arXiv subjects

Faqiang Yuan

Publications and source records attributed to Faqiang Yuan.

7 recordsLinked to original sources

arXiv2026

Loop Quantum Kaluza-Klein Cosmology and Inflation

We present the detailed analyses of five-dimensional loop quantum Kaluza-Klein cosmology based on the symmetric reduction of the connection formulation of the full theory. The previous results in a particular scenario are extended to more general cases. The effective scalar constraint for the geometric sector of the model is derived by the systematic semi-classical analysis in both the canonical and path-integral formulations, incorporating the quantum fluctuations as a subleading-order correction. The resulting effective scalar constraint not only exhibits the correct classical limit of the quantum system, but also serves as the basis for investigating the following three distinct effective scenarios through the incorporation of matter contributions: (i) vacuum, (ii) minimally coupling with a scalar field, and (iii) coupling with the dust. In all the three effective scenarios, the big bang and potential past big rip singularities in the classical model are naturally resolved by including the leading-order quantum correction of holonomies. Moreover, the visible universe undergoes a super-inflationary phase after overcoming the classical big bang singularity, during which the phenomenologically desired 55 e-folds can be achieved by appropriate initial conditions. In the case where the subleading-order quantum fluctuation term is included as a constant, the evolutions of the five-dimensional universe in all the three effective scenarios not only achieve sufficient inflation in the visible dimensions, but also exhibit re-collapse behaviors at certain large scales. Hence the cosmic inflation may originate from the interplay between compact extra dimensions and quantum geometric effects.

gr-qc↗

arXiv2026

Effective dynamics of Janis-Newman-Winicour spacetime

The effective dynamics of the Janis-Newman-Winicour spacetime inspired by loop quantum gravity is studied. Two different schemes are considered to regularize the Hamiltonian constraint for the quantum dynamics. In the $μ_0$ scheme in which the quantum parameters are treated as constants, the equations of motion generated by the effective Hamiltonian are solved analytically. The resulting quantum-corrected effective spacetime obviously extends the effective spacetime previously obtained in the literature. In the new effective spacetime, the naked singularity and the central singularity presented in the classical JNW spacetime are resolved by a series of quantum bounces. In the scheme of choosing the quantum parameters as Dirac observables, the effective dynamics is also solved in the light of the solution in $μ_0$ scheme. It turns out that the resulting effective spacetime has singularities due to the appearance of the zero points of the time reparametrization functions. Hence, the effective theory in this scheme does not remain valid throughout the full spacetime.

gr-qc↗

arXiv2026

Deparametrization and quantization of scalar-tensor gravity and its cosmological model

The degree of freedom of the scalar field in scalar-tensor gravity is employed as "time" to deparametrize the Hamiltonian constraint of the theory. The deparametrized system is then nonperturbatively quantized by the approach of loop quantum gravity. This results in a discrete time evolution of the physical states with respect to the gravitational degree of freedom in the quantum theory. In the corresponding Brans-Dicke cosmological model, the physical solutions to the quantum Hamiltonian constraint are obtained in the light of the deparametrization. The quantum dynamics indicates that the classical big bang singularity is replaced by a quantum bounce.

gr-qc↗

arXiv2026

Effective Dynamics of Loop Quantum Kaluza-Klein Cosmology

The five-dimensional loop quantum Kaluza-Klein cosmology is constructed based on the symmetric reduction of the connection formulation of the full theory. Through semiclassical analysis, the effective scalar constraint for the cosmological model coupled with a dust field is derived, incorporating the quantum fluctuations of geometry as a subleading order correction. It demonstrates that the quantum model has the correct classical limit. The explicit solutions to the equations of motion show that the big bang and past big rip singularities in the classical model are avoided by a quantum bounce and a quantum collapse respectively in the effective model. In a particular scenario, the dynamical compactification of the extra dimension is realized, while the observable four-dimensional universe transitions through three distinct epochs: (i) a super-inflationary phase generating 55 e-folds, (ii) a decelerated expansion era, and (iii) a late-time accelerated expansion phase driven by quantum fluctuations. These results suggest that both cosmic inflation and dark energy may originate from the interplay between the compact extra dimension and quantum geometric effects.

gr-qc↗

arXiv2025

Quantum Effective Dynamics of Papapetrou Spacetime

In this paper, we investigate the quantum effective dynamics of Papapetrou spacetime by using methods from loop quantum gravity. The Papapetrou spacetime is an extension of the Schwarzschild spacetime with an extra coupled anti-scalar field. By solving the equations of motion generated by the Hamiltonian constraint for Papapetrou spacetime, we can construct the quantum effective metric. The resulting effective metric for the quantum-corrected Papapetrou spacetime demonstrates the quantum effects will give rise to a new wormhole throat, while the classical wormhole throat disappear in the case of extremely small mass.

gr-qc↗

arXiv2023

Energy extraction via Comisso-Asenjo mechanism from rotating hairy black hole

It was demonstrated by Comisso and Asenjo that the magnetic reconnection in the ergosphere is a promising mechanism to extract energy from the rotating Kerr black hole. In this work, we investigate the role of Comisso-Asenjo mechanism in energy extraction from the newly suggested rotating hairy black holes which have an extra hair due to the additional surrounding sources, such as dark matter or dark energy. We examine how the hairy parameters characterized the hair affect the magnetic reconnection process in addition to other important variables of the Comisso-Asenjo process, including the parameter spaces that permit energy extraction, the power, efficiency and power ratios with respect to the Blandford-Znajek mechanism.

gr-qc↗

arXiv2023

Energy extraction from rotating regular black hole via Comisso-Asenjo mechanism

Recently, it has been demonstrated by Comisso and Asenjo that magnetic reconnection processes in the ergosphere of a Kerr black hole can provide us with a promising mechanism for extracting the rotational energy from it. In this paper, we study the energy extraction from the the newly proposed rotating regular black holes via this Comisso-Asenjo mechanism. This novel rotating regular black hole has an exponential convergence factor $e^{-k/r}$ on the mass term characterized by the regular parameter $k$ in the exponent. We explore the effects of this regular parameter on the magnetic reconnection as well as other critical parameters determining the Comisso-Asenjo process. The parameter spaces allowing energy extraction to occur are investigated. The power, efficiency and the power ratio to the Blandford-Znajek mechanism are studied. The results show that the regularity of the rotating black hole has significant effects on the energy extraction via the Comisso-Asenjo mechanism.

gr-qc↗