arXiv subjects

Wen-Cong Gan

Publications and source records attributed to Wen-Cong Gan.

At least 19 recordsLinked to original sources

arXiv2026

Critical Scalarization for a Self-Gravitating Bosonic Condensate

We establish a minimal nonrelativistic realization of scalarization in a self-gravitating bosonic condensate coupled to a scalar response field. A local effective-mass shift and nonlinear saturation generate a multibranch structure with two distinct transition routes. In the linearly stable regime, a finite perturbation drives a first-order transition with type-I logarithmic scaling near threshold. Beyond the linear onset, small perturbations grow tachyonically, and the response time controls both the growth and the subsequent breathing dynamics. These results identify a common mechanism for scalarization across relativistic compact objects and nonrelativistic condensates, and point toward laboratory analogues in coherent media.

gr-qc↗

arXiv2026

Quantum Damping of Cosmological Shear: A New Prediction from Loop Quantum Cosmologies

We study the dynamics of the Bianchi I universe in modified loop quantum cosmology (mLQC-I) and uncover a robust mechanism for isotropization: the shear is dynamically suppressed after the bounce and decays rapidly in the quantum post-bounce regime, independently of the equation of state of standard matter sources. This naturally drives the Universe toward a homogeneous and isotropic expanding phase without fine-tuning. Our results show that mLQC-I provides a new quantum-gravitational mechanism for suppressing anisotropies, absent in other bounce models.

gr-qc↗

arXiv2026

Genericness of quantum damping of cosmological shear in modified loop quantum cosmology

In arXiv:2603.18175, the authors argue, based on numerical studies of particular cases, that the quantum damping of cosmological shear in a modified loop quantum cosmological model (mLQC-I) that was recently found in arXiv:2510.14021 is not generic and that the universe never becomes truly classical. In this brief Note, we revisit these claims by carefully examining the underlying assumptions and the class of initial conditions considered. We show that the examples analyzed in arXiv:2603.18175 correspond to configurations that do not represent physically admissible collapsing Bianchi I universes, as they involve mixed expanding-contracting directions and lead to effectively lower-dimensional post-bounce geometries. Restricting to physically relevant initial conditions corresponding to genuine three-dimensional contraction, we find that the quantum damping of cosmological shear is a robust dynamical feature. This conclusion is supported by both numerical and perturbative analyses, which demonstrate that the post-bounce evolution admits an isotropic attractor, with anisotropies decaying exponentially and independently of the matter content, provided that the weak energy condition is satisfied. We further outline a plausible post-bounce mechanism for the onset of classicalization.

gr-qc↗

arXiv2026

On Coordinate Singularities Induced by Trapping Horizons

The trapping (or apparent) horizon serves as a key tool for tracing the complete evolution of black holes. We investigate a class of coordinate singularities induced by such trapping (or apparent) horizons in a spherically symmetric, dynamic spacetime, which are distinct from the well-known coordinate singularities associated with the Killing horizon. In particular, we clarify the geometric structure of this coordinate singularity by means of the Kodama vector field, thereby avoiding unphysical artifacts. We further employ the evolving Ellis drainhole as an analytical model to illustrate key details of this phenomenon.

gr-qc↗

arXiv2024

A new quantization scheme of black holes in effective loop quantum gravity

Loop quantum cosmology has achieved great successes, in which the polymerization plays a crucial role. In particular, the phase-space-variable dependent polymerization turns out to be the unique one that leads to consistent quantization of the homogeneous and isotropic universe. However, when applying the same scheme to the quantization of black holes, it meets resistances, when the Kantowski-Sachs (KS) gauge is adopted. In this paper, we continue to study the quantum effects of the polymerization near the location that a classical black hole horizon used to be, from the point of view of effective loop quantum gravity in the KS gauge. In particular, we find a phase-space-variable dependent polymerization scheme that leads to negligible quantum effects near the location of the classical black hole horizon, but significantly alters the spacetime structure near the origin, so that the classical singularity is finally replaced by a finite and regular transition surface. The final geodesically-complete spacetime consists of the regular transition surface that connects a black hole in one side and an anti-trapped region in the other side. In the anti-trapped region, no white hole horizons are found and the spacetime is extended to infinity, at which the geometric radius of the two-spheres becomes infinitely large.

gr-qc↗

arXiv2024

Nonexistence of quantum black and white hole horizons in an improved dynamic approach

In this paper, we study the quantum geometric effects near the locations where classical black hole horizons used to appear in Einstein's classical theory, within the framework of an improved dynamic approach, in which the internal region of a black hole is modeled by the Kantowski-Sachs (KS) spacetime and the two polymerization parameters are functions of the phase space variables. Our detailed analysis shows that the effects are so strong that black and white hole horizons of the effective quantum theory do not exist at all and instead are replaced by transition surfaces, across which the metric coefficients and their inverses are smooth and remain finite, as are the corresponding curvatures, including the Kretschmann scalar. These surfaces always separate trapped regions from anti-trapped regions. The number of such surfaces is infinite, so the corresponding KS spacetimes become geodesically complete, and no black and white hole-like structures exist in this scheme.

gr-qc↗

arXiv2024

Entanglement island and Page curve for one-sided charged black hole

In this paper, we extend the method of calculating the entanglement entropy of Hawking radiation of black holes using the "in" vacuum state, which describes one-sided asymptotically flat neutral black hole formed by gravitational collapse, to dynamic charged black holes. We explore the influence of charge on the position of the boundary of island $\partial I$ and the Page time. Due to their distinct geometric structures, we discuss non-extremal and extremal charged black holes separately. In non-extremal cases, the emergence of island saves the bound of entropy at late times, and the entanglement entropy of Hawking radiation satisfies the Page curve. Moreover, we also find that the position of the boundary of island $\partial I$ depends on the position of the cutoff surface (observers), differing from the behavior in eternal charged black holes. In extremal black holes, when the island exists, the entanglement entropy is approximately equal to the Bekenstein-Hawking entropy, while the entanglement entropy becomes ill-defined when island is absent. Our analysis underscores how different geometric configurations significantly influence the behavior of entropy.

gr-qc↗

arXiv2024

On the improved dynamics approach in loop quantum black holes

In this paper, we consider the Böhmer-Vandersloot (BV) model of loop quantum black holes obtained from the improved dynamics approach. We adopt the Saini-Singh gauge, in which it was found analytically that the BV spacetime is geodesically complete. We show that black/white hole horizons do not exist in this geodesically complete spacetime. Instead, there exists only an infinite number of transition surfaces, which always separate trapped regions from anti-trapped ones. Comments on the improved dynamics approach adopted in other models of loop quantum black holes are also given.

gr-qc↗

arXiv2023

A Note on Entanglement Entropy for Primary Fermion Fields in JT Gravity

In this paper we analyse and discuss 2D Jackiw-Teitelboim (JT) gravity coupled to primary fermion fields in asymptotically anti-de Sitter (AdS) spacetimes. We get a particular solution of the massless Dirac field outside the extremal black hole horizon and find the solution for the dilaton in JT gravity. Two dimensional JT gravity spacetime is conformally flat, we calculate the two point correlators of primary fermion fields under the Weyl transformations. The key point of this work is to present a standard technique which is called resolvent rather than CFT methods. We redefine the fields in terms of the conformal factor as the fermion fields, and we use the resolvent technique to derive the renormalized entanglement entropy for massless Dirac fields in JT gravity.

hep-th↗

arXiv2023

Page curves and Entanglement Islands for the Step-Function Vaidya Model of Evaporating Black Holes

It was proposed recently that the fine-grained entropy of the Hawking radiation can be expressed by the semiclassical island formula, which reproduces the unitary Page curve. In this paper, we choose the ``in'' vacuum state and apply the quantum extremal surface construction to study the Page curve for the step-function Vaidya model of evaporating black holes in four dimensions, which is produced by the spherical null shells. Metrics of the three regions of this spacetimes are obtained. In addition, the entanglement islands for the step-function Vaidya model of evaporating black holes at very late times are studied. When cutoff surface $A$ is located in Minkowski region III with $u_A < u_H$ at very late times, we find that the location of the boundary of island $\partial I$ depends on the value of $8M-v_A+v_I$. Specifically, $\partial I$ is inside, at or outside the horizon when $8M-v_A+v_I$ is less than, equal to or larger than zero respectively. Moreover, when cutoff surface $A$ is located in Minkowski region III with $u_A > u_H$ after the black hole evaporates completely, we find that entanglement island still exists and $\partial I$ is located on an equal-time Cauchy surface of the observer $A$ when $r_{(A)}^2\geq64G_Nκc $.

hep-th↗

arXiv2022

Understanding quantum black holes from quantum reduced loop gravity

We systematically study the top-down model of loop quantum black holes (LQBHs), recently derived by Alesci, Bahrami and Pranzetti (ABP). To understand the structure of the model, we first derive several well-known LQBH solutions by taking proper limits. These include the Böhmer-Vandersloot and Ashtekar-Olmedo-Singh models, which were all obtained by the so-called bottom-up polymerizations within the framework of the minisuperspace quantizations. Then, we study the ABP model, and find that the inverse volume corrections become important only when the radius of the two-sphere is of the Planck size. For macroscopic black holes, the minimal radius obtained at the transition surface is always much larger than the Planck scale, and hence these corrections are always sub-leading. The transition surface divides the whole spacetime into two regions, and in one of them the spacetime is asymptotically Schwarzschild-like, while in the other region, the asymptotical behavior sensitively depends on the ratio of two spin numbers involved in the model, and can be divided into three different classes. In one class, the spacetime in the 2-planes orthogonal to the two spheres is asymptotically flat, and in the second one it is not even conformally flat, while in the third one it can be asymptotically conformally flat by properly choosing the free parameters of the model. In the latter, it is asymptotically de Sitter. However, in any of these three classes, sharply in contrast to the models obtained by the bottom-up approach, the spacetime is already geodesically complete, and no additional extensions are needed in both sides of the transition surface. In particular, identical multiple black hole and white hole structures do not exist.

gr-qc↗

arXiv2022

Unitary Constraints on Semiclassical Schwarzschild Black Holes in the Presence of Island

We reconsider $D\geq4$ dimensional asymptotically flat eternal Schwarzschild black hole, and focus on the situation where the inner boundary of the radiation region is chosen to be near the horizon (i.e. $β\ll1$). The tension between the near-horizon condition and the short-distance approximation emerges in large dimensions in $[JHEP 06 (2020) 085]$. We remove this tension by introducing a more proper near horizon condition, thus the resulting island solution is well-behaved in any $D\geq4$ dimensional spacetime. Interestingly, a novel constraint is obtained in this situation as required by the existence of the island solution, which directly leads to the constraints on the size of the Schwarzschild black hole, the position of the inner boundary for the radiation region, or the value of $c\cdot\tilde{G}_{N}$ in any $D\geq4$ dimension. When considering the large $D$ limit, the constraint on the size of the Schwarzschild black hole obtained in this situation is in agreement with the result given in $[Phys.Rev.D 102 (2020) 2, 026016]$. We interpret these as the unitary constraints implied by the presence of island in semiclassical gravity.

hep-th↗

arXiv2022

Island and Page curve for one-sided asymptotically flat black hole

Great breakthrough in solving black hole information paradox took place when semiclassical island rule for entanglement entropy of Hawking radiation was proposed in recent years. Up to now, most papers which discussed island rule of asymptotic flat black hole with $D \ge 4$ focus on eternal black hole. In this paper, we take one more step further by discussing island of "in" vacuum state which describes one-sided asymptotically flat black hole formed by gravitational collapse in $D \ge 4$. We find that island $I$ emerges at late time and saves entropy bound. And boundary of island $\partial I$ depends on the position of cutoff surface. When cutoff surface is far from horizon, $\partial I$ is inside and near horizon. When cutoff surface is set to be near horizon, $\partial I$ is outside and near horizon. This is different from the case of eternal black hole in which $\partial I$ is always outside horizon no matter cutoff surface is far from or near horizon. We will see that different states will manifestly affect $S_{\text{ent}}$ in island formula when cutoff surface is far from horizon and thus have different result for Page time.

hep-th↗

arXiv2022

Properties of the spherically symmetric polymer black holes

In this paper we systematically study a five-parameters class of spherically symmetric polymer black/white hole solutions, and find that only three independent combinations are physical and uniquely determine the spacetime properties. After exploring the whole 3-dimensional (3D) phase space, we find that the model has very rich physics, and depending on the choice of these parameters, various possibilities exist, including: (i) spacetimes that have the standard black/white hole structures, that is, spacetimes that are free of spacetime curvature singularities and possess two asymptotically flat regions, which are connected by a transition surface (throat) with a finite and non-zero geometric radius. The black/white hole masses measured by observers in the two asymptotically flat regions are all positive, and the surface gravity of the black (white) hole is positive (negative). (ii) Spacetimes that have wormhole-like structures, in which the two masses are all positive, but no horizons exist.(iii) Spacetimes that still possess curvature singularities, which can be either hidden inside trapped regions or naked. However, such spacetimes correspond to only some limit cases, and the necessary (but not sufficient) condition is that at least one of the two "polymerization" parameters vanishes. In addition, even for solar mass black/white holes, quantum gravitational effects can be still very large at the black/white hole horizons, again depending on the choice of the parameters.

gr-qc↗

arXiv2020

Information loss paradox revisited: farewell firewall?

Unitary evolution makes pure state on one Cauchy surface evolve to pure state on another Cauchy surface. Outgoing Hawking radiation is only subsystem on the late Cauchy surface. The requirement that Hawking radiation to be pure amounts to require purity of subsystem when total system is pure. We will see this requirement will lead to firewall even in \textit{flat} spacetime, and thus is invalid. Information is either stored in the entanglement between field modes inside black hole and the outgoing modes or stored in correlation between geometry and Hawking radiation when singularity is resolved by quantum gravity effects. We will give a simple argument that even in semi-classical regime, information is (at least partly) stored in correlation between geometry and Hawking radiation.

gr-qc↗

arXiv2018

Characterizing quantum phase transition by teleportation

In this paper we provide a novel way to explore the relation between quantum teleportation and quantum phase transition. We construct a quantum channel with a mixed state which is made from one dimensional quantum Ising chain with infinite length, and then consider the teleportation with the use of entangled Werner states as input qubits. The fidelity as a figure of merit to measure how well the quantum state is transferred is studied numerically. Remarkably we find the first-order derivative of the fidelity with respect to the parameter in quantum Ising chain exhibits a logarithmic divergence at the quantum critical point. The implications of this phenomenon and possible applications are also briefly discussed.

quant-ph↗

arXiv2018

Quantum information metric of conical defect

A concept of measuring the quantum distance between two different quantum states which is called quantum information metric is presented. The holographic principle (AdS/CFT) suggests that the quantum information metric $G_{λλ}$ between perturbed state and unperturbed state in field theory has a dual description in the classical gravity. In this work we calculate the quantum information metric of a theory which is dual to a conical defect geometry and we show that it is $n$ times the one of its covering space. We also give a holographic check for our result in the gravity side. Meanwhile, it was argued that $G_{λλ}$ is dual to a codimension-one surface in spacetime and satisfies $G_{λλ}=n_{d}\cdot\mbox{Vol}(Σ_{max})/L^{d}$. We show that the coefficient $n_d$ for conical defect should be rescaled by $n^2$ from the one for AdS. A limit case of conical defect --- the massless BTZ black hole--- is also considered. We show that the quantum information metric of the massless BTZ black hole disagrees with the one obtained by taking the vanishing temperature limit in BTZ black hole. This provides a new arena in differiating the different phases between BTZ spacetime and its massless cousin.

hep-th↗

arXiv2017

Holevo Bound of Entropic Uncertainty in Schwarzschild Spacetime

For a pair of incompatible quantum measurements, the total uncertainty can be bounded by a state-independent constant. However, such a bound can be violated if the quantum system is entangled with another quantum system (called memory); the quantum correlation between the systems can reduce the measurement uncertainty. On the other hand, in a curved spacetime, the presence of the Hawking radiation can increase the uncertainty in quantum measurement. The interplay of quantum correlation in the curved spacetime has become an interesting arena for studying quantum uncertainty relations. Here we demonstrate that the bounds of the entropic uncertainty relations, in the presence of memory, can be formulated in terms of the Holevo quantity, which limits how much information can be encoded in a quantum system. Specifically, we considered two examples with Dirac fields, near the event horizon of a Schwarzschild black hole, the Holevo bound provides a better bound than the previous bound based on the mutual information. Furthermore, if the memory moves away from the black hole, the difference between the total uncertainty and the Holevo bound remains a constant, not depending on any property of the black hole.

quant-ph↗