We study RNAdS black holes in the presence of quintessence matter. We find that the presence of quintessence influences significantly the metric function and the Hawking temperature. For this kind of black holes phase transitions are present. We spot the phase transition points using the corresponding heat capacities, as well as the Helmholtz free energy for the canonical thermodynamic ensemble. Calculating the Regge and Wheeler potential for scalar perturbations, we find out that no bound states are present for stable RNAdS black holes in the presence of quintessence matter.

Greece

George Koutsoumbas

Andri Machattou

Eleftherios Papantonopoulos

Formation of bound states in quintessence alternative theories

In this work, we study nonconformally Ricci-flat gravitational instantons in four-dimensional conformal gravity, both in vacuum and in the presence of nonlinear conformal matter. First, the one-parameter extension of the Kerr-NUT-AdS metric is analyzed. We obtain their conserved charges by using the Noether-Wald formalism. It turns out that they receive corrections from the linear modes present in conformal gravity, which are properly identified. Then, we perform the analytic continuation into the Euclidean section and find the curve in parameter space along which this solution becomes regular and globally (anti)self-dual. Using the Dunajski-Tod theorem, we show that the self-dual metric is not conformally Ricci-flat. Then, the backreaction of nonlinear conformal matter is considered. In particular, we find new gravitational instantons in the presence of conformally coupled scalar fields and ModMax electrodynamics. We compute the partition function and conserved charges, which turn out to be finite by virtue of the conformal invariance of the theory. As a byproduct, we also obtain a generalization of the Riegert metric dressed with nonlinear conformal matter as a particular limit of these instantons. For all cases, we analyze the global properties, the curve in parameter space where the solutions are (anti)-self-dual, and the on-shell Euclidean action, among other features.

Chile

Cristóbal Corral

Borja Diez

Nonconformally Ricci-flat instantons in conformal gravity with and without nonlinear matter fields

We study the motion of particles in the background of a scalar–tensor theory of gravity in which the scalar field is kinetically coupled to the Einstein tensor, and we present the null geodesic structure for asymptotically flat, AdS, and dS Horndeski black holes, studying the effect of the cosmological constant on the orbits. Also, we consider three classical tests of gravity in the solar system, namely the bending of the light, the gravitational redshift, and the Shapiro time delay, in order to constraint the coupling parameters of the scalar field to gravity. Calculating the Lyapunov exponent, we explore the stability of these geodesics for various values of the cosmological constant.

D. Carvajal

P. González

Marco Olivares

Yerko Vásquez

Study of null geodesics and their stability in Horndeski black holes

Using Euclidean methods we investigate the thermodynamics of stealth black hole solutions, which are solutions that geometrically are indistinguishable from those of general relativity; however, they are accompanied by a nontrivial additional field that does not backreact to the metric. We find that in general, one can observe shifts in the mass and/or the entropy of such black holes; hence, at least at the thermodynamic level, the stealth solutions may be distinguished by those of general relativity. We also point out an example, the bona fide stealth, where a nontrivial scalar field can accompany an (A)dS-Schwarzschild black hole without altering the thermodynamic quantities. We point out that thermodynamic discrepancies arise due to the additional boundary terms emanating from the stealth fields, where care should be given in order for the theory at hand to possess a well-defined variational procedure.

Athanasios Bakopoulos

Thanasis Karakasis

Thermodynamics of stealth black holes

We consider higher-order derivative gauge field corrections that arise in the fundamental context of dimensional reduction of string theory and Lovelock-inspired gravities and obtain an exact and asymptotically flat black hole solution, in the presence of nontrivial dilaton configurations. Specifically, by considering the gravitational theory of Euler-Heisenberg nonlinear electrodynamics coupled to a dilaton field with specific coupling functions, we perform an extensive analysis of the characteristics of the black hole, including its geodesics for massive particles, the energy conditions, thermodynamical and stability analysis. The inclusion of a dilaton scalar potential in the action can also give rise to asymptotically (anti) de Sitter spacetimes and an effective cosmological constant. Moreover, we find that the black hole can be thermodynamically favored when compared to the Gibbons-Maeda-Garfinkle-Horowitz-Strominger black hole for those parameters of the model that lead to a larger black hole horizon for the same mass. Finally, it is observed that the energy conditions of the obtained black hole are indeed satisfied, further validating the robustness of the solution within the theoretical framework, but also implying that this self-gravitating dilaton-nonlinear-electrodynamics system constitutes another explicit example of bypassing modern versions of the no-hair theorem without any violation of the energy conditions.

United Kingdom

Nick E. Mavromatos

Theodoros Nakas

Exact black holes in string-inspired Euler-Heisenberg theory

We study the motion of uncharged particles and photons in the background of a magnetically charged Euler–Heisenberg (EH) black hole (BH) with scalar hair. The spacetime can be asymptotically (A)dS or flat. After investigating particle motions around the BH and the behavior of the effective potential of the particle radial motion, we determine the contribution of the BH parameters to the geodesics. Photons follow null geodesics of an effective geometry induced by the corrections of the EH non-linear electrodynamics. Thus, after determining the effective geometry, we calculate the shadow of the BH. We also analyze in detail the case of extremal BH spacetimes and we find that the radius of both the event horizon and shadow exhibits discontinuities when varying with respect to either the magnetic charge or the scalar charge. These extremal scenarios exist for sufficiently small values of the EH parameter, and imply the formation of a second photon ring around the BH shadow. Upon comparing the theoretically calculated BH shadow to the images of the shadows of M87* and Sgr A* observed by the Event Horizon Telescope collaboration, we impose constraints on the BH parameters, namely the scalar hair ( $$\nu $$ <math> <mi>ν</mi> </math> ), the magnetic charge ( $$Q_{m}$$ <math> <msub> <mi>Q</mi> <mi>m</mi> </msub> </math> ) and the EH parameter ( $$\alpha $$ <math> <mi>α</mi> </math> ). For sufficiently large values of the EH parameter, our model is in agreement with the EHT observations for approximately every value for the BH magnetic charge.

Dionysios Theodosopoulos

Motion of particles around a magnetically charged Euler–Heisenberg black hole with scalar hair and the Event Horizon Telescope

We consider scalar field perturbations in the background of black holes immersed in perfect fluid dark matter (PFDM). We find, by using the sixth-order Wentzel–Kramers–Brillouin (WKB) approximation, that the longest-lived modes are those with a higher angular number for a scalar field mass smaller than a critical value, known as the anomalous decay rate of the quasinormal modes, while beyond this critical value the opposite behavior is observed. Moreover, we show that it is possible to recover the real part of the quasinormal frequencies (QNFs), the imaginary part of the QNFs, and the critical scalar field mass of the Schwarzschild background for different values of the PFDM intensity parameter k. For values of k smaller than these values, the abovementioned quantities are greater than the Schwarzschild background. However, beyond these values of k, these quantities are smaller than the Schwarzschild background.

Ramón Bécar

Massive scalar field perturbations of black holes surrounded by dark matter

We explore the consistent truncation of conserved charges in quadratic curvature gravity (QCG) with anti–de Sitter asymptotics to the linear order in the Weyl tensor. The QCG action is given by the most general curvature-squared corrections to Einstein gravity, and it is suitably rendered finite by the addition of extrinsic counterterms (Kounterterms). The conserved charges derived from this action are, as a consequence, nonlinear in the spacetime Riemann tensor. A detailed analysis of the falloff of generic static solutions leads to a charge proportional to the electric part of the Weyl tensor, without loss of information on the energy of the system. The procedure followed provides firmer ground to the extension of the notion of Conformal Mass to higher-curvature gravity, as it appears as associated to a renormalized action. We observe that criticality condition in QCG poses an obstruction to the charge linearization, in contrast to previous results in Lovelock gravity, where degeneracy condition plays a key role.

Olivera Miskovic

Rodrigo Olea

Yoel Parra-Cisterna

Asymptotic black holes and conformal mass in AdS quadratic curvature gravity

In the framework of the dynamical Chern–Simons gravity, we study the scalar field perturbations of the Reissner–Nordström–Melvin spacetime, which describes a charged black hole permeated by a uniform magnetic field. In the presence of the magnetic field, the scalar field acquires an effective mass whose square takes negative value in the half domain of the angular direction. This inevitably introduces the tachyonic instability and associated spontaneous scalarization as long as the coupling constant between the scalar field and the Chern–Simons invariant exceeds a threshold value. We study the object pictures of the time evolutions of the scalar field perturbations at the linear level, and find that the presence of the magnetic field will dramatically change the waveforms and associated ringdown modes. Nonlinear evolutions for the unstable perturbations are also performed in the decoupling limit, which demonstrate the scalar cloud as the final fate. Influences of the coupling constant and the black hole charge on the wave dynamics are also studied.

China

Shao-Jun Zhang

Bin Wang

United States

Anzhong Wang

Magnetic-induced spontaneous scalarization in dynamical Chern–Simons gravity

The anomalous decay rate of the quasinormal modes occurs when the longest-lived modes are the ones with higher angular number. Such behaviour has been recently studied in different static spacetimes, for uncharged scalar and fermionic perturbations, being observed in both cases. In this work we consider the propagation of charged massive scalar fields in the background of Reissner-Nordström-de Sitter black holes and we mainly study the effect of the scalar field charge in the spectrum of quasinormal frequencies, as well as, its effect on the anomalous decay rate. Mainly, we show that the anomalous behaviour is present for massive charged scalar fields as well, and a critical value of scalar field mass exists, beyond which the behaviour is inverted. However, there is also a critical value of the parameter qQ of the charge of the scalar field and of the charge of the black hole, which increases when the cosmological constant increases, and beyond the critical value the anomalous behaviour of the decay rate could be avoided for the fundamental mode.