arXiv subjects

Jutta Kunz

Publications and source records attributed to Jutta Kunz.

At least 55 records · Page 3Linked to original sources

arXiv2023

Geodesic Motion in a Swirling Universe: The complete set of solutions

We study the geodesic motion in a space-time describing a swirling universe. We show that the geodesic equations can be fully decoupled in the Hamilton-Jacobi formalism leading to an additional constant of motion. The analytical solutions to the geodesic equations can be given in terms of elementary and elliptic functions. We also consider a space-time describing a static black hole immersed in a swirling universe. In this case, full separation of variables is not possible and the geodesic equations have to be solved numerically.

gr-qc↗

arXiv2023

Are slowly rotating Ellis-Bronnikov wormholes stable?

We investigate the radial perturbations of Ellis-Bronnikov wormholes ($\mathrm{l}=0$) in a slowly rotating background expanded up to second order in rotation. We find indications that simple wormhole solutions such as Ellis-Bronnikov in General Relativity can be stabilized by rotation, thus favoring a viable traversable wormhole. This opens up the intriguing question whether the many other wormhole solutions with or without the support of exotic matter can become linearly mode stable when the wormhole rotates.

gr-qc↗

arXiv2023

Modified Gravity and Cosmology: An Update by the CANTATA Network

General Relativity and the $Λ$CDM framework are currently the standard lore and constitute the concordance paradigm. Nevertheless, long-standing open theoretical issues, as well as possible new observational ones arising from the explosive development of cosmology the last two decades, offer the motivation and lead a large amount of research to be devoted in constructing various extensions and modifications. All extended theories and scenarios are first examined under the light of theoretical consistency, and then are applied to various geometrical backgrounds, such as the cosmological and the spherical symmetric ones. Their predictions at both the background and perturbation levels, and concerning cosmology at early, intermediate and late times, are then confronted with the huge amount of observational data that astrophysics and cosmology are able to offer recently. Theories, scenarios and models that successfully and efficiently pass the above steps are classified as viable and are candidates for the description of Nature. This work is a Review of the recent developments in the fields of gravity and cosmology, presenting the state of the art, high-lighting the open problems, and outlining the directions of future research. Its realization was performed in the framework of the COST European Action ``Cosmology and Astrophysics Network for Theoretical Advances and Training Actions''.

gr-qc↗

arXiv2023

Magnetized Thick Disks around Boson Stars

The effects of magnetic fields on accretion disks around compact objects are of high importance in the study of their general properties and dynamics. Here we analyze the influence of magnetic fields on thick accretion disks around rotating boson stars. We assume a uniform constant specific angular momentum distribution and a polytropic equation of state. The purely hydrodynamical thick disk solutions are extended to magnetized solutions by adding a toroidal magnetic field and then analyzed in terms of a magnetization parameter. We consider one-centered solutions as well as two-centered solutions and focus on retrograde tori, since they are more distinctive due to their unique properties. Our computed solutions indicate that strong magnetic fields influence the characteristics of thick disks around rotating boson stars and possibly affect their unique features.

gr-qc↗

arXiv2023

Charged hairy black holes in the gauged Einstein-Friedberg-Lee-Sirlin model

We obtain charged spherically symmetric black holes in the two-component scalar Einstein-Maxwell-Friedberg-Lee-Sirlin model with a symmetry breaking potential. These asymptotically flat black holes carry resonant scalar Q-hair. As expected, these hairy black holes give rise to non-uniqueness. When comparing these solutions with the corresponding charged boson stars and Reissner-Nordström black holes, we find a different pattern in the case of a massive real scalar component and a massless one. We demonstrate that, as the real component becomes massless, the resonant hairy black holes bifurcate from Reissner-Nordström black holes for sufficiently small gravitational coupling.

hep-th↗

arXiv2023

Light Ring behind Wormhole Throat: Geodesics, Images and Shadows

The geodesics of the Ellis-Bronnikov wormhole with two parameters are studied. The asymmetric wormhole has only one light ring and one innermost stable circular orbit located on one side of the wormhole throat. Consequently, certain light rays can be reflected back by the wormhole. Additionally, the same wormhole can have different appearances on both sides of the throat. We present novel images of the wormhole with a light ring behind the throat in a scenario with an accretion disk as the light source and in a backlit wormhole scenario, which are distinct from the images of other compact objects and have the potential to be observed.

gr-qc↗

arXiv2023

Phases of rotating black objects in d = 5 Einstein-Gauss-Bonnet theory

We consider several different classes of asymptotically flat, rotating black objects in d = 5 Einstein-Gauss-Bonnet (EGB) theory. These are first the black holes with two equal-magnitude angular momenta, in which case extremal configurations are studied as well. Numerical evidence is also given for the existence of EGB generalizations of the Myers-Perry black holes with a single plane of rotation and of the Emparan-Reall balanced black rings. All solutions approach asymptotically the Minkowski background and present no singularities outside and on the horizon. The numerical results suggest that for any mass of the solutions and any topology of the horizon, the rotating configurations exist up to a maximal value of the GB coupling constant, while the solutions with a spherical horizon topology still satisfy the Einstein gravity bound on angular momentum.

gr-qc↗

arXiv2023

Quadrupole instability of static scalarized black holes

The addition of a Ricci coupling to Einstein-scalar-Gauss-Bonnet theories makes general relativity a cosmological attractor. Previous work considered a quadratic coupling function with two independent coupling constants in such theories and showed that static, spherically symmetric, spontaneously scalarized black holes are radially stable beyond a critical value of the Ricci coupling constant. Here we demonstrate that these black holes are affected by a quadrupole instability which leads to two new branches of static, axially symmetric scalarized black holes. We discuss the properties of these solutions and provide embedding diagrams.

gr-qc↗

arXiv2023

Mixed neutron-star-plus-wormhole systems: Rotating configurations

We present rapidly rotating neutron stars featuring wormholes in their centers. They arise in general relativity in the presence of a ghost scalar field. The nuclear matter is described by a polytropic equation of state, yielding realistic masses and radii for the neutron stars. The wormholes possess small circumferential radii of size up to 3 km. With increasing wormhole size, the masses and radii of the stars decrease, while the domain of existence of these rotating mixed neutron-star-plus-wormhole systems retains the characteristic properties of a rotating neutron star domain. The question of stability of the mixed configurations under consideration is briefly discussed.

gr-qc↗

arXiv2022

Polar modes and isospectrality of Ellis-Bronnikov wormholes

We consider polar perturbations of static Ellis-Bronnikov wormholes and derive the coupled set of perturbation equations for the gravitational and the scalar field. For massless wormholes the perturbations decouple, and we obtain two identical master equations for the scalar and gravitational modes, which moreover agree with the master equation for the axial modes. Consequently there is isospectrality with threefold degenerate modes. For a finite mass of the background wormhole solutions, the equations are coupled. We then obtain two distinct branches of polar quasinormal modes for a given multipole number l, associated with the presence of the two types of fields. We calculate the quasi-normal mode frequencies and decay rates for the branches with l=2, 3 and 4. For a given l the real frequencies of the two branchesget the closer, the higher the multipole number gets.

gr-qc↗

arXiv2022

Einstein-scalar field solutions in AdS spacetime: clouds, boundary conditions, and scalar multipoles

We consider an Einstein-scalar field model which is a consistent truncation of ${\cal N}=8$ $D=4$ gauged supergravity, the scalar field possessing a potential which is unbounded from below and a tachyonic mass above the Breitenlohner-Freedman bound. We investigate the spherically symmetric asymptotically anti-de Sitter soliton and black hole solutions, with the aim of clarifying the asymptotics and the possible boundary conditions at infinity. The emerging picture is contrasted with that found for an Einstein-scalar field model with the same scalar mass and a quartic self-interaction term. We also provide arguments for the existence of solitonic solutions which can be viewed as non-linear continuation of the (probe) scalar multipolar clouds, with emphasis on the dipole case. Apart from numerical results, exact solutions are found for solitons with a monopole and dipole scalar field, as perturbations around the AdS background.

hep-th↗

arXiv2022

Robotic observation pipeline for small bodies in the solar system based on open-source software and commercially available telescope hardware

The observation of small bodies in the Space Environment is an ongoing important task in astronomy. While nowadays new objects are mostly detected in larger sky surveys, several follow-up observations are usually needed for each object to improve the accuracy of orbit determination. In particular objects orbiting close to Earth, so called Near-Earth Objects are of special concern as a small but not negligible fraction of them can have a non-zero impact probability with Earth. Telescopes are often hosted by amateur observatories. With upcoming new NEO search campaigns by very wide field of view telescopes, like the Vera C. Rubin Observatory, NASA's NEO surveyor space mission and ESA's Flyeye telescopes, the number of NEO discoveries will increase dramatically. This will require an increasing number of useful telescopes for follow-up observations at different geographical locations. While well-equipped amateur astronomers often host instruments which might be capable of creating useful measurements, both observation planning and scheduling, and also analysis are still a major challenge for many observers. In this work we present a fully robotic planning, scheduling and observation pipeline that extends the widely used open-source cross-platform software KStars/Ekos for INDI devices. The method consists of algorithms which automatically select NEO candidates with priority according to ESA's NEOCC. It then analyses detectable objects (based on limiting magnitudes, geographical position, and time) with preliminary ephemeris from the Minor Planet Center. Optimal observing slots during the night are calculated and scheduled. Immediately before the measurement the accurate position of the minor body is recalculated and finally the images are taken. Besides the detailed description of all components, we will show a complete robotic hard- and software solution based on our methods.

astro-ph.IM↗

arXiv2022

$ϕ$-modes of neutron stars in a massless scalar-tensor theory

Scalar-tensor theories allow for a rich spectrum of quasinormal modes of neutron stars. The presence of the scalar field allows for polar monopole and dipole radiation, as well as for additional higher multipole modes led by the scalar field. Here we present these scalar-led $ϕ$-modes for the lowest multipoles, $l=0$, 1 and 2 for a massless scalar-tensor theory of the Brans-Dicke type, motivated by $R^2$ theory, and compare with those of a minimally coupled scalar field in general relativity. We consider a set of six realistic equations of state and extract universal relations for the modes.

gr-qc↗

arXiv2022

Hyperelliptic functions and motion in general relativity

Analysis of black hole spacetimes requires study of the motion of particles and light in these spacetimes. Here exact solutions of the geodesic equations are the means of choice. Numerous interesting black hole spacetimes have been analyzed in terms of elliptic functions. However, the presence of a cosmological constant, higher dimensions or alternative gravity theories often necessitate an analysis in terms of hyperelliptic functions. Here we review the method and current status for solving the geodesic equations for the general hyperelliptic case, illustrating it with set a of examples of genus $g=2$.

gr-qc↗

arXiv2022

Neutron Stars

Neutron stars are highly compact astrophysical objects and therefore of utmost relevance to learn about theories of gravity. Whereas the proper equation of state of the nuclear matter inside neutron stars is not yet known, and a wide range of equations of state is still compatible with observations, this uncertainty can be overcome to a large extent, when dimensionless neutron star properties are considered. In this case universal relations between neutron star properties and for the gravitational radiation in the form of quasi-normal modes arise. These universal relations can be rather distinct for alternative theories of gravity as compared to General Relativity. Moreover, the presence of new degrees of freedom in alternative theories of gravity leads to further types of gravitational radiation, that may be revealed by pulsar observations and gravitational wave detectors. Here an introduction to neutron stars, their properties and universal relations is presented, followed by two examples of alternative theories of gravity featuring interesting effects for neutrons stars.

gr-qc↗

arXiv2022

Symmetric wormholes in Einstein-vector-Gauss-Bonnet theory

We construct wormholes in Einstein-vector-Gauss-Bonnet theory where a real massless vector field is coupled to the higher curvature Gauss-Bonnet invariant. We consider three coupling functions which depend on the square of the vector field. The respective domains of existence of wormholes possess as their boundaries i) black holes, ii) solutions with a singular throat, iii) solutions with a degenerate throat and iv) solutions with cusp singularities. Depending on the coupling function wormhole solutions can feature a single throat or an equator surrounded by a double throat. The wormhole solutions need a thin shell of matter at the throat, in order to be symmetrically continued into the second asymptotically flat region. These wormhole spacetimes allow for bound and unbound particle motion as well as light rings.

gr-qc↗

arXiv2022

Compact Objects in Alternative Gravities

We address neutron stars and black holes in alternative gravities, after recalling their basic properties in General Relativity. Among the plethora of interesting alternative gravities we here focus on an interesting set of scalar-tensor theories. We discuss the phenomenon of spontaneous scalarization, that is matter induced for neutron stars and curvature induced for black holes. Along with other relevant physical properties, we address the quasi-normal modes of these compact objects. In particular, we consider \textit{universal relations} of neutron stars to largely reduce the dependence on the equation of state, and we briefly address the shadow of black holes.

gr-qc↗

arXiv2022

Radial perturbations of scalar-Gauss-Bonnet black holes beyond spontaneous scalarization

Spontaneous scalarization of black holes in scalar-Gauss-Bonnet (sGB) gravity is a very interesting phenomenon allowing black holes to circumvent the no-hair theorem and acquire scalar hair while leaving the weak field regime of the theory practically unaltered. It was recently shown that if we allow for a different form of the coupling between the scalar field and the Gauss-Bonnet invariant, a new type of scalarization is possible beyond the standard one that can be excited only nonlinearly. In this case the spectrum of black hole solutions can be more complicated, naturally opening the question about their stability. The goal of the present paper is to study this question including the possible loss of hyperbolicity of the radial perturbation equation. We show that one of the nonlinearly scalarized phases of the Schwarzschild black hole can be stable and hyperbolic for large enough black hole masses. These results establish the studied hairy black holes as viable astrophysical candidates.

gr-qc↗