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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., Comment: 28 pages, 6 figures, 16 tables

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., Comment: 13 pages, 4 figures

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., Comment: 7 pages, 3 figures

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., Comment: 19 pages, 4 figures, 3 tables; v2: equations added, results unchanged, matches published version

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., Comment: 14 pages, 2 figures; accepted for publication in Universe (Special Issue "Black Holes in Einstein-Gauss-Bonnet Theories")

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., Comment: 5 pages, 4 figures

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., Comment: 10 pages, 1 figure, minor corrections to content, version published in PRD

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., Comment: 27 pages, 16 figures

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., Comment: 17 pages, 10 figures; Special Issue Alternative Gravities and Fundamental Cosmology (Altecosmofun'21)

We present wormholes with a Newman-Unti-Tamburino (NUT) charge that arise in certain higher curvature theories, where a scalar field is coupled to a higher curvature invariant. For the invariants we employ i) a Gauss-Bonnet term and ii) a Chern-Simons term, which then act as source terms for the scalar field. We map out the domain of existence of wormhole solutions by varying the coupling parameter and the scalar charge for a set of fixed values of the NUT charge. The domain of existence for a given NUT charge is then delimited by the set of scalarized nutty black holes, a set of wormhole solutions with a degenerate throat and a set of singular solutions., Comment: 10 pages, 2 figures

We analyze the properties of the circular orbits for massive particles in the equatorial plane of symmetric rotating Ellis wormholes. In particular, we obtain the orbital frequencies and the radial and vertical epicyclic frequencies, and consider their lowest parametric, forced and Keplerian resonances. These show that quasi-periodic oscillations in accretion disks around symmetric rotating Ellis wormholes have many distinct properties as compared to quasi-periodic oscillations in accretion disks around rotating Teo wormholes and the Kerr black hole. Still we can distinguish some common features which appear in wormhole spacetimes as opposed to black holes. The most significant ones include the possibility of excitation of stronger resonances such as lower order parametric and forced resonances and the localization of these resonances deep in the region of strong gravitational interaction near the wormhole throat, which will lead to further amplification of the signal., Comment: 21 pages, 16 figures

Black holes represent outstanding astrophysical laboratories to test the strong gravity regime, since alternative theories of gravity may predict black hole solutions whose may differ distinctly from those of General Relativity. When higher curvature terms are included in the gravitational action as, for instance, in the form of the Gauss-Bonnet term coupled to a scalar field, scalarized black holes result. Here we discuss several types of scalarized black holes and some of their properties., Comment: Proceedings of the International Online Workshop on Relativistic Astrophysics and Gravitation, Scalarized Black Holes, Tashkent, Uzbekistan, May 2021

General Relativity and the $\Lambda$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''., Comment: 543 pages

We construct spontaneously vectorized black holes where a real vector field is coupled to the Gauss-Bonnet invariant. We employ three coupling functions for the vector field, and determine the respective domains of existence of the vectorized black holes. These domains of existence are bounded by the marginally stable Schwarzschild black holes and the critical vectorized black holes. We also address the effects of a mass term. For a given black hole mass the horizon radius is smaller for the vectorized black holes than for the Schwarzschild black holes. Since the vector field vanishes at the horizon, there is no contribution from the Gauss-Bonnet term to the entropy of the vectorized black holes., Comment: 12 pages, 8 figures

We construct scalarized wormholes with a NUT charge in higher curvature theories. We consider both Einstein-scalar-Gauss-Bonnet and Einstein-scalar-Chern-Simons theories, following a recent paper by Brihaye et al. [1], where spontaneously scalarised Schwarzschild-NUT solutions were studied. By varying the coupling parameter and the scalar charge we determine the domain of existence of the scalarized nutty wormholes, and their dependence on the NUT charge. In the Gauss-Bonnet case the known set of scalarized wormholes [2] is reached in the limit of vanishing NUT charge. In the Chern-Simons case, however, the limit is peculiar, since with vanishing NUT charge the coupling constant diverges. We focus on scalarized nutty wormholes with a single throat and study their properties. All these scalarized nutty wormholes feature a critical polar angle, beyond which closed timelike curves are present., Comment: 14 pages, 4 figures

We construct black hole solutions with spin-induced scalarization in a class of models where a scalar field is quadratically coupled to the topological Gauss-Bonnet term. Starting from the tachyonically unstable Kerr solutions, we obtain families of scalarized black holes such that the scalar field has either even or odd parity, and we investigate their domain of existence. The scalarized black holes can violate the Kerr rotation bound. We identify "critical" families of scalarized black hole solutions such that the expansion of the metric functions and of the scalar field at the horizon no longer allows for real coefficients. For the quadratic coupling considered here, solutions with spin-induced scalarization are entropically favored over Kerr solutions with the same mass and angular momentum., Comment: 5 pages, 3 figures. Matches version published in PRL and fixes a small typo (F_0 should read F_1 in equation 6)

We construct wormholes in Einstein-scalar-Gauss-Bonnet theories with a potential for the scalar field that includes a mass term and self-interaction terms. By varying the Gauss-Bonnet coupling constant we delimit the domain of existence of wormholes in these theories. The presence of the self-interaction enlarges the domain of existence significantly. There arise wormholes with a single throat and wormholes with an equator and a double throat. We determine the physical properties of these wormholes including their mass, their size and their geometry., Comment: 16 pages, 8 figures

Besides scalarized black holes and wormholes, Einstein-scalar-Gauss-Bonnet theories allow also for particle-like solutions. The scalar field of these particle-like solutions diverges at the origin, akin to the divergence of the Coulomb potential at the location of a charged particle. However, these particle-like solutions possess a globally regular metric, and their effective stress energy tensor is free from pathologies, as well. We determine the domain of existence for particle-like solutions in a number of Einstein-scalar-Gauss-Bonnet theories, considering dilatonic and power-law coupling functions, and we analyze the physical properties of the solutions. Interestingly, the solutions may possess pairs of lightrings, and thus represent ultra-compact objects. We determine the location of these lightrings, and study the effective potential for the occurrence of echoes in the gravitational-wave spectrum. We also address the relation of these particle-like solutions to the respective wormhole and black-hole solutions, and clarify the limiting procedure to recover the Fisher solution (also known as Janis-Newman-Winicourt-Wyman solution)., Comment: 29 pages, 15 figures

We construct rotating black holes in Einstein-scalar-Gauss-Bonnet theory with a quadratic coupling function. We map the domain of existence of the rotating fundamental solutions, we construct radially excited rotating black holes (including their existence lines), and we show that there are angularly excited rotating black holes. The bifurcation points of the radially and angularly excited solutions branching out of the Schwarzschild solution follow a regular pattern.

We present a new type of ultracompact objects, featuring lightrings and echoes in the gravitational-wave spectrum. These particle-like solutions arise in Einstein-scalar-Gauss-Bonnet theories in four spacetime dimensions, representing globally regular spacetime manifolds. The scalar field diverges at the center, but the effective stress-energy tensor is free from pathologies. We determine their domain of existence and compare with wormhole solutions, black holes and the Fisher solution., Comment: 7 pages, 4 figures, the analysis on the observable effects was extended, a new plot, comments and references were added, version published in Physics Letters B

We consider rotating wormhole solutions supported by a complex phantom scalar field with a quartic self-interaction, where the phantom field induces the rotation of the spacetime. The solutions are regular and asymptotically flat. A subset of solutions describing static but not spherically symmetric wormholes is also obtained., Comment: 13 pages, 6 figures

All known five dimensional, asymptotically flat, static black rings possess conical singularities. However, there is no fundamental obstruction forbidding the existence of balanced configurations, and we show that the Einstein--Klein-Gordon equations admit (numerical) solutions describing static asymptotically flat black rings, which are regular on and outside the event horizon. The scalar field is 'phantom', which creates the self-repulsion necessary to balance the black rings. Similar solutions are likely to exist in other spacetime dimensions, the basic properties of a line element describing a four dimensional, asymptotically flat black ring geometry being discussed., Comment: 12 pages, 5 figures

Within general relativity, we determine the energy conditions needed for the existence of a toroidal $T^2$ wormhole. For this purpose, we employ the conditions of the positiveness of the second derivatives of the relevant components of the metric, which describe an increase in the linear sizes (or the area) of the cross section of the throat. The corresponding inequalities for the central energy density and pressures of the matter and for the metric are obtained., Comment: 9 pages

Novel wormholes are obtained in Einstein-scalar-Gauss-Bonnet theory for several coupling functions. The wormholes may feature a single-throat or a double-throat geometry and do not demand any exotic matter. The scalar field may asymptotically vanish or be finite, and it may possess radial excitations. The domain of existence is fully mapped out for several forms of the coupling function., Comment: Latex File, 15 pages, 7 figures, expanded version to appear in Physical Review D

We consider Ellis wormholes immersed in rotating matter in the form of an ordinary complex boson field. The resulting wormholes may possess full reflection symmetry with respect to the two asymptotically flat spacetime regions. However, there arise also wormhole solutions where the reflection symmetry is broken. The latter always appear in pairs. We analyse the properties of these rotating wormholes and show that their geometry may feature single throats or double throats. We also discuss the ergoregions and the lightring structure of these wormholes., Comment: 6 pages, 3 figures

In this work we present full sets of solutions for rotating charged boson stars with different coupling values. By adopting local comoving coordinates, we are able to find expressions for the effective hydrodynamic quantities of the fluids as seen by this class of observers. We show that not only is the energy density non zero at the center, for the uncharged case it has a local maximum at the core from which it decreases until the point of local minimum where its variation is discontinuous. For the first time, it is reported how rotating boson stars, charged and uncharged, are completely anisotropic fluids featuring three different pressures. Furthermore, the character of the electromagnetic fields is analyzed.

We consider a topologically nontrivial thin-shell wormhole with a throat in the form of a $T^2$ torus. It is shown that: (i)~such a wormhole is stable with respect to excitations of the throat; (ii)~not all energy conditions are violated for such wormholes; (iii)~if any of the energy conditions is violated, this violation occurs only partially in some region near the throat, and in other regions the violation is absent. Also, we discuss the differences between spherical $S^2$ wormholes and toroidal $T^2$ wormholes under investigation., Comment: 7 pages, 1 figure

The innermost stable circular orbits (ISCOs) around rapidly rotating neutron stars are studied in dilatonic Einstein-Gauss-Bonnet theory. Universal relations for properly scaled ISCO properties are extended from General Relativity to dilatonic Einstein-Gauss-Bonnet theory and additional relations are obtained., Comment: 14 pages, 11 figues

We compute the axial quasi-normal modes of static neutron stars in scalar tensor theory. In particular, we employ various realistic equations of state including nuclear, hyperonic and hybrid matter. We investigate the fundamental curvature mode and compare the results with those of General Relativity. We find that the frequency of the modes and the damping time are reduced for the scalarized neutron stars. In addition, we confirm and extend the universal relations for quasi-normal modes known in General Relativity to this wide range of realistic equations of state for scalarized neutron stars and confirm the universality of the scaled frequency and damping time in terms of the scaled moment of inertia as well as compactness for neutron stars with and without scalarization.

We consider four-dimensional wormholes immersed in bosonic matter. While their existence is based on the presence of a phantom field, many of their interesting physical properties are bestowed upon them by an ordinary complex scalar field, which carries only a mass term, but no self-interactions. For instance, the rotation of the scalar field induces a rotation of the throat as well. Moreover, the bosonic matter need not be symmetrically distributed in both asymptotically flat regions, leading to symmetric and asymmetric rotating wormhole spacetimes. The presence of the rotating matter also allows for wormholes with a double throat., Comment: 24 pages, 11 figures

We consider spinning generalizations of the Ellis wormhole in scalar-tensor theory. Analogous to other compact objects these wormholes can carry a non-trivial scalarization. We determine the domain of existence of the scalarized wormholes and investigate the effect of the scalarization on their properties. Depending on the choice of the coupling function, they may possess multiple throats and equators in the Jordan frame, while possessing only a single throat in the Einstein frame.

We consider a traversable wormhole solution of Einstein's gravity conformally coupled to a massless scalar field, a solution derived by Barcelo and Visser based on the JNWW spacetime. We study the geodesic motion of time-, light- and space-like particles in this spacetime. We solve the equations of motion analytically in terms of the Weierstra\ss \ functions and discuss all possible orbit types and their parameter dependence. Interestingly, bound orbits occur for time-like geodesics only in one of the two worlds. Moreover, under no conditions there exist time-like two world bound orbits., Comment: 18 pages, 23 figures

We demonstrate that rotating matter sets the throat of an Ellis wormhole into rotation, allowing for wormholes which possess full reflection symmetry with respect to the two asymptotically flat spacetime regions. We analyze the properties of this new type of rotating wormholes and show that the wormhole geometry can change from a single throat to a double throat configuration. We further discuss the ergoregions and the lightring structure of these wormholes., Comment: 8 pages, 5 figues

We show that under certain conditions an axisymmetric rotating spacetime contains a ring of points in the equatorial plane, where a particle at rest with respect to an asymptotic static observer remains at rest in a static orbit. We illustrate the emergence of such orbits for boson stars. Further examples are wormholes, hairy black holes and Kerr-Newman solutions.

We study compact configurations with a nontrivial wormholelike spacetime topology supported by a complex ghost scalar field with a quartic self-interaction. For this case, we obtain regular asymptotically flat equilibrium solutions possessing reflection symmetry. We then show their instability with respect to linear radial perturbations., Comment: 11 pages, 5 figures, minor corrections to content, new references added, version published in PRD

Axisymmetric rotating radially excited boson stars are analyzed. For several fixed parameter sets, the full sets of solutions are obtained. In contrast to the nodeless boson stars, the radially excited sets of solutions do not exhibit a spiraling behavior. Instead they form a loop, when the boson frequency is varied from its maximal value given by the boson mass to a minimal value and back. Thus for all allowed boundary data of the scalar field at the origin, there are two distinct solutions, except for the endpoints. While one endpoint corresponds to the trivial solution, the other one represents the most compact solution. The energy density and the pressures of the solutions are analyzed. A decomposition of the scalar field into spherical harmonics is performed, and stability considerations are presented., Comment: 23 pages, 18 figues

We consider the effect of scalarization on static and slowly rotating neutron stars for a wide variety of realistic equations of state, including pure nuclear matter, nuclear matter with hyperons, hybrid nuclear and quark matter, and pure quark matter. We analyze the onset of scalarization, presenting a universal relation for the critical coupling parameter versus compactness. We find that the onset and the magnitude of the scalarization are strongly correlated with the value of the gravitational potential (the metric component $g_{tt}$) at the center of the star. We also consider the moment-of-inertia--compactness relations and confirm universality for the nuclear matter, hyperon and hybrid equations of state., Comment: 17 pages, 9 figures, matches version published in PRD

Einstein-dilaton-Gauss-Bonnet gravity is a theoretically well-motivated alternative theory of gravity emerging as a low-energy 4-dimensional model from heterotic string theory. Its rotating black hole solutions are known numerically and can have macroscopic deviations from the Kerr black holes of Einstein's gravity. Einstein-dilaton-Gauss-Bonnet gravity can thus be tested with observations of astrophysical black holes. In the present paper, we simulate observations of the reflection spectrum of thin accretion disks with present and future X-ray facilities to understand whether X-ray reflection spectroscopy can distinguish the black holes in Einstein-dilaton-Gauss-Bonnet gravity from those in Einstein's gravity. We find that this is definitively out of reach for present X-ray missions, but it may be achieved with the next generation of facilities., Comment: 10 pages, 6 figures. v2: fixed some typos

When bosonic matter in the form of a complex scalar field is added to Ellis wormholes, the phenomenon of spontaneous symmetry breaking is observed. Symmetric solutions possess full reflection symmetry with respect to the radial coordinate of the two asymptotically flat spacetime regions connected by the wormhole, whereas asymmetric solutions do not possess this symmetry. Depending on the size of the throat, at bifurcation points pairs of asymmetric solutions arise from or merge with the symmetric solutions. These asymmetric solutions are energetically favoured. When the backreaction of the boson field is taken into account, this phenomenon is retained. Moreover, in a certain region of the solution space both symmetric and asymmetric solutions exhibit a transition from single throat to double throat configurations., Comment: 21 pages, 8 figures

We study the shadows of the fully non-linear, asymptotically flat Einstein-dilaton-Gauss-Bonnet (EdGB) black holes (BHs), for both static and rotating solutions. We find that, in all cases, these shadows are smaller than for comparable Kerr BHs, i.e. with the same total mass and angular momentum under similar observation conditions. In order to compare both cases we provide quantitative shadow parameters, observing in particular that the differences in the shadows mean radii are never larger than the percent level. Therefore, generically, EdGB BHs cannot be excluded by (near future) shadow observations alone. On the theoretical side, we find no clear signature of some exotic features of EdGB BHs on the corresponding shadows, such as the regions of negative (Komar, say) energy density outside the horizon. We speculate that this is due to the fact that the Komar energy interior to the light rings (or more precisely, the surfaces of constant radial coordinate that intersect the light rings in the equatorial plane) is always smaller than the ADM mass, and consequently the corresponding shadows are smaller than those of comparable Kerr BHs. The analysis herein provides a clear example that it is the light ring impact parameter, rather than its "size", that determines a BH shadow., Comment: 16 pages, 5 figures, Minor changes, version to published in Phys. Lett. B

Generalizations of the Schwarzschild and Kerr black holes are discussed in an astrophysically viable generalized theory of gravity, which includes higher curvature corrections in the form of the Gauss-Bonnet term, coupled to a dilaton. The angular momentum of these black holes can slightly exceed the Kerr bound. The location and the orbital frequency of particles in their innermost stable circular orbits can deviate significantly from the respective Kerr values. Study of the quasinormal modes of the static black holes gives strong evidence that they are mode stable against polar and axial perturbations. Future gravitational wave observations should improve the current bound on the Gauss-Bonnet coupling constant, based on observations of the low-mass x-ray binary A 0620-00., Comment: 8 pages, 7 figures

We here review asymptotically flat rotating black holes in the presence of non-Abelian gauge fields. Like their static counterparts these black holes are no longer uniquely determined by their global charges. In the case of pure SU(2) Yang-Mills fields, the rotation generically induces an electric charge, while the black holes do not carry a magnetic charge. When a Higgs field is coupled, rotating black holes with monopole hair arise in the case of a Higgs triplet, while in the presence of a complex Higgs doublet the black holes carry sphaleron hair. The inclusion of a dilaton allows for Smarr type mass formulae., Comment: 33 pages, 7 figures

We give a detailed account of the properties of spinning Ellis wormholes, supported by a phantom field. The general set of solutions depends on three parameters, associated with the size of the throat, the rotation and the symmetry of the solutions. For symmetric wormholes the global charges possess the same values in both asymptotic regions, while this is no longer the case for non-symmetric wormholes. We present mass formulae for these wormholes, study their quadrupole moments, and discuss the geometry of their throat and their ergoregion. We demonstrate, that these wormholes possess limiting configurations corresponding to an extremal Kerr black hole. Moreover, we analyze the geodesics of these wormholes, and show that they possess bound orbits., Comment: 24 pages, 17 figures

We show that the general framework proposed in arXiv:1410.0581 for the study of asymptotically flat vacuum black objects with $k+1$ equal magnitude angular momenta in $D\geq 5$ spacetime dimensions (with $0\leq k\leq \big[\frac{D-5}{2} \big]$) can be extended to the case of Einstein-Maxwell-dilaton (EMd) theory. This framework can describe black holes with spherical horizon topology, the simplest solutions corresponding to a class of electrically charged (dilatonic) Myers-Perry black holes. Balanced charged black objects with $ S^{n+1} \times S^{2k+1}$ horizon topology can also be studied (with $D=2k+n+4$). Black rings correspond to the case $k=0$, while the solutions with $k>0$ are black ringoids. The basic properties of EMd solutions are discussed for the special case of a Kaluza-Klein value of the dilaton coupling constant. We argue that all features of these solutions can be derived from those of the vacuum seed configurations., Comment: 16 pages, 7 figures; invited paper for the "Entropy" special issue "Black Hole Thermodynamics II", editor David Kubiznak

Recently, two of us have found numerically rotating Ellis wormholes as solutions of 4-dimensional Einstein gravity coupled to a phantom field. In this paper, we investigate possible observational signatures to identify similar objects in the Universe. These symmetric wormholes have a mass and are compact, so they may look like black holes. We study the iron line profile in the X-ray reflection spectrum of a thin accretion disk around rotating Ellis wormholes and we find some specific observational signatures that can be used to distinguish these objects from Kerr black holes. We simulate some observations with XIS/Suzaku assuming typical parameters for a bright AGN and we conclude that current X-ray missions cannot apply strong constraints., Comment: 10 pages, 6 figures. v2: refereed version with simulated observations

The last three years have again seen new exciting developments in the area of higher dimensional black objects. For black objects with noncompact higher dimensions, the solution space was exlored further within the blackfold approach and with numerical schemes, yielding a large variety of new families of solutions, while limiting procedures created so-called super-entropic black holes. Concerning compact extra dimensions, the sequences of static nonuniform black strings in five and six dimensions were extended to impressively large values of the nonuniformity parameter with extreme numerical precision, showing that an oscillating pattern arises for the mass, the area or the temperature, while approaching the conjectured double-cone merger solution. Besides the presentation of interesting new types of higherdimensional solutions, also their physical properties were addressed in this session. While the main focus was on Einstein gravity, a significant number of talks also covered Lovelock theories., Comment: Report on the session Black Holes in Higher Dimensions of the MG14 meeting; references added