arXiv subjects

Jutta Kunz

Publications and source records attributed to Jutta Kunz.

At least 19 recordsLinked to original sources

arXiv2026

Electroweak balls: non-topological solitons in the Weinberg-Salam theory

We construct a new class of smooth, finite-energy solitons in the bosonic $SU(2)\times U(1)$ Weinberg-Salam theory, which we dub $electroweak$ $balls$. Their localization mechanism is analogous in spirit to that of $Q$-balls: the charged vector fields possess a harmonic time dependence while the energy-momentum tensor remains time independent. We explicitly construct both spherically symmetric electric-type solutions and axisymmetric magnetic-type solutions, and show that they form families characterized by a finite frequency interval, a mass gap, and a two-branch structure. These configurations provide electroweak counterparts of Proca-Higgs balls, with the vector-boson masses generated by the Higgs mechanism rather than introduced explicitly. The construction is not tied to the measured parameters of the Standard Model. More generally, it applies to bosonic electroweak-type sectors with different gauge couplings, Higgs self-coupling and symmetry-breaking scale, and hence potentially very different characteristic particle and soliton mass scales. For the families studied here, we do not find solutions at the measured Standard Model couplings and mass ratios.

hep-th↗

arXiv2026

An Exact Single-Rotating Near-Horizon Geometry in Einstein-Gauss-Bonnet Gravity

We construct a five-dimensional singly rotating near-horizon solution in Einstein-Gauss-Bonnet gravity. We show that the Gauss-Bonnet term removes the local curvature singularity, yielding finite curvature invariants throughout the spacetime, provided the rotation parameter remains below a certain value set by the Gauss-Bonnet coupling. To our knowledge, this is the first analytic example of a singly rotating five-dimensional solution in this framework with finite curvature invariants over a nontrivial region of parameter space. We analyze the geometry across this space, identifying regular noncompact, singular, and marginal regimes. Finally, we study the thermodynamic properties, finding that while higher-derivative corrections regularize the local curvature behavior, they also introduce unique challenges to the standard thermodynamic description of Killing horizons.

gr-qc↗

arXiv2026

Black holes in alternative theories of gravity

Black holes, with their strong gravitational fields, provide an important testing ground for theories of gravity beyond General Relativity. Among the many proposed alternatives, considerable recent work has focused on scalar-tensor theories in which a scalar field couples to higher-curvature terms. The black hole solutions that arise in such theories can differ significantly from the Schwarzschild and Kerr solutions of General Relativity. Characteristic properties of these black holes include instabilities, shadows, and gravitational wave spectra, which can be used to constrain the couplings of the underlying theories.

gr-qc↗

arXiv2026

Static multipolar Einstein-vector-Gauss-Bonnet black holes

We construct and analyze static multipolar black holes in Einstein-vector-Gauss-Bonnet theory with a quadratic coupling function. The vectorized solutions bifurcate from the Schwarzschild black hole at discrete values of the Gauss-Bonnet coupling, obtained from a perturbative eigenvalue problem on the Schwarzschild background and labelled by the angular multipole number $\ell$. We restrict to the fundamental radial branches. The electric sector contains the known spherically symmetric $\ell=0$ branch as well as axisymmetric branches with $\ell>0$. These electric branches extend to larger values of the coupling. The magnetic sector features a sequence of axisymmetric branches, including the previously found magnetic dipole branch at $\ell=0$. The magnetic branches instead exist only on finite intervals of the coupling, ending at critical solutions. Away from the bifurcation point, nonlinearities generate additional multipoles, but even-$\ell$ and odd-$\ell$ moments remain separated.

gr-qc↗

arXiv2026

Nonradial perturbations of static charged wormholes

We investigate the nonradial quasinormal-mode spectrum of static charged Ellis--Bronnikov wormholes in Einstein--Maxwell theory minimally coupled to a phantom scalar field. The background solutions are known in closed form and comprise three classes: subcritical, critical and supercritical, which all approach the extremal Reissner--Nordström geometry at the boundary of their domain of existence. We derive the linear perturbation equations for axial and polar sectors, including the coupled gravitational, electromagnetic and phantom-scalar degrees of freedom, and compute the corresponding spectra by means of a Chebyshev spectral method. The uncharged limit reproduces the known Ellis--Bronnikov spectrum and exhibits the expected electromagnetic isospectrality. For charged configurations we track the axial and polar branches across the three families of solutions and identify the effect of the charge on the damping times and oscillation frequencies. In particular, we find that charge can substantially reduce damping rates as the extremal Reissner--Nordström limit is approached. We also uncover a nonradial polar instability, most clearly visible in the fundamental $l=2$ branch for sufficiently large wormhole masses. This instability is distinct from the familiar radial Ellis--Bronnikov instability and shows that the nonradial sector imposes additional constraints on the dynamical viability of charged wormholes.

gr-qc↗

arXiv2026

Hierarchy of Angular Instabilities in Scalarized Black Holes

We investigate the stability of scalarized black holes in Einstein-scalar-Gauss-Bonnet-Ricci theory along their fundamental branches. We show that initially stable solutions first lose nonspherical stability in the eikonal regime, while lower multipoles remain stable. As the branch is continued, instability extends systematically toward lower multipoles, forming an ordered hierarchy of deformation instabilities extending down to the quadrupole mode, while the dipole sector remains stable. The instability thresholds obey a common scaling law and approach finite eikonal limits, defining the boundary of the angularly stable region. We demonstrate that the previously identified quadrupole and angular-Laplacian instabilities are connected by a continuous hierarchy of instability thresholds spanning the angular sectors of the theory. This hierarchy is distinct from radial stability, which changes only at branch turning points, and reveals a previously unexplored angular organization of instabilities in scalarized black holes.

gr-qc↗

arXiv2026

Stationary Einstein-vector-Gauss-Bonnet black holes

We study spontaneously vectorized black holes in Einstein-vector-Gauss-Bonnet theory with a quadratic coupling function. Besides the static, spherically symmetric black holes carrying an electric charge, there are uncharged static, axially symmetric black holes that possess a magnetic dipole moment. Both types possess radial excitations. The magnetic black holes are prolate. They are hotter than the Schwarzschild black holes and possess lower free energy. The domain of existence of the rotating vectorized black holes is bounded by the Kerr black holes, the spherically and axially symmetric static black holes, and the critical solutions.

gr-qc↗

arXiv2026

Light rings and optical appearances of naked singularities, solitons, and black holes in beyond Horndeski gravity

We investigate the geodesic structure and optical appearance of compact objects with primary scalar hair in shift- and parity-symmetric beyond Horndeski gravity. The analytic solution considered here depends on a theory parameter and a dimensionless mass parameter \cite{Bakopoulos:2023sdm}. For a fixed theory parameter, varying the mass traces a family of static spacetimes that can interpolate between timelike naked singularities, regular solitons, regular black holes, Reissner-Nordström-like black holes, multi-horizon black holes, and Schwarzschild-like black holes. We classify these branches by their horizon structure and analyze null and timelike geodesics, focusing on light rings, innermost stable circular orbits, and static spheres. We then compute thin-disk optical images by ray tracing. We find that the number of horizons is not directly encoded in the image: horizonless objects can show shadow-like central depressions, while multi-horizon black holes can closely resemble single-horizon black holes when their exterior light ring and disk structures are similar. Thus, the optical appearance is governed mainly by the photon potential and the disk inner edge, with the deeper horizon structure leaving only an indirect imprint. Quantitative radial-profile diagnostics confirm that the degeneracy is mainly morphological: the profiles differ at fixed impact parameter, but become much closer after rescaling by the critical impact parameter. These results provide a concrete example of how distinct compact object branches in beyond Horndeski gravity can share similar observational signatures.

gr-qc↗

arXiv2026

Boson Stars surrounded by Polish Doughnuts in Scalar-Tensor Theory

We investigate thick accretion disks (Polish Doughnuts) around rotating self-interacting boson stars in general relativity and scalar-tensor theories, focusing on spontaneously scalarized solutions and their general relativistic counterparts. Using equilibrium models with constant specific angular momentum, we analyze disk structures across the parameter space, with emphasis on the phase transition between GR and scalarized configurations. We find that scalarization induces qualitative changes in the spacetime that significantly affect disk morphology. In particular, scalarized boson stars can lack innermost circular orbits, allowing stable motion down to the center and enabling highly compact, quasi-spherical disks. For the most massive scalarized solutions, a non-monotonic angular momentum profile further permits two-centered disk configurations connected by a cusp. Overall, disks around scalarized boson stars are more compact and more strongly bound than those in general relativity, highlighting distinctive features that may serve as observational signatures of alternative gravity theories in the strong-field regime.

gr-qc↗

arXiv2026

Spin-Induced Nonlinear Scalarization of Kerr Black Holes in Einstein-scalar-Gauss-Bonnet Gravity

We investigate spin-induced scalarization of Kerr black holes in an Einstein-scalar-Gauss-Bonnet (EsGB) model that does not admit a linear tachyonic instability of the scalar-free solution. The scalarization mechanism is therefore genuinely nonlinear. We first analyze the decoupled scalar dynamics on fixed Kerr backgrounds and show that sufficiently rapid rotation modifies the Gauss-Bonnet invariant such that a negative near-horizon region develops near the poles. This region provides a geometric trapping mechanism for nonlinear scalar growth, which becomes effective above a threshold spin $χ=0.5$. We then construct stationary scalarized black hole solutions with full backreaction and determine their domain of existence. We find that the solutions occupy a finite low-mass high-spin wedge in the spin-mass plane. This is in contrast to spin-induced spontaneous scalarization, where the scalarized solutions form a narrow band. In this wedge, toward the high-spin end, the scalar hair becomes stronger, and the solutions approach a near-extremal regime, while toward the low-spin boundary, the scalar field is strongly suppressed and approaches a weak-hair limit as $χ\to 0.5$.

gr-qc↗

arXiv2026

Phase Structure of Scalarized Black Holes in Einstein-Scalar-Gauss-Bonnet Gravity

We revisit scalarized black holes in Einstein-scalar-Gauss-Bonnet gravity and analyze the thermodynamic phase transition between the Schwarzschild solution of general relativity and scalarized black holes. Restricting to spherically symmetric configurations, we investigate several classes of scalar-Gauss-Bonnet coupling functions. For the simplest quadratic coupling that triggers spontaneous scalarization, the scalarized solutions are thermodynamically disfavored and no phase transition occurs. For an exponential coupling, the phase structure depends strongly on the coupling parameter, allowing for the absence of a transition, a continuous second-order transition, or a discontinuous first-order transition. For couplings leading to purely nonlinear scalarization, we find either a first-order transition or no transition. These results reveal a rich phase structure of scalarized black holes controlled by the scalar-Gauss-Bonnet coupling.

gr-qc↗

arXiv2026

Dyonic Einstein-Maxwell-scalar black holes: the cold, the hot and the plunge

We investigate dyonic nonlinearly scalarized black holes in Einstein-Maxwell-scalar theory. The domain of existence of scalarized dyonic black holes consists of three branches. The cold branch and the hot branch bifurcate at a minimal value of the charge, analogous to the purely electrically charged scalarized black holes. However, the presence of both charges allows for regular extremal black holes, leading to a third branch featuring a sudden plunge in Hawking temperature. In fact, the presence of both electromagnetic charges introduces a factor $Δ(ϕ)$ in the source term of scalar field equations that vanishes when the coupling function $f(ϕ)$ equals the ratio of the charges for some value of the scalar field $ϕ_c$. The scalar field of extremal black holes assumes precisely this value at the horizon, $ϕ_H=ϕ_c$. We demonstrate the plunge for the coupling function $f(ϕ)=\exp(αϕ^3)$.

gr-qc↗

arXiv2026

Radial perturbations of charged wormholes

Ellis-Bronnikov wormholes suffer from an unstable radial mode. Here we investigate the evolution of the unstable mode(s) for charged wormholes. We show that the instability remains in the presence of charge, but exhibits a very fast decrease to zero. We hereby make a full study of the spectrum of the unstable radial modes. For so-called supercritical wormholes, two purely imaginary unstable modes merge and continue with degenerate imaginary parts and opposite real parts. By analogy, we conjecture an analogous behavior for rotating chargeless wormholes.

gr-qc↗

arXiv2026

Existence of nonlinearly scalarized black holes in Einstein-scalar-Gauss-Bonnet theory with polynomial couplings

Nonlinearly scalarized black holes are investigated in Einstein-scalar-Gauss-Bonnet (EsGB) theory with polynomial coupling functions $ζ(ϕ)$ satisfying $ζ''(0) = 0$, where $ζ'(ϕ) = 0$ features besides $ϕ=0$ solutions with constant $ϕ_{\rm s} \ne 0$. We determine the threshold amplitudes for Gaussian pulses, above which Schwarzschild black holes (SBHs) %become unstable and may transition to scalarized black holes for two coupling functions: $ζ(ϕ)=αϕ^4-βϕ^8$ and $ζ(ϕ)=αϕ^4-βϕ^6$. In contrast, for the quartic coupling function $ζ(ϕ)=αϕ^4$ SBHs are stable. Treating $ζ(ϕ)R_{GB}^2$ as an effective potential $V_\text{eff}$ provides an explanation for the ``plateau" and the divergence observed in the time evolution. We then construct the branches of nonlinearly scalarized black holes in the probe limit and with backreaction. While the pattern of the solution branches in the probe limit exhibits universal features, the presence of backreaction reveals a distinct dependence on the coupling strength $β$.

gr-qc↗

arXiv2026

Photon rings and shadows of Kerr black holes immersed in a swirling universe

We discuss photon rings around as well as shadows of Kerr black holes immersed in a swirling spacetime (KBHSU). We find that the spin-spin interaction between the angular momentum of the black hole and the swirling of the background leads to new interesting effects as it breaks the symmetry between the upper and lower hemispheres. We find that a pair of light rings exists for all values of the parameter space. Using a topological argument, we prove that there should be, indeed, two light rings and that, additionally, these light rings are unstable. In comparison to the Schwarzschild black hole immersed in a swirling universe, the light rings typically all possess different radii. Interestingly, as the value of the swirling parameter is increased at fixed angular momentum of the black hole the two disconnected patches of the ergoregions eventually merge. The light ring at this merger possesses no angular velocity (as measured by an observer at infinity) and is called a \textit{light point}. To our knowledge, this is the first time the existence of such a light point in a black hole space-time is reported. Finally, we also present the shadows of KBHSU for various parameter values and observe that, due to the presence of the swirling background, the shadows are twisted.

gr-qc↗

arXiv2026

Two types of boson stars in $U(1)$ gauged $3+1$-dimensional $O(3)$ sigma-model

We investigate boson stars in an $O(3)$ scalar field theory with a symmetry-breaking potential. By constructing numerically spherically symmetric solutions, we demonstrate that the model gives rise to a rich set of field configurations. The negative coupling constant of the scalar self-interactions allows for two types of boson stars: Type I solutions represent the usual boson stars, that emerge from the vacuum as the boson frequency is decreased below the boson mass, whereas type II boson stars emerge from a set of static soliton solutions. Depending on the strengths of the gravitational and the electric coupling constants, both types or only one type is present. At a critical set of coupling constants, both types undergo a bifurcation. There the spirals of both types disconnect from their branches and reconnect with each other, while the remaining branches of both types also reconnect with each other.

hep-th↗

arXiv2025

Phase transitions of boson stars in scalar-tensor theories

In scalar-tensor theories, compact objects may experience spontaneous scalarization. Recently, it was shown that matter-induced spontaneous scalarization of neutron stars is predominantly associated with a first-order phase transition. Here we consider matter-induced spontaneous scalarization of boson stars. Employing a repulsive quartic potential for the bosonic matter, we find only first-order phase transitions.

gr-qc↗

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↗