Article
Astronomy & Astrophysics
Nahomi Kan, Takuma Aoyama, Kiyoshi Shiraishi
Summary: This paper examines a bicosmology scenario proposed by Falomir et al., which introduces noncommutativity to solve cosmological problems. By considering exact classical solutions with constant noncommutativity and hypothesizing the appearance of noncommutativity when the scale factors are small, the researchers investigate the behavior of the model.
CLASSICAL AND QUANTUM GRAVITY
(2023)
Article
Mathematics, Interdisciplinary Applications
Seyed Meraj Mousavi Rasouli, Emanuel de Oliveira W. Costa, Paulo Moniz, Shahram Jalalzadeh
Summary: The Wheeler-DeWitt equation in a flat and compact Friedmann-Lemaitre-Robertson-Walker cosmology during the pre-inflation epoch is studied in the contexts of standard and fractional quantum cosmology. By applying the Wentzel-Kramers-Brillouin (WKB) approximation within the semiclassical regime, it is shown that the simple fractional scenario yields fascinating consequences that are distinct from the standard counterparts: (i) The expected de Sitter behavior of the inflationary universe with constant potential is replaced by power-law inflation. (ii) The non-locality of Riesz's fractional derivative leads to power-law inflation, which depends on the fractal dimension of the compact spatial section of space-time, regardless of the energy scale of the inflaton.
FRACTAL AND FRACTIONAL
(2022)
Article
Astronomy & Astrophysics
Alexander Yu Kamenshchik, Alessandro Tronconi, Giovanni Venturi
Summary: This paper compares two different approaches for solving the Wheeler-DeWitt equation in the presence of homogeneous matter and perturbations. The standard Born-Oppenheimer decomposition and a more general decomposition are compared in the case of a minimally coupled inflaton model, and the consistency of the latter decomposition is checked against the former.
CLASSICAL AND QUANTUM GRAVITY
(2021)
Article
Astronomy & Astrophysics
Benno August Ludwig Bodmann, Cesar Augusto Zen Vasconcellos, Peter Otto Hess Bechstedt, Jose Antonio de Freitas Pacheco, Dimiter Hadjimichef, Moises Razeira, Gervasio Annes Degrazia
Summary: This article investigates a formulation of quantum gravity using the Horava-Lifshitz theory of gravity, which is General Relativity augmented by counter-terms to render the theory regularized. By introducing an energy-dependent effective potential, which describes the space-time curvature associated with the embedding geometry and its coupling with the cosmological constant and matter fields, solutions of the Wheeler-DeWitt equation for the wave function of the Universe are obtained. A new scenario for the origin of the Universe is proposed, featuring a smooth transition region between the contraction and expansion phases.
Article
Astronomy & Astrophysics
Marcello Rotondo
Summary: This note discusses the difficulties in the canonical gravity equation proposed by Wheeler and DeWitt, particularly the lack of an explicit time. The authors suggest a simple approach to avoid this issue by explicitly incorporating a classically meaningful notion of time before quantization, resulting in a Wheeler-DeWitt equation with time. A working solution for the de Sitter minisuperspace is presented.
Article
Astronomy & Astrophysics
Benno Bodmann, Dimiter Hadjimichef, Peter Otto Hess, Jose de Freitas Pacheco, Fridolin Weber, Moises Razeira, Gervasio Annes Degrazia, Marcelo Marzola, Cesar A. Zen Vasconcellos
Summary: This contribution investigates the effects of non-commutativity on the structure of the branch-cut scale factor and the acceleration of the Universe. A phase-space transformation is proposed to describe the behavior of the Wheeler-DeWitt wave function of the Universe within a non-commutative algebraic quantum gravity formulation. Numerical solutions indicate the acceleration of the early Universe in the context of the non-commutative branch-cut gravity formulation.
Article
Astronomy & Astrophysics
S. Jalalzadeh
Summary: We investigate the quantum cosmology of a closed spatially homogeneous and isotropic Friedmann-Lemaitre-Robertson-Walker (FLRW) minisuperspace model with electromagnetic radiation as matter content. We solve the corresponding Wheeler-DeWitt equation by utilizing Riemann's zeta function regularization method. We demonstrate that the regularized vacuum energy of the electromagnetic field can overcome factor ordering, boundary conditions, and singularity problems.
Article
Astronomy & Astrophysics
Steven Carlip
Summary: The cosmological constant may be large at the Planck scale but hidden by quantum fluctuations of spacetime. Evidence from a simplified model suggests that a foamy structure can effectively hide a large cosmological constant and persist over time evolution.
Article
Physics, Multidisciplinary
Chia-Min Lin
Summary: This work provides simple analytical solutions for the Wheeler-DeWitt equation applied to de Broglie-Bohm quantum cosmology, focusing on specific potentials of a scalar matter field. One solution describes a rolling scalar field with a quadratic potential and negative cosmological constant, while the other solution describes a flat Minkowski universe with a hilltop potential.
CHINESE JOURNAL OF PHYSICS
(2023)
Article
Astronomy & Astrophysics
Thomas Thiemann, Madhavan Varadarajan
Summary: In this paper, a rigorous implementation of Quantum Spin Dynamics (QSD) in the context of Loop Quantum Gravity (LQG) is derived. The U(1)(3) model for Euclidean vacuum LQG is considered, and the fine details of the algorithm are worked out. It is shown that there are infinitely many solutions, which are generically not normalizable and exhibit propagation.
Review
Astronomy & Astrophysics
Maurizio Gasperini
Summary: In this paper, we present a short review of the applications of the Wheeler-De Witt equation to cosmological models based on the low-energy string effective action. We discuss the potential quantum transitions between the phases of pre-big bang and post-big bang evolution in a context characterized by an initial regime of asymptotically flat, low energy, weak coupling evolution. We show that the birth of our Universe can be represented as a quantum process of tunneling or anti-tunneling from an initial state asymptotically approaching the string perturbative vacuum.
Review
Physics, Multidisciplinary
M. Basil Altaie, Daniel Hodgson, Almut Beige
Summary: This review presents the problem of time in quantum physics, discusses the need to consider time as an observable, and elaborates on recent suggestions to resolve the issue. It also discusses the attitude of covariant loop quantum gravity, which ignores time. The review aims to provide a conceptual foundation for reforming the unification of relativity theory and quantum physics.
FRONTIERS IN PHYSICS
(2022)
Article
Physics, Multidisciplinary
Salman Sajad Wani, James Q. Quach, Mir Faizal
Summary: In this paper, by using Fisher information, the problem of time is addressed. It is demonstrated that the Hamiltonian constraint operator cannot observe the change in time due to the absence of time FI. An alternative operator on the world-sheet, which contains time FI, is identified, suggesting that the timelessness of quantum gravity arises from the use of an inappropriate operator. The proposed criterion for a physically meaningful operator in quantum gravitational processes is that it should contain non-vanishing time FI. Although these results are explicitly derived for the world-sheet of bosonic strings, it is argued that they hold for any quantum geometry associated with time-reparametrization-invariant classical geometry.
Review
Astronomy & Astrophysics
Teodor Borislavov Vasilev, Mariam Bouhmadi-Lopez, Prado Martin-Moruno
Summary: This review discusses the cosmological doomsdays predicted by some phantom dark-energy models, namely the big rip, the little rip, and the little sibling of the big rip, in the framework of metric f(R) theories of gravity. It is suggested that quantum gravity effects may potentially smooth or avoid these classically predicted singularities.
Article
Astronomy & Astrophysics
Xiangdong Zhang, Gaoping Long, Yongge Ma
Summary: This study examines a one-parameter regularization freedom of the Hamiltonian constraint for loop quantum gravity, analyzing the physical phenomena involved. It is found that for a positive value of the regularization parameter, an asymptotic de Sitter branch connected to the standard Friedmann branch can be obtained.
