arXiv subjects

Jutta Kunz

Publications and source records attributed to Jutta Kunz.

At least 37 records · Page 2Linked to original sources

arXiv2025

Quasinormal modes of rotating black holes in shift-symmetric Einstein-scalar-Gauss-Bonnet theory

We employ a recently developed spectral method to obtain the spectrum of quasinormal modes of rapidly rotating black holes in alternative theories of gravity and apply it to the black holes of shift-symmetric Einstein-scalar-Gauss-Bonnet theory. In this theory the quasinormal modes were recently obtained by employing perturbation theory in quadratic order in the Gauss-Bonnet coupling constant. Here we present the full non-perturbative results for the spectrum within the domain of existence of rotating black holes and compare with the perturbative results. We also compare with the quasinormal mode spectrum of rapidly rotating Einstein-dilaton-Gauss-Bonnet black holes.

gr-qc↗

arXiv2025

Quadrupole perturbations of slowly spinning Ellis-Bronnikov wormholes

We study the axial and polar perturbations of slowly rotating Ellis-Bronnikov wormholes in General Relativity, applying a perturbative double expansion. In particular, we derive the equations for $l=2$, $M_z=2$ perturbations of these objects which are parametrized by an asymmetry parameter. The equations constitute an astrophysically interesting sector of the perturbations that contribute dominantly to the gravitational wave radiation. Moreover, calculation of these modes may exhibit potential instabilities in the quadrupole sector.

gr-qc↗

arXiv2025

Matter Accumulations and Accretion Tori around Wormholes

We study circular orbits and accretion structures around symmetric wormholes. As exemplary solutions we choose three different wormhole spacetimes, namely rotating traversable wormholes from the Teo class, the rotating Simpson-Visser metric with the parameter spectrum corresponding to wormholes and a static wormhole from beyond Horndeski theories. We show the existence of a spectrum of circular orbits at the wormhole throat for each of these wormhole solutions and analyze the boundaries of this spectrum across the respective wormhole parameter range. For each of the solutions we identified that vast regions of the parameter space correspond to stable orbits. The presence of this orbit spectrum can be linked through accretion disk models to matter accumulations which may form at the throat. We present here examples of such disk solutions by implementing the Polish Doughnut model. In some cases, these matter accumulations are encapsulating the throat and could imply central bright regions of the wormhole spacetime. Furthermore, the combined analysis of the equatorial Keplerian orbits and the throat orbits, leads to a set of different disk configurations, where matter accumulations at the throat may be present with outer tori around them, in some cases also as a connected structure. Our results may hint to possible instabilities of wormholes, especially if they have an ergoregion. Moreover, wormholes with such disks, may appear more star-like when it comes to their observational signature, due to the centralized and more spherical emission profile associated with the possible matter accumulations at the throat.

gr-qc↗

arXiv2025

Circular Orbits and Photon Orbits at Wormhole Throats

In this work we study timelike circular orbits and photon orbits at the throat of stationary and axisymmetric wormholes. Our minimal requirements on the spacetime are the existence of a global radial coordinate l, which connects both sides of the wormhole, two times differentiable metric components with respect to l at the wormhole throat and vanishing first derivatives of the metric components at the wormhole throat, which is the case for symmetrical wormholes. We derive expressions in terms of the metric components for the specific angular momentum $\ell$ of a test particle, that describe a possible spectrum of solutions for bound circular orbits at the wormhole throat. We identify a phase transition in the parameter space, which occurs if an ergoregion is present. Furthermore, we showcase expressions for the parameter space in terms of physical properties of the spacetime in the form of the throat circumference $C_T$ , the angular frequency of the wormhole $ω$ and the gravitational redshift $z$. An analysis of these expressions and the characteristics of the photon orbits gives constraints on wormhole properties, as it hints to possible instabilities, such as for fast rotating wormholes and wormholes with an ergoregion. Through the use of accretion disk models, the existence of possible stable circular orbits could be linked to accretion disks surrounding the wormhole throat.

gr-qc↗

arXiv2025

Quasinormal mode spectrum of rotating black holes in Einstein-Gauss-Bonnet-dilaton theory

Quasinormal modes are excited during the ringdown phase of black holes after merger. Determination of quasinormal modes of rapidly rotating black holes in alternative theories of gravity has remained a challenge for a long time. Here we discuss in detail our recently developed method to extract the quasinormal modes for rapidly rotating black holes in Einstein-Gauss-Bonnet-dilaton theory. We first obtain numerically the exact rapidly rotating background solutions, which also clarify their domain of existence. Then we solve the equations for the linear perturbations of the metric and the dilaton field by employing an appropriate set of boundary conditions and a spectral decomposition of the perturbation functions. The resulting spectrum agrees well with the known limits obtained for slow rotation and weak coupling, while it exhibits larger deviations for stronger coupling.

gr-qc↗

arXiv2025

Quasinormal modes of rapidly rotating Einstein-Gauss-Bonnet-dilaton black holes

Quasinormal modes of rapidly rotating black holes are crucial in understanding the ringdown phase after a merger. While for Kerr black holes these modes have been known for a long time, their calculation has remained a challenge in alternative theories of gravity. We obtain the spectrum of quasinormal modes of rapidly rotating black holes in Einstein-Gauss-Bonnet-dilaton theory without resorting to perturbation theory in the coupling constant. Our approach is based on a spectral decomposition of the linear perturbations of the metric and the scalar field. The quasinormal modes agree excellently with the perturbatively known slow rotation and weak coupling limits. For large coupling, though, the spectrum changes significantly.

gr-qc↗

arXiv2024

Hairy dyonic Reissner-Nordström black holes in an Einstein-Maxwell-Friedberg-Lee-Sirlin type model

We construct spherically symmetric dyonic black holes in a generalized Maxwell-Friedberg-Lee-Sirlin type model with a complex scalar doublet and a symmetry breaking potential {for the real scalar field}, minimally coupled to Einstein gravity in asymptotically flat space. We analyze the properties of the hairy black holes and determine their domain of existence. Our discussion focuses mostly on the case of a long-ranged massless real scalar field. Our results indicate that in this case, depending on the coupling constants, the resonant hairy dyonic black holes may bifurcate from Reissner-Nordström black holes at maximal chemical potential, while the limiting solutions at minimal chemical potential may be related to the Penney solution.

hep-th↗

arXiv2024

Black Holes Inside and Out 2024: visions for the future of black hole physics

The gravitational physics landscape is evolving rapidly, driven by our ability to study strong-field regions, in particular black holes. Black Holes Inside and Out gathered world experts to discuss the status of the field and prospects ahead. We hope that the ideas and perspectives are a source of inspiration. Structure: Black Hole Evaporation - 50 Years by William Unruh The Stability Problem for Extremal Black Holes by Mihalis Dafermos The Entropy of Black Holes by Robert M. Wald The Non-linear Regime of Gravity by Luis Lehner Black Holes Galore in D > 4 by Roberto Emparan Same as Ever: Looking for (In)variants in the Black Holes Landscape by Carlos A. R. Herdeiro Black Holes, Cauchy Horizons, and Mass Inflation by Matt Visser The Backreaction Problem for Black Holes in Semiclassical Gravity by Adrian del Rio Black Holes Beyond General Relativity by Enrico Barausse and Jutta Kunz Black Holes as Laboratories: Searching for Ultralight Fields by Richard Brito Primordial Black Holes from Inflation by Misao Sasaki Tests of General Relativity with Future Detectors by Emanuele Berti Black Holes as Laboratories: Tests of General Relativity by Ruth Gregory and Samaya Nissanke Simulating Black Hole Imposters by Frans Pretorius Black Hole Spectroscopy: Status Report by Gregorio Carullo VLBI as a Precision Strong Gravity Instrument by Paul Tiede Testing the nature of compact objects and the black hole paradigm by Mariafelicia De Laurentis and Paolo Pani Some Thoughts about Black Holes in Asymptotic Safety by Alessia Platania Black Hole Evaporation in Loop Quantum Gravity by Abhay Ashtekar How the Black Hole Puzzles are Resolved in String Theory by Samir D. Mathur Quantum Black Holes: From Regularization to Information Paradoxes by Niayesh Afshordi and Stefano Liberati

gr-qc↗

arXiv2024

Gravitating Skyrmions with localized fermions

We consider self-gravitating Skyrmions in the presence of Dirac fermions, that carry spin and isospin. By varying the gravitational and the Yukawa coupling constants, we investigate the spectral flow of the fermion eigenvalue associated with a zero mode in the absence of gravity. We demonstrate that the backreaction of the fermion can strongly influence the Skyrmion-fermion configurations. In particular, the energy conditions may be violated, and regular anti-gravitating asymptotically flat solutions with negative ADM mass may emerge.

hep-th↗

arXiv2024

Motion of charged particles in an electromagnetic swirling universe: The complete set of solutions

We discuss the motion of electrically and magnetically charged particles in the electromagnetic swirling universe. We show that the equations of motion can be decoupled in the Hamilton-Jacobi formalism, revealing the existence of a fourth constant of motion. The equations of motion can be analytically integrated. The solutions are presented in terms of elementary and elliptic functions. In addition, we discuss the possible orbits for both uncharged particles (in which case the motion is geodesic) and charged particles, respectively. A typical orbit is bounded in the radial direction and escapes to infinity in the $z-$ direction. However, the presence of the electromagnetic fields also leads to the existence of planar orbits.

gr-qc↗

arXiv2024

Fermion states localized on a self-gravitating Skyrmion

We investigate self-gravitating solutions of the Einstein-Skyrme theory coupled to spin-isospin Dirac fermions and consider the dependence of the spectral flow on the effective gravitational coupling constant and on the Yukawa coupling. It is shown that the effects of the backreaction of the fermionic mode may strongly deform the configuration. Depending on the choice of parameters, solutions with positive, negative and zero ADM mass may arise. The occurrence of regular anti-gravitating asymptotically flat solutions with negative ADM mass is caused by the violation of the energy conditions.

hep-th↗

arXiv2024

Einstein-(complex)-Maxwell static boson stars in AdS

We consider a model with two real Maxwell fields (or equivalently, a complex Maxwell field) minimally coupled to Einsteins gravity with a negative cosmological constant in four spacetime dimensions. Assuming a specific harmonic dependence of the vector fields, we show the existence of asymptotically anti-de Sitter (AdS) self-gravitating boson-star-like solitonic solutions, which are static and axially symmetric. Analytical solutions are found in the test-field limit, where the Maxwell equations are solved on a fixed AdS background. The fully nonlinear solutions are constructed numerically.

gr-qc↗

arXiv2024

Quasinormal modes of rapidly rotating Ellis-Bronnikov wormholes

We present for the first time a study of the quasinormal modes of rapidly rotating Ellis-Bronnikov wormholes in General Relativity. We compute the spectrum of the wormholes using a spectral decomposition of the metric perturbations on a numerical background. We focus on the $M_z=2,3$ sector of the perturbations, and show that the triple isospectrality of the symmetric and static Ellis-Bronnikov wormhole is broken due to rotation, giving rise to a much richer spectrum than the spectrum of Kerr black holes. We do not find any instabilities for $M_z=2,3$ perturbations.

gr-qc↗

arXiv2024

Quasinormal modes of Kerr black holes using a spectral decomposition of the metric perturbations

We report a new method to calculate the quasinormal modes of rotating black holes, using a spectral decomposition to solve the partial differential equations that result from introducing linear metric perturbations to a rotating background. Our approach allows us to calculate a large sector of the quasinormal mode spectrum. In particular, we study the accuracy of the method for the $(l=2)$-led and $(l=3)$-led modes for different values of the $M_z$ azimuthal number, considering the fundamental modes as well as the first two excitations. We show that our method reproduces the Kerr fundamental modes with an accuracy of $10^{-6}$ or better for $a/M<0.8$, while it stays below $0.1\%$ for $a/M<0.98$.

gr-qc↗

arXiv2024

Parameter Constraints on Traversable Wormholes within Beyond Horndeski Theories through Quasi-Periodic Oscillations

{\it Hunting} compact astrophysical objects such as black holes and wormholes, as well as testing gravity theories, are important issues in relativistic astrophysics. In this sense, theoretical and observational studies of quasiperiodic oscillations (QPOs) observed in (micro)quasars become helpful in exploring their central object, which can be a black hole or a wormhole. In the present work, we study the throat properties of traversable wormholes beyond Horndeski theory. Also, we investigate the circular motion of test particles orbiting the wormhole. We analyze the test particle's effective potential and angular momentum for circular orbits. Frequencies of radial and vertical oscillations of the particles around stable circular orbits have also been studied and applied in explaining the quasiperiodic oscillations mechanism in the relativistic precession (RP) model. Finally, we obtain constraint values for the parameters of Horndeski gravity and the mass of the wormhole candidates using QPOs observed in the microquasars GRO J1655-40, GRS 1915+105 \& XTE J1550-564 and at the center of Milky Way galaxy through Monte-Carlo-Markovian-Chain (MCMC) analyses.

gr-qc↗

arXiv2024

Radial perturbations of Ellis-Bronnikov wormholes in slow rotation up to second order

We consider slowly rotating Ellis-Bronnikov wormholes and investigate their radial perturbations ($\mathrm{l}=0$), expanding up to second order in rotation. We present the detailed derivations in the general case, including symmetric and non-symmetric wormholes. The calculations show that the unstable mode present in the static case becomes less unstable with increasing rotation, until it reaches zero and then disappears. This indicates that wormhole solutions may become linearly mode stable at sufficiently fast rotation.

gr-qc↗

arXiv2024

Comparison of Magnetized Thick Disks around Black Holes and Boson Stars

Boson stars are considered as promising candidates for black hole mimickers. Similar to other compact objects they can form accretion disks around them. The properties of these disks could possibly distinguish them from other compact objects like black holes in future observations. Retrograde thick disks around boson stars and the influence of strong magnetic fields on them were already studied and it was shown that they can harbor very distinct features compared to black hole disks. However, the case of prograde thick disks is mostly unexplored, since they may appear much more similar to black hole disks. In this work we will investigate similarities and differences regarding prograde thick disks around non-selfinteracting rotating boson stars and rotating black holes. We assume thereby a polytropic equation of state and a constant specific angular momentum distribution of the disks. We classify the various conceivable boson star and black hole solutions by a dimensionless spin parameter $a$ and compare their corresponding disk solutions. The influence of toroidal magnetic fields on the disks is analyzed by selected disk properties, as the rest-mass density distribution and the Bernoulli parameter. Disk solutions are characterized by their degree of magnetization represented by the magnetization parameter $β_{mc}$. We found that strong magnetic fields can strengthen the differences of disk solutions or oppositely even lead to a correlation in disk properties, depending on the spin parameter of the boson star and black hole solutions. We identify the vertical thickness of the boson star disks as the main differentiating factor, since for most solutions the vertical density distribution is far more outreaching for boson star disks compared to black hole disks.

gr-qc↗

arXiv2024

Shadow images of compact objects in beyond Horndeski theory

A beyond Horndeski theory is considered that admits wormholes, black holes and naked singularities. In this theory the shadow images of the black holes and the exotic compact objects (ECOs), illuminated by an optically and geometrically thin disk, are investigated. The results show that the three kinds of objects cast unlike shadow images, in particular, because the different objects possess a different number of light rings. The different boundaries of the accretion disk also affect the images. This may provide further insight into the nature of the shadow images of massive compact objects.

gr-qc↗