Following the generalised form of non-stationary axisymmetric space-time (Chandrasekhar 1983), we assume a bifocale elliptic symmetry and attempt to show that this particular metric can be a solution to Einstein's field equations. We discuss the form of the metric and the curvature implied, as well as its possible physical meaning and applications.

Black-hole spectroscopy is arguably the most promising tool to test

gravity in extreme regimes and to probe the ultimate nature of black

holes with unparalleled precision. These tests are currently limited by

the lack of a ringdown parametrization that is both robust and accurate.

We develop an observable-based parametrization of the ringdown of

spinning black holes beyond general...

I will discuss an action principle for non-relativistic gravity, as has recently been obtained from a covariant large speed of light expansion of Einstein's theory of gravity. This action reproduces Newtonian gravity as a special case, but goes beyond it by allowing for gravitational time dilation while retaining a non-relativistic causal structure. As a consequence, it can be shown that the...

We present a smooth extension of the Schwarzschild exterior geometry, where the singular interior is superceded by a vacuum phase with vanishing metric determinant. Unlike the Kruskal-Szekeres continuation, this explicit solution to the first-order field equations in vacuum has no singularity in the curvature two-form fields, no horizon and no global time. The underlying non-analytic structure...

We discuss the weak gravity conjecture (WGC) from black hole entropy in the Einstein-Maxwell-dilaton system or string theory. The WGC is strongly motivated by theorems forbidding global symmetries which arise in the vanishing-charge limit, and implies the fact that not only all non-BPS black holes but also extremal one without supersymmetry should be able to decay. It is shown that the large...

We show that the non-minimal conformally-coupled (CC) scalar-tensor theory and the Palatini theory with kinetic coupling of the scalar to the Ricci tensor (PKC) are the same. This is demonstrated by showing that both theories coincide in the Einstein frame. Using this duality as generating technique, we construct the PKC counterpart to the BBMB black hole of the CC theory. It turns out to be...

In the last decade, many observations of bright quasars at $z > 5$, have revealed the existence of Supermassive Black Holes (SMBHs), giants of billion solar masses shining close to their Eddington limit. The mechanism of their

formation at these early epochs represents currently an open problem in galaxy evolution.

Several scenarios have been proposed to overcome this problem, such as the...

Neutron stars are one of the most compact and densest astrophysical objects known in nature, they result from the supernova explosion of a massive star. Many of the neutron stars have very strong magnetic fields, which lead to the emission of radio and X-ray radiation. This work is devoted to study the effects of strong magnetic fields in the structure of neutron stars, within the framework of...

Using concomitantly the Generalized Second Law of black hole thermodynamics and the holographic Bekenstein entropy bound embellished by Loop Quantum Gravity corrections to quantum black hole entropy, we show that the boundary area of the remnant from the binary black hole merger in GW150914 is bounded from below. This lower bound is more general than the bound from application of Hawking's...

Scalar fields can give rise to confined structures, such as Q-balls or boson

tars, which can serve as interesting models for cold dark-matter. The existence and stability of objects in a given theory is relevant for a wide range of topics, from planetary science to a description of fundamental particles. Taking as starting point a theory describing a time-dependent scalar field, in this...

We consider the interaction between a plane wave and a (counter-moving) black hole. We show that energy is transferred from the black hole to the wave, giving rise to a *negative* absorption cross-section. Moving black holes absorb radiation *and* deposit energy in external radiation. Due to this effect, a black hole hole of mass $M$ moving at relativistic speeds in a cold medium will appear...

In this talk I will present new solutions describing neutron stars and black holes in the tensor-multi-scalar theories of gravity. Some astrophysical implications of the solutions will be also discussed.

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. 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.

The study of the effects of higher curvature terms is a major research theme of contemporary gravitational physics. In this talk, I will present a comprehensive study of the higher curvature gravity and various observational & theoretical constraints. The inclusion of these terms leads to exciting new possibilities, e.g., gravitational and electromagnetic perturbations following different...

Einstein’s theory has been the standard theory of gravity for nearly a century. Alternatives to and extensions of it have been proposed to address various issues. With advances in technology, these theories are becoming testable, especially in the strong field regime around black holes. In this talk, I will describe a theory agnostic approach to probe the nature of black holes. I will provide...

The experimental confirmation of the polarization of the Lambda hyperons observed in relativistic heavy ion collisions experiments [1] has renewed the interest in anomalous transport of fermions due to the spin-orbit coupling (e.g., through the chiral vortical effect [2]). Using a non-perturbative technique [3], exact expressions are derived for the thermal expectation values of the...

We generalize the recent derivation http://arxiv.org/abs/arXiv:1908.10617 of the Smarr formulas for Einstein-Maxwell stationary axisymmetric asymptotically locally flat spacetimes with line singularities to the Einstein-Maxwell-dilaton (EMD) theory with an arbitrary dilaton coupling constant. The line singularities include the Dirac and Misner strings for spacetimes with magnetic and NUT...

Kerr black holes are known to support massive bosonic test fields whose phase angular velocity fulfills the synchronization condition, i.e. the threshold of superradiance. The presence of these real-frequency bound states at the linear level, commonly dubbed stationary clouds, is intimately linked to existence of Kerr black holes with bosonic hair at the non-linear level. These configurations...

The strong cosmic censorship conjecture has recently regained a lot of attention in charged and rotating black holes immersed in de Sitter space. Such spacetimes possess Cauchy horizons in the internal region of the black hole. The stability of Cauchy horizons is intrinsically connected to the decay of small perturbations

exterior to the event horizon. As such, the validity of strong cosmic...

We use loop quantum gravity inspired holographic thermal stability criteria to establish the existence of regions in parameter space of charged rotating black holes away from extremality, where partial fulfillment of the stability criteria is possible. Physical implications of our results will be discussed.

Axion and axion-like particle are the candidates of the dark matter. Due to the super-radiant instability, these fields are amplified, and can localize around Kerr BH, as axion clouds. Since the axion clouds emit gravitational waves, it is important to analyze several properties of the axion cloud around BHs. Here, we study the axion cloud around binary BH (BBH). The axion cloud around a BH of...

Historically the Teukolsky equation corresponding to gravitational perturbations is solved for the Weyl scalars. However, reconstructing the metric from these scalars involves solving a fourth order PDE to obtain the Hertz potential and then another second order PDE to construct the metric perturbation. Solving the (adjoint) Teukolsky equation for the Hertz potential directly simplifies the...

We are illustrating a procedure for computing the total probabilities corresponding to the processes of fermion pair production in electric fields and in the field of a magnetic dipole on de Sitter space-time. The total probabilities are preserving the dependence on the expansion parameter, proving the fact that the results are consistent with the ones obtained for probability densities. The...

We propose, as a novelty in the literature, the modelling of wormholes within the particular case of the $f(R,T)$ gravity, namely $f(R,T)=R+\alpha R^{2}+\lambda T$, with $R$ and $T$ being the Ricci scalar and trace of the energy-momentum tensor, respectively, while $\alpha$ and $\lambda$ are constants. Although such a functional form application can be found in the literature, those concern to...