### Conveners

#### Black Holes in Alternative Theories of Gravity: Block 1

- Jutta Kunz (University of Oldenburg)

#### Black Holes in Alternative Theories of Gravity: Block 2

- Jutta Kunz (University of Oldenburg)

### Description

Black holes are excellent probes to study strong gravitational fields and thus to test Einstein's theory of general relativity and its contenders by comparing their predictions with observations. Alternative theories of gravity may lead to distinctive features for black holes, which include their multipoles, quasi-normal modes or shadows.

We study asymptotically flat black holes with massive graviton hair within the ghost-free bigravity theory. There have been contradictory statements in the literature about their existence, but we have been able to construct such solutions within a carefully designed numerical scheme. The masses of stable hairy black holes that would be physically relevant range form stellar values up to...

As gravitational waves are now being nearly routinely measured with interferometers, the question

of using them to probe new physics becomes increasingly legitimate. In this article, we rely on a

well established framework to investigate how the complex frequencies of quasinormal modes are

affected by different models. The tendencies are explicitly shown for both the pulsation and...

The existence of black holes is one of the key predictions of general relativity (GR) and therefore a basic consistency test for modified theories of gravity. In spherical symmetry, only two classes of GR solutions are compatible with the formation of a regular apparent horizon in finite time of a distant observer. In this talk, I will demonstrate how to derive constraints that any...

As the electron in the hydrogen atom, a bosonic field can bind itself to a black hole occupying a discrete infinite set of states. When (i) the spacetime is prone to superradiance and (ii) a confinement mechanism is present, some of such states are infinitely long–lived. These equilibrium configurations, known as stationary clouds, are states “synchronized” with a rotating black hole's event...

From a phenomenological point of view, the singularities of ordinary black hole solutions hint at a breakdown of general relativity. The Bardeen black hole is a prototype of regular black hole solutions, i.e. those which are geodesically complete and avoid a curvature singularity.

The Bardeen solution is generally interpreted as a magnetically or electrically charged solution to gravity...

This abstract is primarily based on my paper EPJC 79 (2019) 877.

$f(R)$ gravity theory is one of the most popular alternate theories of gravity, which has been used to describe various phenomena in cosmology and astrophysics. In this talk, I will explore the properties of spacetime in the presence of $f(R)$ gravity. First, I will describe how it is used to obtain a spherically symmetric...

The evidence is mounting that the Universe is currently undergoing a phase of accelerated expansion. One possible alternative is the modification in gravity in the largest possible scales. This leads to the many questions related to black-holes: violation of Birkhoff theorem and no-hair theorem. To confirm/infirm, we need to obtain exact black-hole solutions in these modified gravity theories....

We examine if there are grounds to entertain the Penrose suggestion as to black holes from a prior cycle of creation appearing in the present cosmos. There are two cases to consider. One a singular start to the Universe or as Karen Freeze and others have modeled a non-singular start. The two cases are different and touch upon the limits of validity of the Penrose singularity theorem. We will...

Following the direct detection of gravitational waves, modification to General Relativity (GR) at strong gravity regimes is one of the most important aspects of gravity research. Chern Simons (CS) gravity is one of the most frequently studied parity-violating models of strong gravity. CS gravity is indistinguishable from GR for all conformally flat space-times and for space-times that possess...

It is shown that the Kerr solution exists in the generalized hybrid metric-Palatini gravity theory and that for certain choices of the function $f(R,\mathcal R)$ that characterizes the theory, the Kerr solution can be stable against perturbations on the scalar degree of freedom of the theory. We start by verifying which are the most general conditions on the function $f(R,\mathcal R)$ that...

We show that a magnetic charge in curved spacetime could be an artefact of a vacuum phase with zero metric determinant at a distance. This phase is characterized by a solution of the first order field equations with nontrivial torsion. The monopole charge has a topological origin, given precisely by a lower-dimensional counterpart of the Nieh-Yan invariant in absence of matter. In this...

We present several families of black hole solutions in metric-affine theories of gravity with independent metric and affine connection. We show that the resolution of the singularity problem is a common feature of many such solutions, and discuss several strategies in terms of their observational discriminators as compared to GR canonical objects.

The nonextremal Kerr black holes have been considered to be holographically dual to two-dimensional (2D) conformal field theories (CFTs). In this talk, we extend the holography to the case of an asymptotically anti--de Sitter (AdS) rotating charged black holes in f(T)=T+αT^2 gravity, where α is a constant. We find that the scalar wave radial equation at the near-horizon region implies the...

We consider gravitational collapse of a spherically symmetric sphere of a fluid with spin and torsion into a black hole. We use the Tolman metric and the Einstein-Cartan field equations with a relativistic spin fluid as a source. We show that gravitational repulsion of torsion prevents a singularity and replaces it with a nonsingular bounce. Quantum particle production during contraction...

Alternative theories of gravity and the parameterized deviation approach allow black hole solutions to have additional parameters beyond mass, charge and angular momentum. Matter fields could be, in principle, affected by the additional parameters of these solutions. We compute the absorption cross section of massless spin-0 waves by static Konoplya-Zhidenko black holes, characterized by a...

A certain type of matter with anisotropic pressures can add to the Reissner-Nordström metric a term proportional to a power of the radial coordinate. Using the standard method of separating variables for the Hamilton-Jacobi equation, we study the shadow of the corresponding rotating solution, obtained through the Newman-Janis algorithm. We define and calculate three observables in order to...

We show that a magnetic charge in curved spacetime could be an artefact of a vacuum phase with zero metric determinant at a distance. This phase is characterized by a solution of the first-order field equations with nontrivial torsion. The monopole charge has a topological origin, given precisely by a lower-dimensional counterpart of the Nieh-Yan invariant in absence of matter. In this...