ratbay Myrzakulov
Keyword(s):  
Mathematical Model ◽  
Cosmological Model ◽  
Main Idea ◽  
Scale Factor ◽  
Specific Form ◽  
Fermion Field ◽  
Fermionic Fields ◽  
Particular Solution ◽  
The Universe ◽  
Symmetry Method

Abstract We investigated the gravity model F (R, T), which interacts with a fermion field in a uniform and isotropic at spacetime FLRW. The main idea and purpose of the work donewas to create a mathematical model and find a particular solution for the scale factor a, since it describes the dynamics of the evolution of the Universe. The solutions for this universe are obtained using the Noether symmetry method. With its help, a specific form of the Lagrangian is obtained. And the possible types of the scale factor were found. The evolution of the resulting cosmological model has been investigated.

Tomographic analysis of quantum and classical de Sitter cosmological models

2019 ◽  
Vol 28 (16) ◽  
pp. 2040009 ◽  
Author(s):  
Cosimo Stornaiolo
Keyword(s):  
Quantum Cosmology ◽  
Classical Limit ◽  
Initial Conditions ◽  
Quantum Tomography ◽  
Wave Functions ◽  
Hamiltonian Constraint ◽  
De Sitter ◽  
Quantum Universe ◽  
Tomographic Analysis ◽  
The Universe

In this work, we show the importance of introducing the quantum tomography formalism to analyze the properties of wave functions in quantum cosmology. In particular, we examine the initial conditions of the universe proposed by various authors in the context of de Sitter’s cosmology studying their classical limit and comparing it with the classical tomogram obtained from the Hamiltonian constraint in General Relativity. This comparison gives us the opportunity to find under which conditions there is a transition from the quantum universe to the classical one. A relevant result is that in these models the decay of the cosmological constant is a sufficient condition for this transition.

COSMIC TIME IN QUANTUM COSMOLOGY: INFLATION-COMPACTIFICATION AS A QUANTUM EFFECT

1993 ◽  
Vol 02 (02) ◽  
pp. 221-247 ◽  
Author(s):  
E.I. GUENDELMAN ◽  
A.B. KAGANOVICH
Keyword(s):  
Cosmological Constant ◽  
Quantum Cosmology ◽  
Quantum Effect ◽  
Cosmic Time ◽  
Expectation Values ◽  
Cosmological Singularity ◽  
Inflationary Phase ◽  
Definition Of ◽  
The Universe ◽  
Kaluza Klein

We consider 1+D-dimensional, toroidally compact Kaluza-Klein theories. In the context of the minisuperspace approach of quantum cosmology, we solve the Wheeler-DeWitt equation in the presence of a negative cosmological constant and dust. Then, it is found that the quantum effects stabilize the volume of the Universe, so that there can be an avoidance of the cosmological singularity. Although cosmic time does not appear explicitly in the Wheeler-DeWitt equation, we find that a cosmic time dependence appears for the expectation values of certain variables. This result is obtained when proper care of some subtle points concerning the definition of averages in this model is taken. The stabilization of the volume, when there is anisotropy in the evolution of the Universe (which turns out to be quantized), is consistent with another effect we find: the existence of a “quantum inflationary phase” for some dimensions and simultaneously the existence of a “quantum deflationary contraction” for the rest.

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 Phenomenology analysis of duration inflation for tachyon field in loop quantum cosmology

2014 ◽  
Vol 23 (11) ◽  
pp. 1450087 ◽  
Author(s):  
Kui Xiao ◽  
Xiao-Kai He ◽  
Fei Huang ◽  
Jian-Yang Zhu
Keyword(s):  
Quantum Cosmology ◽  
Initial Conditions ◽  
Scale Factor ◽  
Loop Quantum Cosmology ◽  
Tachyon Field ◽  
Exponential Potential ◽  
Quadratic Potential ◽  
Slow Rolling

Assuming that the e-folding number is just determined by the change of the scale factor, the tachyonic inflation theory in loop quantum cosmology (LQC) has been discussed. Considering the tachyon field with exponential potential and inverse quadratic potential, we find that the evolutionary pictures of super inflation are affected by the potentials and the initial conditions. However it cannot provide enough e-folding number, no matter which condition is chosen. Therefore a slow-rolling inflation is necessary. The e-folding number for slow-rolling inflation depends on the values of the parameter α of the exponential potential and the initial conditions. To get enough e-folding number, α should be small. Based on the slow-rolling inflation happens immediately when the super inflation ends, and the scale factor continuously grows during the whole inflation stage, we consider an e-folding number provided by the whole inflationary stage, and we find that it is easier to get enough e-folding number when the scale factor increases during all the inflation phase.

Quantelung der Zeit und Quantenmechanik

1985 ◽  
Vol 40 (5) ◽  
pp. 456-461
Author(s):  
M. Börner
Keyword(s):  
Initial Conditions ◽  
Classical Equation ◽  
Equation Of Motion ◽  
Expectation Values ◽  
Functional Connection ◽  
Probability Densities ◽  
Necessary Existence ◽  
The Moment ◽  
The Universe

If the universe as a whole can be described as an ordered succession of discrete (Eigen-)states, the parameter of this order, a number t (t ∈ ℤ.) plays the role of a quantizised time. Then a particle (with mass m) as a substructure of the universe no longer follows a classical equation of motion with the moment p and the position x. The functional connection between these two quantities is rather a distribution. Especially there no longer exists the classical union of differential equation-initial conditions-path. The path is now only understandable as average, introducing a continuously running time. A central part for finding p̄ and x̄ as such averages, is played by the expectation values of these new quantities. Since the expectation values depend on all discrete points x(t) ( − ∞ ≦ t ≦ + ∞), we find sumrelations, which we can approximate by integrals. The integration extends over all d.x resp. dp-elements, which are loaded with the probability of their appearance. Following this procedure p and x become operators. If we postulate p̄ and p̄ to fulfil Newtons law, we find the ψxand ψp functions, constituting the resp. probability densities, to be governed by Shroedingers equation. The necessary existence of a quantum mechanics can thus be a reference to the existence of a noncontinuous time.

Bouncing scenario in f(R,T) gravity

2020 ◽  
Vol 35 (13) ◽  
pp. 2050095 ◽  
Author(s):  
Parbati Sahoo ◽  
Snehasish Bhattacharjee ◽  
S. K. Tripathy ◽  
P. K. Sahoo
Keyword(s):  
Equation Of State ◽  
Initial Conditions ◽  
Scale Factor ◽  
Energy Conditions ◽  
Geometrical Parameters ◽  
Singularity Problem ◽  
Eos Parameter ◽  
Qualitative Understanding ◽  
The Stability ◽  
The Universe

This paper presents modeling of matter bounce in the framework of [Formula: see text] gravity, where [Formula: see text]. We start by defining a parametrization of scale factor which is non-vanishing. The geometrical parameters such as the Hubble parameter and deceleration parameter are derived, from which expressions of pressure, density and Equation of State (EoS) parameter and a qualitative understanding of the initial conditions of the universe at the bounce are ascertained. We found that the initial conditions of the universe are finite owing to the non-vanishing nature of the scale factor thus eliminates the initial singularity problem. Furthermore, we show the violation of energy conditions near the bouncing region and analyze the stability of our model with respect to linear homogeneous perturbations in Friedmann–Lemaître–Robertson–Walker (FLRW) spacetime. We found that our model and hence matter bounce scenarios in general are highly unstable at the bounce in the framework of [Formula: see text] gravity but the perturbations decay out rapidly away from the bounce safeguarding its stability at late-times.

scholarly journals The perturbed universe in the deformed algebra approach of loop quantum cosmology

2016 ◽  
Vol 25 (08) ◽  
pp. 1642003 ◽  
Author(s):  
Julien Grain
Keyword(s):  
Quantum Gravity ◽  
Quantum Cosmology ◽  
Initial Conditions ◽  
Power Spectra ◽  
Quantum Corrections ◽  
Loop Quantum Cosmology ◽  
Algebra Approach ◽  
Deformed Algebra ◽  
Contracting Phase ◽  
The Universe

Loop Quantum Cosmology (LQC) is a tentative approach to model the universe down to the Planck era where quantum gravity settings are needed. The quantization of the universe as a dynamical spacetime is inspired by Loop Quantum Gravity (LQG) ideas. In addition, LQC could bridge contact with astronomical observations, and thus potentially investigate quantum cosmology modelings in the light of observations. To do so however, modeling both the background evolution and its perturbations is needed. The latter described cosmic inhomogeneities that are the main cosmological observables. In this context, we present the so-called deformed algebra approach implementing the quantum corrections to the perturbed universe at an effective level by taking great care of gauge issues. We particularly highlight that in this framework, the algebra of hypersurface deformation receives quantum corrections, and we discuss their meaning. The primordial power spectra of scalar and tensor inhomogeneities are then presented, assuming initial conditions are set in the contracting phase preceding the quantum bounce and the well-known expanding phase of the cosmic history. These spectra are subsequently propagated to angular power spectra of the anisotropies of the cosmic microwave background. It is then shown that regardless of the choice for the initial conditions inside the effective approach for the background evolution (except that they are set in the contracting phase), the predicted angular power spectra of the polarized [Formula: see text]-modes exceed the upper bound currently set by observations. The exclusion of this specific version of LQC establishes the falsifiability of the approach, though one shall not conclude here that either LQC or LQG excluded.

scholarly journals Delayed Inflation of a Closed Cosmological Model Driven by the False Vacuum in Five Dimensions

1990 ◽  
Vol 43 (1) ◽  
pp. 117
Author(s):  
C Wolf
Keyword(s):  
Cosmological Model ◽  
Unique Feature ◽  
Initial Conditions ◽  
Inflationary Cosmology ◽  
Higgs Potential ◽  
False Vacuum ◽  
Intermediate Point ◽  
Five Dimensions ◽  
Model Driven ◽  
The Universe

An inflationary cosmology in five dimensions is discussed that has the unique feature that if initial conditions are tuned appropriately with the values of the unstable maximum of the Higgs potential the universe is trapped in a Lemaitre-type equilibrium point after which it expands in an exponential manner. Such a quasi-static intermediate point may be responsible for the homogeneity of the universe at late times.

scholarly journals Anisotropic Bianchi Type-III Bulk Viscous Fluid Universe in Lyra Geometry

2013 ◽  
Vol 2013 ◽  
pp. 1-5 ◽  
Author(s):  
Priyanka Kumari ◽  
M. K. Singh ◽  
Shri Ram
Keyword(s):  
Cosmological Model ◽  
Viscous Fluid ◽  
Bianchi Type ◽  
Lyra Geometry ◽  
Scale Factor ◽  
Field Equations ◽  
Type Iii ◽  
Bulk Viscous Fluid ◽  
Bulk Viscous ◽  
The Universe

An anisotropic Bianchi type-III cosmological model is investigated in the presence of a bulk viscous fluid within the framework of Lyra geometry with time-dependent displacement vector. It is shown that the field equations are solvable for any arbitrary function of a scale factor. To get the deterministic model of the universe, we have assumed that (i) a simple power-law form of a scale factor and (ii) the bulk viscosity coefficient are proportional to the energy density of the matter. The exact solutions of the Einstein’s field equations are obtained which represent an expanding, shearing, and decelerating model of the universe. Some physical and kinematical behaviors of the cosmological model are briefly discussed.

Sign in / Sign up

Export Citation Format

Share Document