arXiv subjects

Yen Chin Ong

Publications and source records attributed to Yen Chin Ong.

At least 19 recordsLinked to original sources

arXiv2026

How Much Geometry Does Horizon Entropy Determine? A Discourse on Entropy, Gravity, and Observable Quantities

Building on Jacobson's thermodynamics derivation of Einstein field equations, the "generalized entropy and varying-G" (GEVAG) framework was proposed to consistently incorporate generalized horizon entropy $S=f(A)/4G$ into both the thermodynamics and the geometry of spacetime. This leads to an effective gravitational coupling $G_\text{eff}=G/f'(A)$. A related "entropy-gravity correspondence" (EGC) instead is equivalent to promoting this horizon relation to a radially varying bulk coupling $G_\text{eff}(r)$. We clarify the similarities and fundamental distinctions between these approaches. Although they share the same horizon consistency relation, their global causal structures and asymptotic geometries can differ substantially. In addition, the radial prescription of EGC introduces an additional ambiguity for multi-center configurations. We further examine a near-horizon extension of GEVAG, which restores the equivalence between thermodynamic and ADM masses that is absent in the horizon-only formulation. We then discuss the implications of these different prescriptions for the Bekenstein bound. The Bekenstein bound provides a surprising link between $G_\text{eff}$ in near horizon GEVAG, EGC, and the one obtained in the recent $f(R)$-reconstruction approach of generalized entropy geometry via Wald entropy. Finally, we raise the most important question: what exactly is the $G$ measured in laboratories?

gr-qc↗

arXiv2026

One Gravity, Many G's: Generalized Horizon Entropy and Gravitational Susceptibility

Effective and thermodynamic derivations of gravity theory in the context of generalized entropy can give rise to multiple, and potentially varying, gravitational couplings, raising the question of which quantity should be identified with the physically measured Newton constant. Motivated by this ambiguity, we formulate the notion of gravitational susceptibility of the horizon. In spacetimes containing multiple horizons, if the entropy is a function on a multi-horizon state space, $S=S(A_1,A_2,\cdots,A_n)$, then the derivatives of the entropy with respect to individual horizon areas define the horizon gravitational response coefficients. Different horizons may possess distinct gravitational responses without requiring multiple fundamental gravitational couplings, analogous to direction-dependent response coefficients in macroscopic media. This framework naturally suggests the possibility that horizon entropies (such as the cosmological apparent horizon and a black hole horizon) can influence one another, which could explain the black hole-cosmology coupled "black hole mass growth" previously discussed in the literature.

gr-qc↗

arXiv2026

Frenet-Serret equations with variable proper acceleration in Minkowski spacetime

We study Frenet-Serret equations for timelike worldlines in Minkowski spacetime with proper-time-dependent curvature and torsion. This corresponds to relativistic motion with non-uniform proper acceleration and, when torsion is included, to trajectories whose Frenet-Serret frame rotates beyond the acceleration plane. Using the Gram-Schmidt construction of the tetrad from the four-velocity and its derivatives, we relate the intrinsic Frenet-Serret parameters to kinematic quantities such as proper acceleration, four-jerk, and four-snap. We then consider simple analytic cases for the jerk invariant and torsion, obtaining explicit curvature profiles and reduced Frenet-Serret equations. These examples clarify how non-constant acceleration and torsion modify the geometry of accelerated relativistic motion.

gr-qc↗

arXiv2026

Corrected Hawking Temperature and Final State of Black Hole Evaporation Under GEVAG Framework

In the GEVAG (Generalized Entropy Varying-G) framework, any generalization to horizon entropy leads to a varying gravitational "constant" $G_\text{eff}$ that is a function the horizon area. In this work, it is shown that if we promote $G_\text{eff}$ to be valid in the neighborhood of the horizon, then Hawking temperature consists of two terms, the second of which is related to the variation of $G_\text{eff}$. When applied to the logarithmic correction of the entropy, as is common across various quantum gravity approaches, the first term in the Schwarzschild black hole temperature exactly agrees with that obtained from utilizing generalized uncertainty principle (GUP), while the second term improves on the GUP result by driving the Hawking temperature to zero as the black hole approaches a minimum mass. This resolves the inconsistency in the GUP result concerning a nonzero temperature minimum mass remnant. This work also derives simple general formulas for both the thermodynamic energy and the Bekenstein bound for any correction to the area law under the same assumption that $G_\text{eff}$ can be extended off-shell in the horizon neighborhood. The (generalized) Bekenstein bound can be interpreted as a statement regarding the renormalization group scaling dimension of the entropy functional $f(A)$ and the naturalness of the theory.

gr-qc↗

arXiv2026

Cosmology with Logarithmic Corrected Horizon Entropy According to the Generalized Entropy and Variable-G Correspondence

According to the GEVAG (Generalized Entropy Varying-G) framework, any modification to the Bekenstein-Hawking area law would also lead to a varying-$G$ gravity theory in which the effective gravitational constant $G_\text{eff}$ becomes area-dependent. Among a myriad of generalized entropy functions explored in the literature, of special interest is the logarithmic correction of quantum gravity. In this work, we apply GEVAG to investigate the effect of logarithmic correction on very early-time cosmology, including the conditions for inflation. We found that if the coefficient of the logarithmic correction term is negative, $G_\text{eff}$ becomes twice that of the current value; whereas, a positive coefficient leads to a very small value of $G_\text{eff}$, which may ameliorate the "arrow of time" problem. In fact, slow-roll inflation becomes more natural in the latter case. We make some comparisons with the constant-$G$ approach and reveal the advantages of the GEVAG approach. For example, it can evade the sudden singularity that could otherwise arise when the coefficient of the logarithmic correction term is negative. We also check the validity of the generalized second law and comment on the range of the various parameters.

gr-qc↗

arXiv2025

Measurement-Induced Perturbations of Hausdorff Dimension in Quantum Paths

In a seminal paper, Abbott et al. analyzed the relationship between a particle's trajectory and the resolution of position measurements performed by an observer at fixed time intervals. They predicted that quantum paths exhibit a universal Hausdorff dimension that transitions from $d=2$ to $d=1$ as the momentum of the particle increases. However, although measurements were assumed to occur at intervals of time, the calculations only involved evaluating the expectation value of operators for the free evolution of wave function within a single interval, with no actual physical measurements performed. In this work we investigate how quantum measurements alter the fractal geometry of quantum particle paths. By modelling sequential measurements using Gaussian wave packets for both the particle and the apparatus, we reveal that the dynamics of the measurement change the roughness of the path and shift the emergent Hausdorff dimension towards a lower value in nonselective evolution. For selective evolution, feedback control forces must be introduced to counteract stochastic wave function collapse, stabilising trajectories and enabling dimensionality to be tuned. When the contribution of the measurement approaches zero, our result reduces to that of Abbott et al. Our work can thus be regarded as a more realistic formulation of their approach, and it connects theoretical quantum fractality with measurement physics, quantifying how detectors reshape spacetime statistics at quantum scales.

quant-ph↗

arXiv2025

Kiselev Black Holes in Anti-de Sitter Spacetime Are Not Viable

While the Kiselev black hole solution does not describe a perfect fluid quintessence, it is possible that dark energy fluid is not isotropic, and so one may hope that the solution may still describe at least locally a black hole embedded in such a dark energy fluid environment. Taking into account that the accelerated expansion of the Universe is caused by a dynamical dark energy fluid, and the actual underlying cosmological constant is negative as has been sometimes suggested, we show that the Kiselev solution is untenable as it is unstable against brane nucleation instability.

gr-qc↗

arXiv2025

Insights in $f(Q)$ cosmology: the relevance of the connection

We explore the role of the affine connection in $f(Q)$ gravity, a modified theory where gravity is governed by non-metricity within the symmetric teleparallel framework. Although the connection is constrained to be flat and torsionless, it is not uniquely determined by the metric, allowing for multiple physically distinct formulations. We analyze three such connections compatible with a homogeneous and isotropic universe to show that they yield markedly different cosmological dynamics, even under the same functional form of $f(Q)$. Using both analytical and numerical methods, including a Born-Infeld type model of $f(Q)$, we demonstrate that specific connections can resolve cosmological singularities like the Big Bang and Big Rip, replacing them with smooth de Sitter phases. Others retain singularities but with notable modifications in their behavior. These findings highlight the physical relevance of connection choice in $f(Q)$ gravity and its potential to address fundamental cosmological questions.

gr-qc↗

arXiv2025

Do Black Holes With Generalized Entropy Violate Bekenstein Bound?

In general yes, but also not quite. It is known that if the Bekenstein-Hawking entropy is replaced by some kind of generalized entropy, then the Bekenstein bound may be grossly violated. In this work, we show that this undesired violation can be avoided if we employ the equivalence between generalized entropy and varying-$G$ gravity (GEVAG). In this approach, modifying entropy necessarily also modifies gravity (as one should expect if gravity is indeed inherently tied to thermodynamics), which leads to an effective gravitational "constant" $G_\text{eff}$ that is area-dependent, and a thermodynamic energy that is distinct from the ADM mass. We show that a relaxed Bekenstein bound of the form $S \leqslant CRE$ is always satisfied, albeit the coefficient $C$ is no longer $2π$.

gr-qc↗

arXiv2025

GUP Effective Metric Without GUP: Implications for the Sign of GUP Parameter and Quantum Bounce

The standard form of generalized uncertainty principle (GUP) predicts that the Hawking temperature is modified near the Planck scale and that the Bekenstein-Hawking entropy receives a logarithmic correction, consistent with other approaches to quantum gravity. However, due to the heuristic arguments in most GUP literature, it is not clear how to obtain the Schwarzschild metric that incorporates GUP correction. In this work, we try a different approach. We will start with the entropy expression with the standard logarithmic correction term, and use the recently proposed "generalized entropy and varying-G correspondence" (GEVAG) to obtain the associated metric. We show that the Hawking temperature obtained from this metric matches the GUP version. In this sense, we have derived in a consistent and reliable manner, a metric tensor that can describe the standard GUP physics, and use it to clarify some shortcomings in the heuristic GUP approach itself. In particular, if the strict Bekenstein bound is imposed, then the GUP parameter is negative. We also speculate on the possibility that instead of a stable remnant, the final stage of black hole evaporation could be a "bounce" due to an effective gravitational repulsion, once higher order corrections are included.

gr-qc↗

arXiv2025

Testing Cotton gravity as dark matter substitute with weak lensing

Harada proposed a modified theory of gravity called Cotton gravity, and argued that it successfully explains the rotation curves of $84$ galaxies without the need of dark matter. In this work we use galaxy-galaxy lensing technique to test whether the modification effect of Cotton gravity can indeed be a viable substitute for dark matter. Using the spherically symmetric solution of Cotton gravity, we obtain the deflection angle via Gauss-Bonnet theorem and the weak lensing shear. We use five galaxy catalogs divided in 5 stellar mass bins from the Sloan Digital Sky Survey Data Release 7 (SDSS DR7), each of which is further divided into blue star forming galaxy and red passive galaxy sub-catalogs. We find that Cotton gravity on its own has significant deviation from the measured galaxy-galaxy lensing signals, thus it cannot replace the role of dark matter. If we consider the combination of dark matter and Cotton gravity, the modification is tightly constrained. Our analysis also applies to other modified gravity theories whose an additional linear term appears in the Schwarzschild solution.

astro-ph.CO↗

arXiv2025

The Case For Black Hole Remnants: A Review

It has been almost 40 years since the proposal of the idea that Hawking radiation of black holes does not lead to a complete evaporation but rather a "remnant" state. Though traditionally viewed with great criticisms especially from the high energy physics community, in recent years, various approaches have demonstrated that black hole remnants remain a viable possibility. In this review, which is primarily aimed as an introduction to the subject, we will discuss some possible routes to forming remnants and their respective properties and challenges.

gr-qc↗

arXiv2025

Generalized Entropy Implies Varying-G: Horizon Area Dependent Field Equations and Black Hole-Cosmology Coupling

When the Bekenstein-Hawking entropy is modified, ambiguity often arises concerning whether the Hawking temperature or the thermodynamic mass should be modified. The common practice, however, is to keep the black hole solution the same as that in general relativity. On the other hand, if Jacobson's method of deriving Einstein equations from thermodynamic is valid in the general settings, then given a generalized entropy one should first derive the corresponding modified gravity, and then look for the compatible black hole solution before investigating its thermodynamics. We comment on some properties and subtleties in this approach. In particular, we point out that generically generalized entropy would lead to a varying effective gravitational "constant" theory, where $G_\text{eff}$ depends on the horizon area. We discuss in what ways such theories are discernible from general relativity despite its seemingly jarring differences, and how to make sense of area-dependent field equations. As a consequence we show that in the Jacobson's approach, the standard quantum gravitational logarithmic correction to Bekenstein-Hawking entropy is equivalent to a running gravitational "constant". A horizon area dependent $G_\text{eff}$ could also lead to a coupling between black hole masses and cosmological expansion, a scenario that has been studied recently in the literature, but so far lacks strong theoretical motivation. In the Tsallis case, we show that the thermodynamic mass for a Schwarzschild black hole is just a constant multiple of its ADM mass, which is considerably simpler than the approach not utilizing the Jacobson's method.

gr-qc↗

arXiv2025

Hawking Temperature and the Inverse-Radius Scale of the Horizon

The Hawking temperature of a Schwarzschild black hole can be heuristically derived by identifying the temperature with the inverse radius of the horizon up to a multiplicative constant. This does not work for more general black holes such as the Kerr and Reissner-Nordström solutions. Expounding on the details of how it fails to work nevertheless uncovers interesting connections with the "spring constant" of black holes and with black hole thermodynamics.

gr-qc↗

arXiv2024

Constrain the linear scalar perturbation theory of Cotton gravity

We perform a cosmological test of Cotton gravity, which describes gravity by cotton tensor. We assume in Cotton gravity the background evolution is the same as the flat FLRW background. We derive the cosmological perturbation theory of the scalar mode at the linear level, where the difference from the $Λ$CDM model is characterized by the parameter $β$. We incorporate Cotton gravity with a neutrino model and perform a Monte Carlo Markov Chain (MCMC) analysis using data from the Cosmic Microwave Background (CMB) and Sloan Digital Sky Survey (SDSS). The analysis constrains parameter $β=-0.00008^{+0.00080}_{-0.00104}$ at the 1-$σ$ confidence level. We conclude that currently, there is no obvious deviation between Cotton gravity and the $Λ$CDM model in the linear cosmological perturbation level for observations.

astro-ph.CO↗

arXiv2024

Black Holes, Complex Curves, and Graph Theory: Revising a Conjecture by Kasner

The ratios $\sqrt{8/9}=2\sqrt{2}/3\approx 0.9428$ and $\sqrt{3}/2 \approx 0.866$ appear in various contexts of black hole physics, as values of the charge-to-mass ratio $Q/M$ or the rotation parameter $a/M$ for Reissner-Nordström and Kerr black holes, respectively. In this work, in the Reissner-Nordström case, I relate these ratios with the quantization of the horizon area, or equivalently of the entropy. Furthermore, these ratios are related to a century-old work of Kasner, in which he conjectured that certain sequences arising from complex analysis may have a quantum interpretation. These numbers also appear in the case of Kerr black holes, but the explanation is not as straightforward. The Kasner ratio may also be relevant for understanding the random matrix and random graph approaches to black hole physics, such as fast scrambling of quantum information, via a bound related to Ramanujan graph. Intriguingly, some other pure mathematical problems in complex analysis, notably complex interpolation in the unit disk, appear to share some mathematical expressions with the black hole problem and thus also involve the Kasner ratio.

gr-qc↗

arXiv2024

Classical Acceleration Temperature (CAT) in a Box

A confined, slow-moving, accelerating electron is shown to emit thermal radiation. Since laboratories face spatial constraints when dealing with rectilinear motion, focusing on a finite total travel distance combines the benefits of simple theoretical analysis with prospects for table-top experimentation. We demonstrate an accelerated moving charge along an asymptotically static worldline with fixed transit distance and slow maximum speed, emitting self-consistent analytic power, spectra, and energy. The classical radiation is Planck distributed with an associated acceleration temperature. This is the first fully parametrized, spectrum-solved, finite-distance worldline.

physics.class-ph↗

arXiv2024

Upper Bound of Barrow Entropy Index from Black Hole Fragmentation

Both classical and quantum arguments suggest that if Barrow entropy is correct, its index $δ$ must be energy dependent, which would affect the very early universe. Based on thermodynamic stability that sufficiently large black holes should not fragment, we argue that Barrow entropy correction must be small except possibly at the Planckian regime. Furthermore, the fact that a solar mass black hole does not fragment implies an upper bound $δ\lesssim O(10^{-3})$, which surprisingly lies in the same range as the bound obtained from some cosmological considerations assuming fixed $δ$. This indicates that allowing $δ$ to run does not raise its allowed value. We briefly comment on the case of Kaniadakis entropy.

gr-qc↗