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19/12/2022, 09:15
In 1972 Bekenstein put forward the idea that a black has an entropy S proportional to its area A in Planck units. Bekenstein’s black hole entropy was published in Nuovo Cimento. Hawking, developing quantum field theory calculations in curved spacetime, soon showed that the constant of proportionality in the entropy formula is one quarter, so that S = A/4, and that indeed a black hole has...
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19/12/2022, 09:30
Using the covariant 1+3 threading spacetime decomposition, we discuss solutions of the Einstein-Cartan theory sourced by a cosmological perfect fluid composed by particles with intrinsic spin. It is shown that, even in the presence of intrinsic spin of matter, the metric tensor is described by a general FLRW solution, however the Weyl tensor might not vanish. The coupling between the intrinsic...
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19/12/2022, 09:45
We discuss and implement a spectral method approach to computing stationary and axisymmetric black hole solutions and their properties in modified theories of gravity. The resulting code is written in the Julia language and is transparent and easily adapted to new settings. We test the code on both general relativity and on Einstein-Scalar-Gauss-Bonnet gravity. It is accurate and fast,...
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19/12/2022, 10:00
We present results on the encounter of two black holes that are initially on a hyperbolic-like orbit simulated with the numerical relativity code SpEC. We discuss the dynamical capture scenario as well as the scattering scenario, in which the black holes remain unbound during the encounter. In dynamical captures the binary becomes bound due to the emission of gravitational waves, which poses a...
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19/12/2022, 10:15
We propose a method to determine the mass and spin of primordial black holes (PBHs) in the mass range 5 × 10^7 − 10^12 kg (Hawking temperatures ∼ 10 MeV – 200 GeV), based on measuring the energy of specific features in the photon Hawking emission spectrum, including both primary and secondary components. This is motivated by scenarios where PBHs in this mass range spin up as they evaporate,...
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19/12/2022, 11:00
Ultracompact objects with light-rings (LRs) but without an event horizon could mimic black holes (BHs) in their strong gravity phenomenology. But are such objects dynamically viable? Stationary and ax- isymmetric ultracompact objects that can form from smooth, quasi-Minkowski initial data must have at least one stable LR, which has been argued to trigger a spacetime instability; but its...
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19/12/2022, 11:15
Generic equilibrium black holes (BHs) and ultra-compact horizonless objects can admit a special set of planar and circular bound null orbits: Light Rings (LRs). These orbits are deeply connected to strong gravitational lensing. For instance, in spherical symmetry, LRs completely determines the photon spheres and the BH shadow image.
In generic equilibrium spacetimes, non-planar bound photon...
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19/12/2022, 11:30
In this talk, I will consider quantum effects on some specific black hole solutions and take into account their gravitational backreaction. In particular, I will describe the holographic construction of the quan- tum BTZ black hole (quBTZ) from an exact four-dimensional bulk solution.
I will present some of the thermodynamic properties of these black holes, focus on the generalized first...
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19/12/2022, 11:45
In the presence of certain non-minimal couplings between a scalar field and the Gauss-Bonnet curvature invariant, Kerr black holes can scalarize, as long as they are spinning fast enough. This provides a distinctive violation of the Kerr hypothesis, occurring only for some high spin range. Motivated by the fact that self-gravitating magnetic fields, by themselves, can promote “spin-induced”...
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19/12/2022, 12:00
In this presentation I will introduce a rigorous and general framework to study systematically self- gravitating elastic materials within general relativity. I will introduce two classes of elastic equations of state that can be used to explore properties of exotic compact objects. I will discuss the general effects of elasticity on the structure of compact objects, including an analysis on...
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19/12/2022, 12:15
Black hole spectroscopy is as a powerful approach to extract space- time information from gravitational wave observed signals. However, quasinormal mode (QNM) spectral instability under high wave- number perturbations has been recently shown to be a common classical general relativistic phenomenon. I will discuss these recent results on the stability of QNM in asymptotically flat black hole...
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19/12/2022, 12:30
GRAVITY is the most advanced optical-infrared instrument in a ground-based observatory. It sharpens the light of four giant 8 m telescopes with advanced adaptive optics and combines it interferometrically, achieving exquisite precision. It was central to the experimental breakthrough justifying part of the 2020 Physics Nobel Prize.
We will conduct a review of past achievements of the...
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19/12/2022, 14:30
The motion of S2, one of the stars closest to the Galactic Center, has been well studied and used to set constraints on the compact object at the center of the Milky Way. This central object is well established to be a supermassive black hole but the nature of its environment is unknown. Here, we investigate the possibility that dark matter in the form of an ultralight scalar field clusters...
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19/12/2022, 14:30
The local thermodynamic stability of a black hole (BH) in the canonical ensemble is defined by the positivity of the specific heat at constant global charges. Schwarzschild BHs in thermal equilibrium with the environment are always unstable against small fluctuations of energy, whereas Reissner-Nordström/Kerr BHs with sufficiently large specific electric charge/angular momentum are stable. One...
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19/12/2022, 14:45
Gravitational waves (GWs) carry linear momentum. As a consequence, due to the asymmetry of the GW emission, the remnant of a black hole of a black-hole merger can inherit a recoil speed, known as “kick”, which can exceed the scape velocity of the host environment. The magnitude of a black-hole kick has fundamental implications in black-hole formation and while its direction is key to...
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19/12/2022, 14:45
We construct the canonical ensemble of a Reissner-Nordström black hole in a cavity for an arbitrary number of dimensions. The system of a charged black hole in a cavity can be described by a partition function given by the Euclidean path integral approach, where we consider the usual Einstein-Maxwell action with the Gibbons-Hawking-York boundary term and an additional boundary term depending...
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19/12/2022, 15:00
In the string axiverse scenario, light primordial black holes may spin up due to the Hawking emission of a large number of light (sub-MeV) axion-like particles (ALPs). We show that this may trigger superradiant instabilities associated with heavier ALPs during the black hole?s evolution, and study the coupled dynamics of superradiance and evaporation. We find, in particular, that the black...
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19/12/2022, 15:00
We use the exact statistical degeneracy formula of single-centred 1/4 BPS dyonic black holes in 4D N = 4 toroidally compactified heterotic string theory, given in terms of the Igusa cusp form of Sp(2,Z) to write down the gravitational path integral that captures black hole degeneracy as a sum over Euclidean backgrounds including orbifolds of the Euclidean AdS2 × S2 attractor geometry. We...
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19/12/2022, 15:15
We construct some new dynamical p-brane solutions to gravity theories on curved backgrounds. These solutions depend on time and are carriers of the p-form field strength. The simplest such solutions have the function of (p + 1)-dimensional worldvolume spacetime, i.e. dynamical p-branes. We discuss the relations between dynamical branes, a new time-dependent solution on complex projective space...
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19/12/2022, 15:15
Black-hole existence has been widely tested in several contexts, with all observations in agreement with general relativity (GR). However, GR predicts the presence of a curvature singularity at the black- hole core, where the theory breaks down. Consequently, there has recently been increased interest in studying regular black-hole geometries, i.e., objects that are completely regular...
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19/12/2022, 15:30
In this talk, I will present some recent results concerning the loop quantisation of the interior region of a Schwarzschild black hole within the framework of loop quantum cosmology and a preliminary study of the physical states of the quantum theory. We start from a classical canonical description based on an extension of the phase space of Kantowski-Sachs cosmologies. Following the programme...
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19/12/2022, 15:30
The Eikonal approximation has provided a simple approach to analyze the scattering angle of particles interacting with a large massive object. With the Eikonal approach, it is possible to study the quantum perturbative scattering amplitudes of a binary system interacting through graviton exchanges. We re- view the effective field theory treatment of general relativity and the higher order...
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19/12/2022, 15:45
Penrose’s singularity theorem proved the utter inevitability of spacetime singularity formation in general relativity during gravitational collapse, where the theory completely loses its predictive powers. It is generally believed that this problem will find a solution in a theory of quantum gravity (QG), whose effects have so far been assumed to act at the Planck length. Instead, this talk...
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19/12/2022, 15:45
We explore the feasibility of using CubeSats for detecting Gravitational Waves (GW). Heavier black hole mergers are dominated by low-frequency emission, however terrestrial gravitational noise contributions prevent the use of ground based low-frequency GW detectors. We present the sources of such noise contributions and the argument for the use of space based low-frequency GW detectors. We...
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19/12/2022, 16:30
The detection of 90 gravitational wave signals produced by coalescing black holes or neutron stars have opened a rich discovery space for astrophysics, fundamental physics and cosmology. In particular, they enable qualitatively new tests of gravity in its most extreme regime that unfolds when black holes collide. To link gravitational wave observations to the underlying theory of gravity we...
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19/12/2022, 16:30
We investigate general relativistic (GR) solutions in the Minimal Theory of Bigravity (MTBG). MTBG does not contain potentially problematic scalar and vector polarizations in the gravitational sectors and can be a promissing model for the origin of the cosmic acceleration of the present day. We show that a pair of Schwarzschild-de Sitter spacetimes written in the spatially-flat coordinates is...
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19/12/2022, 16:45
In the framework of Metric-Affine Gravity, the existing correspondence between the Einstein tensor and the energy-momentum tensor of matter provided by General Relativity is extended towards a post- Riemannian description in terms of the torsion and nonmetricity fields, which are sourced by the spin, dilation and shear currents of matter. In this talk, we discuss the dynamical role of the...
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19/12/2022, 16:45
New light fundamental fields are natural candidates for all or a fraction of dark matter. Self-gravitating structures of such fields might be common objects in the universe, and could comprise even galactic halos. These structures would interact gravitationally with black holes, a process of the utmost importance since it dictates their lifetime, the black hole motion, and possible...
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19/12/2022, 17:00
I will present a systematic study of the dynamics and gravitational-wave emission of head-on collisions of spinning vector boson stars, known as Proca stars. To this aim we build a catalogue of about 800 numerical-relativity simulations of such systems. We have found that the wave-like nature of bosonic stars has a large impact on the gravitational-wave emission. In particular, we show that...
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19/12/2022, 17:00
After a brief description of what is a traversable wormhole we describe the connection between traversability, the Casimir effect and Yukawa deformations. With the help of an equation of state we also discuss other different solutions generated by the Casimir source. Speculations on these results are also presented including the effect of an additional electric charge.
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19/12/2022, 17:15
In this presentation we report on our study of the evolution in time of spherical excited scalar boson stars under the framework of General Relativity. We see that spherically symmetric excited scalar boson stars can be made dynamically stable (up to timescales of tμ ~ 10^4, where μ is the mass of the scalar particle) with a quartic self-interaction, for certain values of the self-interaction...
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19/12/2022, 17:15
Compact boson star solutions can be constructed on such a scale that binary systems emit gravitational waves in the same frequency range of stellar-mass black hole binaries. Nonetheless, already at the inspiral stage, the resulting signal is significantly different due to tidal and spin-induced effects which affect the waveform. We constructed a post-Newtonian expanded template for the...
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19/12/2022, 17:30
Virial (a.k.a. scaling) identities are integral identities that are useful for a variety of purposes in non- linear field theories, including establishing no-go theorems for solitonic and black holes solutions as well as checking the accuracy of numerical solutions. In this presentation, we provide an algorithm for the derivation of such integral identities. We show that a complete treatment...
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19/12/2022, 17:30
We compute the emission of linear momentum (kicks) by both gravitational and electromagnetic radiation in fully general-relativistic numerical evolutions of quasi-circular charged black hole binaries. We derive analytical expressions for slowly moving bodies and explore numerically a variety of mass ratios and charge-to-mass ratios. We find that for the equal mass case our analytical...
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19/12/2022, 17:45
In the present work, we investigate the consequences of running gravitational coupling on the properties of the three-dimensional BTZ black hole. We take as starting point the functional form of gravitational coupling obtained in the context of asymptotic safe gravity theory. By using the standard scale setting relation where k ∼ ξ / r^n, we compute the solution of the Einstein field...
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19/12/2022, 17:45
Carter-Penrose diagrams are a useful tool to understand the causal properties of spacetimes. I will consider the spherically symmetric collapse of a massless scalar field in the context of numerical relativity. The evolution takes place on hyperboloidal slices, which are smooth and spacelike, and reach future null infinity. This is the collection of the endpoints of future-directed null...
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20/12/2022, 09:00
This year we celebrate the 50 years of the highly influential paper ``Floating Orbits, Superradiant Scattering and the Black-hole Bomb'' by William Press and Saul Teukolsky [Nature 238, 211-212 (1972)]. In this talk I will briefly review the fascinating ideas put forward in this paper and highlight the main implications that it had for black hole physics, astrophysics and even particle physics.
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20/12/2022, 09:15
A number of well-known black object solutions in (electro)vacuum general relativity (GR) are plagued by conical singularities which provide the force balance that allows the existence of such configurations. The basic example is the Bach-Weyl (or double-Schwarzschild) solution, which describes two static black holes in four dimensional vacuum GR, with a deficit angle along the section in...
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20/12/2022, 09:30
As of now, almost a hundred gravitational-wave (GW) signals have been observed and interpreted as resulting from the coalescence of compact binaries by LIGO-Virgo. While these signals seem to be in perfect agreement with the theory of general relativity (GR) in vacuum, it is crucial to quantify any possible small deviations. By looking at the GW signal, how sure are we that these binaries are...
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20/12/2022, 09:45
A system of equations that serves as a model for the Einstein field equation in generalised harmonic gauge called the good-bad-ugly system is studied in the region close to null and spatial infinity in Minkowski spacetime. This analysis is performed using H. Friedrich’s cylinder construction at spatial infinity and defining suitable conformally rescaled fields. The results are translated to...
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20/12/2022, 10:00
We study the impact of accumulated asymmetric bosonic dark matter on neutron star properties, including tidal deformability, maximum masses, radii, etc. The conditions at which dark matter particles tend to condensate in the core of a star or create an extended halo are presented. We show that at some values of mass, interaction scale, and relative fraction, the dark matter core becomes...
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20/12/2022, 10:15
KORAL is a general relativistic radiation magnetohydrodynamics (GRRMHD) code used to study accretion onto compact objects and related phenomena. Radiation is treated using the M1 closure scheme. We present some examples of results obtained with KORAL in our group. We also discuss our ongoing project where we intend to set up a simulation of an optically thin and geometrically thick advection...
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20/12/2022, 11:00
We explore holographic attributes of four-dimensional near-extremal Reissner-Nordstrom black hole solutions in ungauged N = 2 supergravity theories at the two-derivative level by recasting them as a specific first-order deformation in solution space, associated with an infinitesimal Harrison transformation, of black holes in an AdS2 space-time. We further show that the nAdS2 attractor...
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20/12/2022, 11:15
Extreme Mass Ratio Inspirals (EMRIs), binary systems with a secondary stellar mass compact object inspiralling into a massive black hole, are among the main targets for LISA, as they harbour the potential for precise strong gravity tests. Although the description of these systems in modified theories of gravity can be dramatically complex, for a vast class of theories with additional scalar...
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20/12/2022, 11:30
In many astrophysical scenarios, the charge of the black hole is often neglected due to unrealistically large values of the charge required for the Reissner-Nordstrom spacetime metric. Black holes, however, might have a small electric charge because of various methods for selective accretion. We study how an imaginary small electric charge on a Schwarzschild black hole affects the ionization...
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20/12/2022, 11:45
In the highly relativistic regime around compact objects signatures of new physics may be unraveled. Some compact objects may in fact be constructed from beyond Standard Model matter. In particular, boson stars are useful toy models for exotic compact objects that could be produced in the Early Universe. Gravitational wave astronomy may also uncover a signature of a new force-mediating boson...
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20/12/2022, 12:00
Using the standard methods of numerical relativity we study axisymmetric gravitational waves undergo- ing gravitational collapse. Because it is known that the 1+log lapse choice breaks down in this situation, we propose a computationally effective alternative to maximal slicing and show that it allows the simulation to proceed until either the gravitational waves disperse or an apparent...
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20/12/2022, 12:15
Perturbation theory of vacuum spherically symmetric spacetimes is a crucial tool for understanding the dynamics of black hole (BH) perturbations as well as BH scattering phenomena. Since the pioneering work of Regge and Wheeler it is known that the equations for the perturbations can be decoupled in terms of (gauge-invariant) master functions that satisfy 1 + 1 wave equations. However, while...
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20/12/2022, 12:30
In this talk, we model the gravitational collapse of stars in massive scalar-tensor gravity. In this theory, the two tensorial gravitational-wave polarization modes are complemented by a massive breathing mode. This latter mode is triggered by the spontaneous scalarization mechanism discovered by Damour and Esposito-Farese; its radiation exhibits a drastically different behaviour dominated by...
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20/12/2022, 14:30
It is broadly believed that quasinormal modes (QNMs) cannot tell the black-hole near-horizon geometry, because usually the low-lying modes are determined by the scattering of perturbations around the peak of the effective potential. Using the general parametrization of the black-hole spacetimes respecting the generic post-Newtonian asymptotic, we will show that tiny modifications of the...
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138. R. Z. Ferreira: Primordial black holes and axions: a tale of (galactic and extragalactic) light20/12/2022, 14:30
Primordial black holes with asteroid-like masses (10^{14−18} kg) can still account for all the dark matter. In this talk, I will discuss a new indirect probe of PBH dark matter. I will show that axions with masses in the 0.1eV-MeV range can form in clouds around such PBHs, via superradiance, and ultimately decay, if coupled to photons. The decay products will contribute to the galactic or...
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20/12/2022, 14:45
It was recently shown that small perturbations due, e.g., to environmental effects (the “flea”) to the effective potential governing gravitational-wave generation and propagation in black hole exteriors (the “elephant”) can lead to arbitrarily large changes in the black hole’s quasinormal spectrum. This raises an important question: is the black hole spectroscopy program robust against...
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20/12/2022, 14:45
The non-singular bouncing cosmology is an alternative paradigm to inflation, where the background energy density vanishes at the bounce point in the context of Einstein’s gravity. Therefore, the non- linear effects in the evolution of density fluctuations (δρ) may be strong in the bounce phase, which potentially provides a mechanism to enhance the abundance of primordial black holes (PBHs). We...
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20/12/2022, 15:00
In the recent years, our understanding of the galactic centre has grown rapidly with the joint efforts of large international collaborations like the GRAVITY, who detected infrared flares close to the innermost stable circular orbit (ISCO) of the supermassive black-hole at the galactic centre, and the Event Horizon Telescope (EHT), which recently published the first picture of a black-hole...
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20/12/2022, 15:00
Ringdown offers a unique possibility to study the spacetime in extreme curvature regimes, to test general relativity and better understand the nature of black holes. The ringdown corresponds to the last phase of black hole binary coalescences, when the newly formed black hole relaxes to its stationary Kerr state by emitting gravitational waves. The linear theory of black hole perturbations...
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20/12/2022, 15:15
We report the results of the first comprehensive study on the shadows of fundamental Kerr black holes with Proca hair. Some of these solutions show striking non-Kerr features, such as chaotic patterns, non- smooth shadow edges, and ghost shadows. We illustrate how fundamental photon orbits can help us understand some of these features. Nonetheless, a wide range of solutions where deviations...
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20/12/2022, 15:15
Recent investigations of the pseudospectrum in black hole spacetimes have shown that quasinormal mode frequencies suffer from spectral instabilities. We extend the pseudospectrum analysis to horizon- less exotic compact objects which possess a reflective surface arbitrarily close to the Schwarzschild radius, and find that their quasinormal modes also suffer from an overall spectral...
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20/12/2022, 15:30
Gravitational waves provide a unique opportunity to test gravity in the dynamical and nonlinear regime. Here, we propose a parametrised test of general relativity that introduces generic deviations to the plunge, merger and ringdown stages of binary-black-hole coalescences. The novel feature of the model is that it can capture signatures of beyond-GR physics in the merger phase of black-hole...
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20/12/2022, 15:30
The coupling between the angular momentum of a compact object and an external tidal field gives rise to the “rotational” tidal Love numbers, which affect the tidal deformability of a spinning self-gravitating body and enter the gravitational waveform of a binary inspiral at high post-Newtonian order. In this talk I will present an unexpected symmetry of these quantities in slowly-rotating...
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20/12/2022, 15:45
The tidal response of a compact object is a key gravitational-wave observable encoding information about its interior. This link is subtle due to the nonlinearities of general relativity. We show that considering a scattering process bypasses challenges with potential ambiguities, as the tidal response is determined by the asymptotic in- and outgoing waves at null infinity. As an application...
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20/12/2022, 15:45
The coalescence of compact binaries and the gravitational waves produced in this process provide an unique view into gravity in its dynamical, nonlinear regime. Proeminent during a black-hole coalescence is the plunge-merger stage, when the black holes collide at a large fraction of the speed of light and around which the gravitational-wave luminosity peaks. How robust are the predictions of...
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20/12/2022, 16:30
The Regularity Transformation (RT-)equations are an elliptic system of partial differential equations which determines coordinate and gauge transformations that remove apparent singularities in space- time by furnishing optimal metric regularity. The resulting gain of one derivative for connections above their Lp curvature suffices to establish Uhlenbeck compactness. By developing an existence...
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20/12/2022, 16:30
We give brief review of our recent works. We consider several different aspects connected with properties of motion inside the Schwarzschild black hole. We elucidate, how (i) to maximize the survival proper time and (ii) how to make a visible part of the outer universe as large as possible before hitting the singularity. We also consider red(blueshift) of photons that an observer absorbs...
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20/12/2022, 16:45
Collapsing stars may be approximated by homogeneous distributions of dust, either in the form of massive spheres or thin shells. Furthermore, using modern mathematical techniques, the dynamics of these systems may be obtained by separating spacetime into interior and exterior regions, focusing on the boundary conditions and thus foregoing solving the field equations for the totality of...
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20/12/2022, 16:45
I will present a new method to study evolving black holes in general relativity. Approximate analytical solutions are obtained by expanding the Einstein field equations close to the trapping horizon for a dynamical spherically symmetric black hole in the presence of a minimally coupled self-interacting scalar field. This is made possible by a new parametrization of the metric, in which the...
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20/12/2022, 17:00
We consider an Abelian-Higgs type model wherein a complex vector field gains mass via spontaneous symmetry breaking. This model can be considered as a UV completion of a complex Proca model with self-interactions. We study the flat spacetime (“balls”) and self-gravitating (“stars”) solitons describing some of their mathematical and physical properties. The stars reduce to the well-known...
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20/12/2022, 17:00
Recently, it has been shown that the radial stability of a light-ring (LR) in a spacetime generated by a stationary, axisymmetric, asymptotically flat object with a Z2 symmetry determines the possibility and radial stability of timelike circular orbits (TCOs) around the LR. In this paper, we generalise this result by also considering the vertical (angular) stability of the orbits through the...
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20/12/2022, 17:15
Before considering any modifications of classical gravitational theory, the question is whether black hole singularities can be cured by coupling to some type of matter fields. We inspect nonlinear electromagnetic theories as candidates for resolving black hole singularities. This problem was first systematically addressed by Bronnikov, whose no-go theorems are focused on spherically symmetric...
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20/12/2022, 17:15
Motivated by processes happening around accreting black holes, a series of papers entitled “Free motion around black holes with discs or rings: Between integrability and chaos” were published in order to classify and evaluate the chaotic behaviour that may occur in geodesic motion around static black holes if these are perturbed by gravity of some additional matter. However the static and...
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20/12/2022, 17:30
We explore the problem of a scalar charge orbiting inside a cavity within Schwarzschild spacetime. This problem is relevant to study stellar binaries in AdS as well as in theories with massive degrees of freedom. We show that the charge suffers a non-trivial self-force that would alter its orbit. However, for appropriate initial configurations of the field this effect vanishes over an orbital...
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20/12/2022, 17:30
The astrophysical black holes are not isolated bodies. One of the most prominent structures which form in the proximity of the black hole is a thin accretion disc. Whether treated in Newton’s theory or in general relativity, in the static and axially symmetric case the gravitational potential is determined by the Laplace equation. Though clear in principle, its solution is only known...
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20/12/2022, 17:45
The numerical solution of a system of hyperbolic PDEs all the way to future null infinity requires the knowledge of asymptotics. The Good-Bad-Ugly-F model is known to mimic the asymptotic properties of Einstein equations in generalized harmonic gauge. In this talk I will present the results of numerical evolution of this system, both in spherical symmetry and full 3D, with the scope of using...
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20/12/2022, 17:45
The motion of particles on general (1+3)-dimensional spacetimes satisfying a set of assumptions can be described by the geodesics of a 2-dimensional manifold. In this work we resort to these 2-dimensional metrics to study circular geodesics of generic static, spherically symmetric, and asymptotically flat (1 + 3)-dimensional spacetimes containing either non-extremal black holes or horizonless...
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