CyberSec Research

This paper presents a new conformal symmetry of stationary, axisymmetric Kerr perturbations. This symmetry is exact but non-geometric (or "hidden"), and each of its generators has an associated infinite family of eigenstate solutions. Tidal perturbations of a black hole form an irreducible highest-weight representation of this conformal group, while the tidal response fields live in a different such representation. This implies that black holes have no tidal deformability, or vanishing Love numbers.

We present a general analysis of the role of shear viscosity in cosmological backgrounds, focusing on isotropic space-time in both Einstein and $f(R)$ gravity. By computing the divergence of the stress-energy tensor in a general class of isotropic (but not necessarily homogeneous) geometries, we show that shear viscosity does not contribute to the background dynamics when the fluid is comoving. This result holds in both the Jordan and Einstein frames, and implies that shear viscosity cannot affect the electromagnetic luminosity distance which is determined by the background light-like geodesics. As an application of our results, we critically examine recent claims that shear viscosity can alter the Hubble evolution and the electromagnetic luminosity distance in Starobinsky gravity. We demonstrate that the continuity equation used in that work is at odds both with the covariant conservation of the stress-energy tensor and the local second law of thermodynamics. We further show that even in models where such modifications could mimic bulk viscosity, the resulting entropy evolution is inconsistent with standard thermodynamic expectations.

Gravitational-wave astronomy has entered a regime where it can extract information about the population properties of the observed binary black holes. The steep increase in the number of detections will offer deeper insights, but it will also significantly raise the computational cost of testing multiple models. To address this challenge, we propose a procedure that first performs a non-parametric (data-driven) reconstruction of the underlying distribution, and then remaps these results onto a posterior for the parameters of a parametric (informed) model. The computational cost is primarily absorbed by the initial non-parametric step, while the remapping procedure is both significantly easier to perform and computationally cheaper. In addition to yielding the posterior distribution of the model parameters, this method also provides a measure of the model's goodness-of-fit, opening for a new quantitative comparison across models.

We present static axially symmetric fluid distributions not producing gravitational field outside their boundaries (i.e. fluid sources which match smoothly on the boundary surface to Minkowski space-time). These solutions provide further examples of ghost stars. A specific model is fully described, and its physical and geometrical properties are analyzed in detail. This includes the multipole moment structure of the source and its complexity factors, both of which vanish for our solution.

We investigate on the diffeomorphism invariance of the effective gravitational action, focusing in particular on the path integral measure. In the literature, two different measures are mainly considered, the Fradkin-Vilkovisky and the Fujikawa one. With the help of detailed calculations, we show that, despite claims to the contrary, the Fradkin-Vilkovisky measure is diffeomorphism invariant, while the Fujikawa measure is not. In particular, we see that, contrary to naive expectations, the presence of $g^{00}$ factors in the Fradkin-Vilkovisky measure is necessary to ensure the invariance of the effective gravitational action. We also comment on results recently appeared in the literature, and show that formal calculations can easily miss delicate points.

In the paper, we study the dynamical behavior of a particle around a charged black hole within the framework of 4D-EGB gravity theory. Based on the spacetime metric of the black hole, we rigorously derived the effective potential of the particle and analyzed the particle's MBO and ISCO. The results show that the key physical quantities of MBO and ISCO-including orbital radius, angular momentum, and energy-all exhibit significant dependence on parameter $\alpha$ and charge $Q$. As these two parameters increase, the energy-angular momentum ($E, L$) region required for particle bound orbits shifts towards lower ($E, L$) areas. Furthermore, in order to assess the validity of the chosen values for parameter $\alpha$ and charge $Q$, a rigorous constraint on these parameters is imposed in this paper, utilizing observational data from the black hole shadows of $M87^{\ast }$ and $Sgr A^{\ast }$ and precession observations of the $S2$ star. Finally, we analyzed the periodic orbits of particle and their generated gravitational waveforms under different parameter values. We found that changes in parameter $\alpha$ and charge $Q$ lead to significant differences in the periodic trajectories of the particle and their corresponding gravitational waveforms.

Black hole (BH) shadow observations and gravitational wave astronomy have become crucial approaches for exploring BH physics and testing gravitational theories in extreme environments. This paper investigates the charged black hole with scalar hair (CBH-SH) derived from the Einstein-Maxwell-conformal coupled scalar (EMCS) theory. We first constrain the parameter space $(Q/M, s/M^2)$ of the BH using the Event Horizon Telescope (EHT) observations of M87* and Sgr A*. The results show that M87* provides stronger constraints on positive scalar hair, constraining the scalar hair $s$ within $0\le s/M^2\le0.4632$ and the charge $Q$ within the range $0\le Q/M\le0.6806$. In contrast, Sgr A* imposes tighter constraints on negative scalar hair. When $Q$ approaches zero, $s$ is constrained within the range $0\geq s/M^2\geq-0.0277$. Overall, EHT observations can provide constraints at most on the order of $\mathcal{O}\left({10}^{-1}\right)$. Subsequently, we construct extreme mass ratio inspiral (EMRI) systems and calculate their gravitational waves to assess the detection capability of the LISA detector for these BHs. The results indicate that for central BHs of $M={10}^6M_\odot$, LISA is expected to detect scalar hair $s/M^2$ at the $\mathcal{O}\left({10}^{-4}\right)$ level and charge $Q/M$ at the $\mathcal{O}\left({10}^{-2}\right)$ level, with detection sensitivity far exceeding the current EHT capabilities. This demonstrates the immense potential of EMRI gravitational wave observations in testing EMCS theory.

This paper develops a synthetic framework for the geometric and analytic study of null (lightlike) hypersurfaces in non-smooth spacetimes. Drawing from optimal transport and recent advances in Lorentzian geometry and causality theory, we define a synthetic null hypersurface as a triple $(H, G, \mathfrak{m})$: $H$ is a closed achronal set in a topological causal space, $G$ is a gauge function encoding affine parametrizations along null generators, and $\mathfrak{m}$ is a Radon measure serving as a synthetic analog of the rigged measure. This generalizes classical differential geometric structures to potentially singular spacetimes. A central object is the synthetic null energy condition ($\mathsf{NC}^e(N)$), defined via the concavity of an entropy power functional along optimal transport, with parametrization given by the gauge $G$. This condition is invariant under changes of gauge and measure within natural equivalence classes. It agrees with the classical Null Energy Condition in the smooth setting and it applies to low-regularity spacetimes. A key property of $\mathsf{NC}^e(N)$ is the stability under convergence of synthetic null hypersurfaces, inspired by measured Gromov--Hausdorff convergence. As a first application, we obtain a synthetic version of Hawking's area theorem. Moreover, we obtain various sharpenings of the celebrated Penrose's singularity theorem: for smooth spacetimes we show that the incomplete null geodesic whose existence is guaranteed by Penrose's argument is actually maximizing; we extend Penrose's singularity theorem to continuous spacetimes; we prove the existence of trapped regions in the general setting of topological causal spaces satisfying the synthetic $\mathsf{NC}^e(N)$.

Gravitational wave (GW) data from observed binary black hole coalescences (BBHC), proven to validate the Hawking Area Theorem (HAT) for black hole horizons, has been demonstrated to unambiguously pick theoretically computed logarithmic corrections to the Bekenstein-Hawking Area Formula, which have a {\it negative} coefficient, when combined with the Generalized Second Law of thermodynamics. We propose a composite, `hybrid' approach to quantum gravity black hole entropy calculation, additively combining results from the non-peturbative, background-independent Loop Quantum Gravity method, with those from the perturbative (one loop), background-dependent semiclassical approach (often called `geometric' entropy) based on Euclidean Quantum Gravity. Our goal is to examine under what conditions, {\it absolute} consistency with HAT-validating GW data analyses is guaranteed. As a consequence of this demand for absolute consistency, nontrivial, albeit indirect, constraints appear to emerge on the Beyond-Standard-Model (BSM) part of the spectrum of perturbative elementary particle fluctuations in a classical black hole background. Some species of the constrained, yet-unobserved BSM particle spectrum are currently under active consideration in particle cosmology as candidates for dark matter.

For locally rotationally symmetric (LRS) spacetimes, we construct two equivalent forms of the Komar current derived from a conformal Killing vector. One is a kinematic construction and the other is in terms of the matter quantities on the spacetime. The required conservation condition for the current is derived and discussed in various instances, and the implications of the conservation of the current, and in the case of a vanishing current, are analyzed. A relationship between the conservation criterion and the presence of trapped surfaces in the spacetime is found and discussed. We also show that for LRS II metrics with constant metric time component, the current is always conserved. In the presence of a conformal Killing horizon, properties of the current are analyzed and restrictions on, and some implications for the physical spacetime variables, in the vicinity of the horizon, are obtained. Finally, with respect to the conformal Killing horizon, the associated Noether charge is shown to be proportional to the surface gravity, establishing the thermodynamic interpretation of the Noether charge.

We compute the scattering angle for a scalar neutral probe undergoing unbound motion around a Topological Star, including self-force effects. Moreover we identify the `electro-magnetic' source of the background as Papapetrou Field compatible with the isometries and characterize Topological Stars by studying their sectional curvature, geometric transport along special curves and the gravitational energy content in terms of the super-energy tensors.

We consider a three-dimensional gravity model that includes (non-linear) Maxwell and Chern-Simons-like terms, allowing for the existence of electrically charged rotating black hole solutions with a static electromagnetic potential. We verify that a Cardy-like formula, based not on central charges but on the mass of the uncharged and non-spinning soliton, obtained via a double Wick rotation of the neutral static black hole solution, accurately reproduces the Bekenstein-Hawking entropy. Furthermore, we show that a slight generalization of this model, incorporating a dilatonic field and extra gauge fields, admits charged and rotating black hole solutions with asymptotic Lifshitz behavior. The entropy of these solutions can likewise be derived using the Cardy-like formula, with the Lifshitz-type soliton serving as the ground state. Based on these results, we propose a generalized Cardy-like formula that successfully reproduces the semiclassical entropy in all the studied cases.

In this work we obtain a numerical self-consistent spherical solution of the semiclassical Einstein equations representing the evaporation of a trapped region which initially has both an outer and an inner horizon. The classical matter source used is a static electromagnetic field, allowing for an approximately Reissner-Nordstr\"om black hole as the initial configuration, where the charge sets the initial scale of the inner horizon. The semiclassical contribution is that of a quantum scalar field in the "in" vacuum state of gravitational collapse, as encoded by the renormalised stress-energy tensor in the spherical Polyakov approximation. We analyse the rate of shrinking of the trapped region, both from Hawking evaporation of the outer apparent horizon, as well as from an outward motion of the inner horizon. We also observe that a long-lived anti-trapped region forms below the inner horizon and slowly expands outward. A black-to-white-hole transition is thus obtained from purely semiclassical dynamics.

We study the linearised Einstein--Maxwell equations on the Reissner--Nordstr\"om spacetime and derive the canonical energy conservation law in double null gauge. In the spirit of the work of Holzegel and the second author, we avoid any use of the hyperbolic nature of the Teukolsky equations and rely solely on the conservation law to establish control of energy fluxes for the gauge-invariant Teukolsky variables, previously identified by the third author, along all outgoing null hypersurfaces, for charge-to-mass ratio $\frac{|Q|}{M} < \frac{\sqrt{15}}{4}$. This yields uniform boundedness for the Teukolsky variables in Reissner--Nordstr\"om.

We study Parity Violating Gravity Theories whose gravitational Lagrangian is a generic function of the scalar curvature and the parity odd curvature pseudoscalar, commonly known as the Holst (or Hojmann) term. Generalizing some previous results in the literature, we explicitly show that if the Hessian of this function is non-degenerate, the initial non-Riemannian Theory is on-shell equivalent to a metric Scalar-Tensor Theory. The generic form of the kinetic coupling function and the scalar potential of the resulting Theory are explicitly found and reported.

In this study, we investigate the relativistic effects of Earth on Hong-Ou-Mandel (HOM) interference experiments conducted in a terrestrial laboratory. Up to the second order, we calculate the relativistic time delay from the null geodesic equation (particle perspective), and the phase shift, along with the associated effective time delay, from the Klein-Gordon equation (wave perspective). Since gravity influences both the temporal and spatial parts of the phase shift, these time delays differ and predict different coincidence probabilities. The previous HOM experiment conducted on a rotating platform shows that the wave perspective can explain the experimental results. We further explore the frame-dragging and redshift effects within an arbitrarily oriented rectangular interferometer in two distinct cases, finding that both effects can be amplified by increasing the number of light loops. Additionally, we emphasize that the next-leading order Sagnac effect, due to the gravitational acceleration, is comparable to the Thomas precession, geodetic effect, and Lense-Thirring effect. To detect the leading-order Sagnac effect and the redshift effect caused by the gravitational acceleration within the current experimental precision, we determine the number of loops photons should travel in the interferometer. Furthermore, we suggest using the difference between any two HOM patterns with different effective time delays as a probe to detect the influence of relativistic effects on quantum systems.

The space-time geometry under investigation is chosen to be a high-dimensional, static, spherically symmetric solution in an asymptotically flat background within the Einstein-power-Yang-Mills-Gauss-Bonnet (EPYMGB) gravity. To address the limitations of previous shadow constraints, we construct a standardized framework based on the Schwarzschild-Tangherlini metric to constrain the characteristic parameters of high-dimensional black holes by leveraging observational shadow data. Additionally, we provide a rigorous derivation of the shadow radius formula for a general high-dimensional spherically symmetric black hole. Subsequently, we systematically and comprehensively present the equations of motion and master variables governing spin-0, spin-1, $p$-form, and spin-2 perturbations in high-dimensional static spherically symmetric flat space-time. Our analysis reveals that the Yang-Mills magnetic charge $\mathcal{Q}$ and the power $q$ have a negligible impact on both the shadow radius and perturbations of the black hole when compared to the Gauss-Bonnet coupling constant ${\alpha}_2$ in various dimensions. Hence, the physical signatures of the parameters $\mathcal{Q}$ and $q$ in the black hole environment remain undetectable through either perturbation analysis or shadow observations. Cross-validation of the allowable range of ${\alpha}_2$ derived from the high-dimensional constraint on shadow radius and the dynamical stability analysis of gravitational perturbations demonstrates excellent agreement between these independent approaches. The conclusion of this cross-analysis further substantiate the accuracy of the high-dimensional shadow constraint formula proposed in this work, and we argue that this formula may serve as a universal tool for constraining the characteristic parameters of other high-dimensional spherically symmetric black hole solutions.

We propose a thermodynamic formalism, within the particle-frame, for the energy-momentum tensor of irreversible anisotropic imperfect fluids subject to causality. Building on the Israel-Stewart extension of Eckart's theory, we further generalize these formalisms to incorporate anisotropic effects while ensuring the preservation of causality. In this framework, the second law of thermodynamics includes an additional term accounting for the system's anisotropy, which we derive explicitly in closed form for both first- and second-order theories. Notably, when anisotropy is removed, our model recovers Eckart's theory at first order and Israel-Stewart's at second order.