arXiv subjects

Zhong-Xi Yu

Publications and source records attributed to Zhong-Xi Yu.

8 recordsLinked to original sources

arXiv2026

Solar-System Bounds on Ricci-flat Spindle Deformations of Schwarzschild

Recently, a new class of deformed black-hole exact solutions was constructed in four-dimensional general relativity. The deformation is controlled by a parameter $B$, which survives after demagnetizing a black hole immersed in an external Bertotti-Robinson magnetic field and changes the global structure of the spacetime into a non-asymptotically flat spindle geometry. Although no astrophysical mechanism for generating such a deformation is currently known, it is natural to ask phenomenologically how large such a geometric deformation could be if it extended over the weak-field solar exterior. Using two classical Solar-System tests, we derive the leading corrections to planetary perihelion precession and to the light travel time in a Shapiro-type configuration. Requiring the $B$-induced perihelion advance to be smaller than the observational uncertainties in the supplementary perihelion precessions of planets gives the strongest bounds, $ |B|\lesssim 10^{-24}\text{--}10^{-23}\ {\rm cm}^{-1}$, while a Cassini time delay estimate gives a complementary null-geodesic sensitivity at the level $ |B|\lesssim 10^{-21}\ {\rm cm}^{-1}$. These results show that any such spindle deformation, if extended to the solar exterior geometry, must be extremely suppressed on Solar-System scales.

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arXiv2026

Black holes and neutron stars in massive Hellings-Nordtvedt theory

Hellings-Nordtvedt theory is a vector-tensor theory in which a vector field $A_\mu$ is nonminimally coupled to curvature through two independent interactions $A^2{\cal R}$ and $A^\mu A^\nu{\cal R}_{\mu\nu}$. When supplemented by a potential whose zero-energy minimum occurs at nonzero $A^2$, the restricted $A^\mu A^\nu{\cal R}_{\mu\nu}$ sector is known to admit black-hole and neutron-star solutions with a monopole-like asymptotic vacuum structure. We examine whether this structure is a generic consequence of the nonzero vector vacuum or instead relies on the special Ricci-tensor coupling. By analyzing the field equations near spatial infinity, we show that the asymptotic vacuum condition is incompatible with generic nonzero values of both couplings and instead selects two allowed single-coupling sectors. The $A^\mu A^\nu{\cal R}_{\mu\nu}$ sector reproduces the known monopole-like asymptotics, whereas the $A^2{\cal R}$ sector admits an asymptotically flat Schwarzschild metric with a nontrivial radial vector field. We further compute the Noether mass in the $A^2{\cal R}$ sector, derive the corresponding Solar-System constraints, and construct neutron-star configurations. Although the weak-field deviation is constrained to be small, neutron stars can still show appreciable departures from both general relativity and the Ricci-tensor-coupling sector in their masses, radii, and moments of inertia. Our results identify that the $A^2{\cal R}$ sector of massive Hellings-Nordtvedt theory as a viable and useful framework for studying strong-field compact objects with a nonzero vector vacuum while remaining compatible with weak-field tests.

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arXiv2025

Revisiting black holes and their thermodynamics in Einstein-Kalb-Ramond gravity

Einstein-Kalb-Ramond (EKR) gravity is an alternative theory in which a rank-two antisymmetric tensor field, the Kalb-Ramond field, is nonminimally coupled to gravity, potentially generating Lorentz-violating backgrounds. In this work, we revisit black hole solutions and thermodynamics in EKR gravity, addressing subtleties overlooked in previous studies. We obtain two distinct classes of exact static black hole solutions with general topological horizons in diverse dimensions, both with and without a cosmological constant, corresponding to different coupling sectors dictated by the field equations. We analyze their thermodynamic properties and, using the Wald formalism, compute the Noether mass and entropy, establishing the first law and clarifying the role of the Noether mass. Finally, we discuss the implications of this definition of mass for observational constraints in EKR gravity.

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arXiv2025

Radial oscillations of neutron stars in Starobinsky gravity and its Gauss-Bonnet extension

Starobinsky gravity, as one of the simplest and best-behaved higher-curvature gravity theories, has been extensively studied in the context of neutron stars over the past few decades. In this work, we investigate the adiabatic radial oscillation stability of neutron stars within the framework of Starobinsky gravity. We find that gravitational modifications can significantly impact stellar stability. Specifically, the higher-derivative nature of the theory causes the exterior spacetime to dynamically respond to fluid oscillations, in contrast to general relativity where Birkhoff's theorem ensures a static exterior. For stellar models with low central densities, the fundamental frequency becomes nearly independent of the central density when the coupling constant is large. For stellar models with high central densities, the transition from stability to instability still approximately occurs near the maximum-mass configuration, similar to the case in general relativity. Our main analysis is conducted in the Jordan frame of the scalar-tensor gravity equivalent to Starobinsky gravity, and we explicitly verify consistency with results obtained in the Einstein frame. We further extend our study to a class of Gauss-Bonnet extensions of Starobinsky gravity.

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arXiv2021

Inverse Chameleon Mechanism and Mass Limits for Compact Stars

As is well known, there are various mass limits for compact stars. For example, the maximum mass for non-rotating white dwarfs is given by the famous Chandrasekhar limit about $1.4 M_\odot$ (solar masses). Although the mass limit for neutron stars is not so clear to date, one of the widely accepted values is about $2.1 M_\odot\,$. Recently, challenges to these mass limits appeared. Motivated by the super-Chandrasekhar mass white dwarfs with masses up to $2.4 \sim 2.8 M_\odot\,$, and compact objects (probably neutron stars) in the mass gap (from $2.5 M_\odot$ or $3 M_\odot$ to $5 M_\odot$) inferred from gravitational waves detected by LIGO/Virgo in the third observing run (O3), we reconsider the mass limits for compact stars in the present work. Without invoking strong magnetic field and/or exotic equation of state (EOS), we try to increase the mass limits for compact stars in modified gravity theory. In this work, we propose an inverse chameleon mechanism, and show that the fifth-force mediated by the scalar field can evade the severe tests on earth, in solar system and universe, but manifest itself in compact stars such as white dwarfs and neutron stars. The mass limits for compact stars in the inverse chameleon mechanism can be easily increased to $3 M_\odot\,$, $5 M_\odot$ or even larger. We argue that the inverse chameleon mechanism might be constrained by the observations of exoplanets orbiting compact stars (such as white dwarfs and neutron stars), and gravitational waves from the last stage of binary compact star coalescence.

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arXiv2020

Lemaitre-Tolman-Bondi Static Universe in Rastall-like Gravity

In this work, we try to obtain a stable Lemaitre-Tolman-Bondi (LTB) static universe, which is spherically symmetric and radially inhomogeneous. However, this is not an easy task, and fails in general relativity (GR) and various modified gravity theories, because the corresponding LTB static universes must reduce to the Friedmann-Robertson-Walker (FRW) static universes. We find a way out in a new type of modified gravity theory, in which the conservation of energy and momentum is broken. In this work, we have proposed a novel modification to the original Rastall gravity. In some sense, our Rastall-like gravity is essentially different from GR and the original Rastall gravity. In this Rastall-like gravity, LTB static solutions have been found. The stability of LTB static universe against both the homogeneous and the inhomogeneous scalar perturbations is also discussed in details. We show that a LTB static universe can be stable in this Rastall-like gravity.

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arXiv2020

Neutron Star as a Mirror for Gravitational Waves

Gravitational wave (GW) has become one of the most active fields in physics and astronomy since the first direct detection of GW event in 2015. As is well known, multiple images of GW events are possible through the gravitational lenses. Here, we propose a novel mirror imaging mechanism for GW events different from the gravitational lens. In the literature, the superconductor was predicted to be highly reflective mirror for GWs. It is well known that neutron stars exhibit superconductivity and superfluidity. In this work, we predict that there are two types of GW mirror imaging phenomena caused by the neutron star located in Milky Way or the same host galaxy of GW source, which might be detected within a life period of man (namely the time delay $Δt$ can be a few years to a few tens of years). It is expected to witness this predicted GW mirror imaging phenomenon in the near future. In the long term, the observations of this novel GW mirror imaging phenomenon might help us to find numerous neutron stars unseen by other means, and learn more about the complicated internal structures of neutron stars, as well as their equations of state.

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arXiv2017

Catalyst type of interactions between dark energy and dark matter

In this paper, we focus on three specific interactions of dark sector in the existence of baryonic matter and radiation. First, we attempt to assume baryonic matter and radiation can affect the conversion between dark energy and dark matter like the way catalyst influences the conversion rate of two materials in some reversible chemical reactions. Then we present phase space analysis for every special interaction model. Finally, for every case, we obtain a stable attractor solution that can alleviate the coincidence problem.

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