It is well known that the Einstein equations can be interpreted as a system of PDEs describing a geometric flow - a continuous family of Riemannian metrics on a given topological manifold.

In this perspective the global structure of solutions to the Einstein equations is analysed using this geometric flow to obtain information on the asymptotic behaviour towards future and past. In this talk...

In this talk, we investigate the gravitational collapse of the Oppenheimer-Snyder dust cloud with spatially constant matter density from a quasi-local perspective. Given a closed two-surface within the star, three versions of the quasi-local energy are discussed.

This talk is based on a recent paper with Xiaokai He [Class. Quantum Grav. 37(2020)185016, arXiv:2005.04659].

I will show how to generalize classical results on linear metric perturbations to any higher orders of perturbation expansion and discuss some possible applications.

During the talk the course and results of gravitational collapse within the Higgs-dark matter sector using the double null formalism will be presented. The employed model consists of two scalar fields non-minimally coupled to gravity, one of which is charged under a U(1) gauge field and represents a stable dark matter candidate. The uncharged scalar may represent a real part of the Higgs...

The problem of quasilocal mass has been extensively studied mainly in four dimensions. Here we report results regarding several quasilocal mass proposals in spacetime dimensions $n \geq 4$. After generalising three distinct quasilocal mass definitions to higher dimensions under appropriate assumptions, we evaluate their small sphere limits along lightcone cuts shrinking towards the lightcone...

The problem of integrals of the motion for the conformal Killing fields in curved space-times equipped with electromagnetic backgrounds will be analysed. In particular, for the pp-wave spacetimes the explicit form of conserved charges will be presented. Relations between these charges and symmetries will be discussed in various Lagrangian and Hamiltonian approaches.

Spacetimes with NUT parameter are known to posses a string-like conical singularity. We present a method for obtaining non-singular Kerr-NUT spacetimes with an arbitrary cosmological constant, via an analogue of the Misner interpretation of Taub-NUT spacetimes. Among the non-singular solution there is a class for which also one of the horizons is projectively non-singular, i.e. its space of...

The widely accepted ADM expression for the energy of an asymptotically flat spacetime satisfies a "natural" consistency test - its second variation is equal to the canonical hamiltonian functional for linearized gravity on a Minkowski background. A viable quasi-local mass candidate should posses a similar property, namely - its second-order approximation should equal the hamiltonian of the...

A family of global shock-wave solutions of the Einstein field equations for a perfect fluid are constructed. These shock-wave solutions consist of an interior self-similar expanding wave and an exterior self-similar static wave, separated by a spherical shock surface. The interior and exterior fluids are assumed to have isothermal equations of state of the form $p = \sigma\rho$ and $\bar{p} =...

I will present the ${\it No Hair}$ result for the solution of Einstein equations describing the non-vacuum regular interior and the vacuum exterior of a spinning black hole. There are only two parameters characterizing the vacuum exterior and two parameters for the non-vacuum regular interior of a spinning black hole.

These two sets of two parameters are connected by the matching condition...

Supermassive black hole mergers are one of the most dramatic phenomena in the Universe. For a few hours, they can emit as much power in gravitational waves as all the stars in the Universe produce in light. Moreover, they are an important element in determining the mass distribution of the entire population of supermassive black holes. However, none has yet been caught in the act, in large...

Neutron stars are an extraordinary laboratory in which to study matter in extreme conditions of density, magnetic energy and gravity. Modelling the interior of these objects requires an understanding of high density physics and phenomena such as superfluidity, which are well known in laboratory settings, but must now be understood in a strong gravity, relativistic setting. This issue is...

GRBs are panchromatic events, very attractive sources of study from very high energy (GeV and TeV) to very low frequency until radio. This gives the unique opportunity to study their emission mechanism in multiwavelength. Relevant cases are the GRB 190114C observed by the Major Atmospheric Gamma Imaging Cherenkov telescopes detected above 0.2 TeV, recording the most energetic photons ever...

The talk will cover the recent numerical investigation of a system composed of a Supermassive Black Hole Binary (SMBHB) and a non-self-gravitating, thin, locally isothermal, viscous disk.

The evolution of such a configuration is relevant not only for the expected gravitational-wave signal, but also for electromagnetic searches for SMBHB candidates. In 2-dimensional, Newtonian, numerical...

Recently, Peissker, Eckart, Zajacek et al. (2020) have reported the discovery of six faint stars in the innermost cluster of the Galaxy, so-called S cluster. These stars, S4711-S4715 and S62, can be monitored in the near-infrared K-band using both photometry and spectroscopy. Their elliptical orbits around the supermassive black hole (Sgr A*) break several records. S4711 with the orbital...

Accretion disks in High mass X-ray binaries (HMXB's) are mostly fed by the stellar wind from there companion star. These winds also affect the observed X-ray spectra arising from the hot coronal flow.

Cygnus X-1 and its companion star, HDE-226868 is one of such HMXBs. It is one of the brightest X-ray sources observed and shows the X-ray intensity variations in both the soft and hard X-rays....

Force-free electrodynamics is a non-linear regime of Maxwell's equations capable to provide the minimal non-trivial level of description for pulsar and black hole magnetospheres. For this system to be hyperbolic it is necessary that the field is magnetically dominated, F^2=B^2-E^2>0. Despite its crucial role in explaining energy and angular momentum extraction from slowly spinning black holes...

Recent observation of the shadow of the Messier 87 (M87) galactic center by the Event Horizon Telescope (EHT) group has triggered a great interest to investigate the causal structure of spacetime around the galactic center. From the recent investigations on gravitational lensing, it is now established that the shadow is not the signature of a black hole alone, it can also be cast by timelike...

In the current new era for cosmological observations, much work is devoted in investigating the possibility of measuring new effects like the secular changes of optical observables (known as drift effects). In this context, the new bi-local geodesic operators (BGO) formalism of light propagation provides a unified framework in which it is possible to describe all possible optical phenomena...

This talk is based on the paper “Self-gravitating perfect-fluid tori around black holes: Bifurcations, ergoregions, and geometrical properties” by W. Dyba, W. Kulczycki, P. Mach, Phys. Rev. D (2020).

We investigate numerical solutions of Einstein field equations corresponding to a stationary, axial symmetric spacetime containing a self-gravitating perfect-fluid torus rotating around a black...

The Hamiltonian energy, and its flux, of weak gravitational waves on a de Sitter background will be discussed. A new renormalized energy will be proposed. Used asymptotic conditions on the linearized metric have been modeled on the asymptotic behavior of the full solutions of the Einstein equations with positive cosmological constant. Considered space of solutions is greater than the solutions...

Gravitational waves from supermassive black hole binary mergers along with an electromagnetic counterpart has the potential to shed `light' on the nature of dark energy in the intermediate redshift regime. An accurate measurement of dark energy parameters at intermediate redshift is extremely essential to improve our understanding of dark energy, and to possibly resolve couple of tensions...

A new exploration of the Universe has recently started through gravitational-wave observations. On August 17, 2017, the first observation of gravitational waves from the inspiral and merger of a binary neutron-star system by the Advanced LIGO and Virgo network, followed 1.7 s later by a weak short gamma-ray burst detected by the Fermi and INTEGRAL satellites initiated the most extensive...

I will report on recent developments of TEOBResumS, an effective-one-body based waveform model for coalescing relativistic binaries. In particular, I will discuss: (i) spin-aligned quasi-circular binaries; (ii) spin-aligned eccentric binaries; (iii) hyperbolic encounters and dynamical capture.

The detection by LIGO and Virgo of gravitational waves from compact binary coalescences has given us access to the genuinely strong-field, dynamical regime of gravity, enabling tests of general relativity in the fully nonlinear domain. Moreover, the nature of gravitational waves can be tested by looking at the way they propagate over large distances. As the sensitivity of the detectors...

The canonical formalism by Arnowitt, Deser and Misner has proven very efficient in the derivation of higher order post-Newtonian (PN) dynamics of compact binaries in general relativity (GR), also including bodies with spin. There is ongoing strong impact on the gravitational wave research, in particular through its offspring, the effective-one-body (EOB) approach. The complete 3PN spinless...

We study the spin-evolution and gravitational-wave luminosity of a newly-born magnetar with an initial spin period of 1 ms and having an inclination α between the magnetic moment axis and the rotation axis. Given any random initial choice for the inclination, we always find α → 90 ◦ in a few milliseconds. As the star rotates under the influence of magnetic dipole radiation and the escaping...

The phenomena of standing waves are mostly studied in the context of mechanical or electromagnetic waves. In the context of General Relativity, the issue of how to define standing gravitational waves was addressed by Bondi and later by Stefani. We investigate an expanding universe filled with standing gravitational waves. We study how freely falling particles in this spacetime behave, namely,...

I will discuss local field theories with global degrees of freedom. The oldest of them is the so-called unimodular gravity introduced by Einstein a century ago. In this theory the cosmological constant is not a constant of nature, but merely a constant of integration. This provides an ideal landscape or ensemble of theories giving a different view on the origin of naturalness issues in modern physics.

The NANOGrav Collaboration has recently reported strong evidence for a stochastic common-spectrum process, which we interpret as a SGWB in the framework of cosmic strings. The possible NANOGrav signal would correspond to a string tension Gμ∈(4×10^{−11},10^{−10}) at the 68% confidence level, with a different frequency dependence from supermassive black hole mergers. The SGWB produced by cosmic...

We study non-minimal Coleman-Weinberg inflation in the Palatini formulation of gravity in the presence of an $R^2$ term. The Planck scale is dynamically generated by the vacuum expectation value of the inflaton via its non-minimal coupling to the curvature scalar $R$. We show that the addition of the $R^2$ term in Palatini gravity makes non-minimal Coleman-Weinberg inflation again compatible...

The conservation law for the total (orbital and spin) angular momentum of a Dirac particle in the presence of gravity requires that spacetime is not only curved, but also has a nonzero torsion. The coupling between the spin and torsion in the Einstein-Cartan theory of gravity generates gravitational repulsion at extremely high densities. We consider gravitational collapse of a spin-fluid...

I would like to present application of the recently proposed curved spacetime Effective Field Theory (cEFT) to a problem of vacuum stability. To model the matter sector we used two scalar fields coupled through the Higgs portal type of interaction. Additionally, both of these fields were coupled non-minimally to gravity. This may be considered as a simplified model describing an interaction...

Production of Z bosons in emission processes by neutrinos in the expanding de Sitter universe is studied. We use perturbative methods to investigate emission processes that are forbidden in flat spacetime electroweak theory by the energy and momentum conservation. The amplitude and probability for the spontaneous emission of a Z boson by a neutrino or an antineutrino are computed analytically,...

We construct models with the Gauss-Bonnet term multiplied to a function of the scalar field leading to inflationary scenario. The consideration is related with the slow-roll approximation. The cosmological attractor approach gives the spectral index of scalar perturbations which is in a good agreement with modern observation and allows variability for tensor-to-scalar ratio. We reconstruct...

What happens at the evaporation of a black of a black hole is not only still unclear, but also -- contrary to what is sometimes claimed -- relevant for the issue of unitarity in blackhole physics. I illustrate what we know about this physics. I show that the problem breaks into three distinct processes, to some extent independent: the quantum physics of the matter reaching the black hole...

Causal Dynamical Triangulations (CDT) is a background independent approach to quantum gravity which introduces a lattice regularization. The framework uses only geometric invariants without referring to any coordinate system.

One of its features is the ability to control the topology of the Universe. The introduction of toroidal spatial topology allows for a definition of hypersurfaces which...

A space of quantum states and an algebra of quantum observables are constructed over the set of all metrics of arbitrary but fixed signature, defined on a manifold. The construction is diffeomorphism invariant, and unique up to natural isomorphisms.

In this talk, I will present the first construction of the theory of quantum gravity completely free of UV divergences. I will show the successful construction in the framework of quantum field theory of gravitational interactions. Moreover, I will relate the enhanced symmetry properties of this theory with fixed points of RG flow and ensuing quantum conformal invariance. The implications for...

The spectral dimension is one of definitions of the effective dimensionality of spacetime that is commonly applied to characterize quantum gravity models. A quite universal prediction is the dimensional reduction to 2 in the UV regime. The notion of spectral dimension can be seen as arising from properties of either a (fictitious) diffusion process or spectral geometry. In the latter context,...

The interior of a Schwarzschild black hole is quantized by the method of loop quantum gravity. The Hamiltonian constraint is solved and the physical Hilbert space is obtained in the model. The properties of a Dirac observable corresponding to the Arnowitt-Deser-Misner mass of the Schwarzschild black hole are studied by both analytical and numerical techniques. It turns out that zero is not in...

I will present quantum model of the dynamics underlying the Belinski-Khalatnikov-Lifshitz (BKL) scenario. The classical BKL scenario concerns generic singularity of general relativity. The quantum BKL scenario indicates that the gravitational singularity can be avoided by a quantum bounce. The latter presents a unitary evolution of considered gravitational system. It is fairly probable that...

It is well-known that quantum groups are relevant to describe the quantum regime of 3d gravity. They encode a deformation of the gauge symmetries (Lorentz symmetries) parametrized by the value of the cosmological constant. Such deformation might be perplexing from a classical picture since the action is defined in terms of plain/undeformed gauge symmetry. I would like to present here a novel...

The identification of black hole event horizon area with entropy by Bekenstein and Hawking suggested a generalized second law of thermodynamics. This was later extended to cosmological horizons. However, there remain some deep unsolved problems about the validity of the generalized law in the cosmological case, and unanswered questions about whether a more comprehensive notion of gravitational...

Gravitational physics is arguably better understood in the presence of a negative cosmological constant than a positive one, yet there exist strong technical similarities between the two settings. These similarities can be exploited to enhance our understanding of the more speculative realm of quantum cosmology, building on robust results regarding anti-de Sitter black holes describing the...

I will present how pre-main sequence and low-mass stars' (M<0.5 solar mass) can be a laboratory for constraining theories of gravity. We will discuss modified gravity impacts on their early evolution, such as Hayashi tracks and radiative core development, together with the measured quantities such as effective temperature, masses, and luminosities as well as fusion of light elements.

I will discuss the AdS/CFT correspondence with finite cut-off and TT-deformations of holographic conformal field theories. After introduction, I will present some recent results for the deformations of 2d conformal field theories on curved backgrounds and their holographic realization in Anti-de Sitter geometries in 3 spacetime dimensions.

Cyclic universes with bouncing solutions are candidates for solving the big bang initial singularity problem. Here I will look for bouncing solutions in the context of modified Gauss-Bonnet gravity theories whose field equations contain up to fourth-order derivatives of the metric tensor. In finding such bouncing solutions I will resort to an order reduction technique that reduces the order of...

We consider a recent proposal to obtain a finite contribution of second derivative order to the gravitational field equations in $D = 4$ dimensions from a renormalized Gauss-Bonnet term in the action. In previous works, it has been shown that the resulting field equations cannot be obtained as the Euler-Lagrange equations from a diffeomorphism-invariant action. Here we use techniques from the...

In this talk, I summarize the process of investigating neutrino oscillations within the framework of extended theories of gravity. Based on the covariant reformulation of Pontecorvo’s formalism, the oscillation probability of neutrinos propagating in static spacetimes described by gravitational actions quadratic in the curvature invariants is evaluated. For the sake of simplicity, calculations...

Horndeski gravity is the most general scalar tensor theory, with a single scalar field, leading to second-order field equations and after the GW170817 it has been severely constrained. In this talk, I will present an analog of Horndeski’s theory in the Teleparallel Gravity framework where gravity is mediated through torsion instead of curvature. It will be shown that, even though, many terms...

We present a formalism which allows for the perturbative derivation of the Extended Uncertainty Principle for arbitrary spatial curvature models. The leading curvature induced correction is proportional to the Ricci scalar evaluated at the expectation value of the position operator. By Born reciprocity this method can be equivalently applied in curved momentum space allowing for a general...

In the recent article Phys. Rev. D 100, no. 4, 043533 (2019) a compact phase space generalization of the flat de Sitter cosmology has been proposed. The main advantages of the compactification is that physical quantities are bounded, and the quantum theory is characterized by finite dimensional Hilbert space. The purpose of this presentation is to discuss the extraction of semiclassical...

The full theory of LQG presents enormous challenge to create physical computable models. In this talk we will present the new modern version of Quantume Reduced Loop Gravity (QRLG). We will show that this framework provide an arena to study the full LQG in a certain limit, where the quantum computations are possible. We will analyze all the major step necessary to build this framework, how is...

In this talk I will present a new model for the description of a gravitating kinetic gas, by coupling the 1-particle distribution function (1PDF) of the gas directly to the gravitational field, lifted to the tangent bundle of spacetime. This procedure takes the influence of the velocity distribution of the kinetic gas particles on their gravitational field fully into account, instead of only...

In this short talk I will briefly discuss the hairy black hole solutions obtained within Einstein-Maxwell-Scalar theory with so called box boundary conditions and the influence of dark sector fields on the modelled system. The phase diagram of the theory will be presented with the explanation of the influence of the hidden sector on the thermodynamics of particular phases (hairy black holes,...