# Gravitational Waves, Black Holes and Fundamental Physics

Europe/Rome
IFPU, Miramare campus, Trieste, Italy

#### IFPU, Miramare campus, Trieste, Italy

Via Beirut 2, Trieste, Italy
Description



# 3rd meeting of the GWVerse COST action

The long-held promise of gravitational-wave astronomy as a new window onto the universe has finally materialized. We have taken but the first steps along a new, exciting avenue that has now opened before us. The harvesting of useful information from gravitational-wave signals and the understanding of its broader implications demand a cross-disciplinary effort. How, when and in which environment were black holes formed? How fast do they spin and how have some of them grown to become supermassive? Can black hole mergers inform us on the nature and distribution of dark matter? Are there new fundamental degrees of freedom? Gravitational waves will allow for precise tests of General Relativity, and of the black hole paradigm itself. However, to be able to collect and interpret the information encoded in the GWs, one has to be equipped with faithful and accurate theoretical models of the predicted waveforms. To accomplish the far-reaching goals of gravitational-wave science it is of paramount importance to bring together expertise over a very broad range of topics, from astrophysics and cosmology, through general-relativistic source modelling to particle physics and other areas of fundamental science.

In 2016, a short time before the announcement of the first gravitational-wave detection, a cross-disciplinary initiative in Europe led to the establishment of the new CA16104 COST networking Action on "Black holes, gravitational waves and fundamental physics'' ("GWverse''). GWverse aims to maintain and consolidate leadership in black-hole physics and gravitational-wave science. The Action supports the training of the next generation of leaders in the field, and the very first "native'' GW/multi-messenger astronomers, ready to tackle the challenges of high-precision GW astronomy with ground and space-based detectors.

The third global meeting of the Action will take place in Trieste, Jan 14-16 2020. A meeting of the board of the action is scheduled for Jan 13.

Scientific Organising CommitteeLeor Barack (Southampton), Enrico Barausse (SISSA), Vitor Cardoso (CENTRA), Valeria Ferrari (Rome Sapienza), Stefano Liberati (SISSA), Samaya Nissanke (GRAPPA), Thomas Sotiriou (Nottingham)

Local Organising Committee: Enrico Barausse, Alexandru Dima, Jaumila Gonzalez, Stefano Liberati, Giovanni Tambalo, Sebastian Volkel.

Participants
• Alberto Mangiagli
• Alex Bentley Nielsen
• Alexandre Toubiana
• Alexandros Karam
• Alexandru Dima
• Ali Seraj
• Alicia M Sintes
• Andrea Lapi
• Andrea Maselli
• Andrea Trombettoni
• Andreja. Gomboc
• Andrew Coates
• Anna Puecher
• Antoine Lehébel
• Anupam Mazumdar
• Banafsheh Shiralilou
• Barry Wardell
• Bert Vercnocke
• Betul Demirkaya
• Beyhan Karakaş
• Béatrice Bonga
• Caner Unal
• Carlo Baccigalupi
• Carlos F. Sopuerta
• Carlos Herdeiro
• Cecília Gergely
• Christopher Eckner
• Christos Chalkias
• Christos Charmousis
• Christos Mermigkas
• Costantino Pacilio
• Daniel Mayerson
• Daniela Doneva
• Daniele Pranzetti
• Dennis Hansen
• Diana-Cristiana Popescu
• Dimitra Tseneklidou
• Dmitry Gal'tsov
• Dorota Rosinska
• Edi Bon
• Eduard Larrañaga
• Elvis Barakovic
• Emil Nissimov
• Enrico Catalano
• Fabio Moretti
• Federica Sassano
• FETHI MUBIN RAMAZANOGLU
• Francesco Di Filippo
• Francisco Duque
• Francois Foucart
• Frederic Vincent
• Gabriel Andres Piovano
• Gabrijela Zaharijas
• Gauri Sharma
• George Pappas
• Georgios Antoniou
• Georgios Loukes-Gerakopoulos
• Georgios Vretinaris
• Germano Nardini
• Giovanni Gandolfi
• Giovanni Tambalo
• Giovany Cruz
• Giulia Ventagli
• Giulio Scelfo
• Glykeria Mangaloudi
• Guilherme Raposo
• Igor Bogush
• Ilia Musco
• IOANNIS BARESIS
• Ippocratis Saltas
• Isabel Suárez Fernández
• Istvan Racz
• Ivan Vasquez
• Jackson Levi Said
• Jaumila Gonzalez
• Kevin Wolz
• Kostas Glampedakis
• Kunihito Uzawa
• Kutay Arınç Çokluk
• Kwinten Fransen
• Kyriakos Destounis
• Laura Sberna
• Leonardo Gualtieri
• Leonardo Trombetta
• Leor Barack
• Lorenzo Annulli
• Lotte ter Haar
• Luca Marzola
• Lucas Gardai Collodel
• Luciano Rezzolla
• Lumen Boco
• Manuel Hohmann
• Marco Crisostomi
• Marco Quaglia
• Marcos Valdes
• Maria Chernyakova
• Marika Giulietti
• Mario Herrero-Valea
• Marta Volonteri
• Martina Toscani
• Mateja Bošković
• Matteo Viel
• Maximiliano Isi
• Md Arif Shaikh
• Mekhi Dhesi
• Miguel Bezares
• Miguel Zilhão
• Monica Colpi
• Nemanja Rakic
• Nicola Franchini
• Niels Obers
• Niels Warburton
• Nikolaos Chatzarakis
• Nikolaos Karnesis
• Nikolas Andreou
• Nuno M. Santos
• Oliver Long
• Orlenys Troconis
• Paolo Creminelli
• Paolo Marcoccia
• Paolo Pani
• Partha Sarathi Majumdar
• Pau Amaro Seoane
• Raffaella Schneider
• Raquel Santos Garcia
• Recai Erdem
• Richard Brito
• Rob Tielemans
• Roberto Oliveri
• Rodrigo Vicente
• Samaya Nissanke
• sandipan Sengupta
• Sayak Datta
• Sebastian Völkel
• Semih Kacmaz
• Sergey Tsygankov
• Simone Speziale
• Sohyun Park
• Sourabh Nampalliwar
• Stamatis Vretinaris
• Stefano Liberati
• Stephen Green
• Sudipta Hensh
• Sudipta Sarkar
• Susana Oliveira Novais
• Swetha Bhagwat
• Taishi Ikeda
• Thanasis Giannakopoulos
• Thomas Sotiriou
• Tommaso De Lorenzo
• Troels Harmark
• Uma Papnoi
• Valeria Ferrari
• Victor E. Ambrus
• Viktor Gakis
• Vitor Cardoso
• Yosef Verbin
• Zeljka Bosnjak
• ﻿Enrico Barausse
Contact
• Tuesday, 14 January
• 08:30 09:00
Registration
• 09:00 09:15
Opening 15m
• 09:15 11:00
Morning session
• 09:15
Cosmic Archaeology with black hole binaries 45m

The existence of massive stellar black hole binaries, with primary black hole (BH) masses greater than 30 -35 Msun was proven by the detection of the gravitational wave (GW) event GW150914 during the first LIGO/Virgo observing run (O1), and successively confirmed by seven additional GW signals discovered by independent analyses of the O1 and O2 data. Recently reported O3 alerts suggest that similar events may have been detected. Building on well established observed galaxy scaling relations and on our current understanding of stellar evolution at different metallicities, we find that these systems may be the relics of stellar populations forming in low-mass dwarf galaxies before the end of cosmic reionization. At the other extreme of the mass spectrum of astrophysical black holes, the existence of a population of almost 200 bright quasars at z > 6 poses crucial questions about their formation and growth processes. Observationally, the most distant quasars provide joint constraints on the mass of black-hole ‘seeds’ and their accretion efficiency. Depending on their birth environmental conditions in the first halos collapsing at very high redshifts, black hole seeds can be light, heavy or with intermediate masses. Using ab-initio structure formation models, we investigate the relative role of different populations of black hole seeds in the formation histories of the first super-massive black holes and how these formation scenarios may be constrained by future electromagnetic and gravitational wave observations. In particular, future gravitational wave facilities, such as LISA and the Einstein Telescope, will be able to improve our knowledge of these ancient systems, fully exploiting their potential as cosmic archaeology probes.

Speaker: Raffaella Schneider
• 10:00
Growth of supermassive black hole seeds in ETG star-forming progenitors via gaseous dynamical friction: perspectives for GW detections 15m

In this talk I will discuss a novel mechanism to grow supermassive black hole seeds in star-forming ETG progenitors at z >1. This envisages the migration and merging of stellar compact remnants, via gaseous dynamical friction, toward the central regions of such galaxies. I will show that this process can build up central BH masses of order 10^4 − 10^6 Msun in a timescale shorter than 10^8 yr, providing heavy seeds before standard disk accretion takes over to become the dominant process for further BH growth. I will discuss the perspectives to detect the merger events between the migrating stellar remnants and the accumulating central supermassive BH via gravitational wave emission with future ground and space-based detectors such as the Einstein Telescope (ET) and the Laser Interferometer Space Antenna (LISA).

Speaker: Dr Lumen Boco (SISSA)
• 10:15
Merger rate of stellar black hole binaries above the pair-instability mass gap 15m

In current stellar evolutionary models, the occurrence of pair-instability supernovae plays a key role in shaping the resulting black hole (BH) mass population, preventing the formation of remnants between about $[60, \, 120] \rm M_\odot$.
We develop a simple approach to describe BHs beyond the pair-instability gap, by convolving the initial mass function and star formation rate with the metallicity evolution across cosmic time and SEVN, a code to evolve single stars up to $\rm M < 350 \, \rm M_\odot$. Under the ansatz that the underlying physics of binary formation does not change beyond the gap, we then construct the cosmic population of merging BH binaries.
We found that LIGO/Virgo at design sensitivity will detect between $\simeq [0.4,\,7] \, \rm yr^{-1}$ with both mass components above the gap, considering the most pessimistic and optimistic scenario. Similarly, the expected rate for third generation ground-based detectors, like Einstein Telescope, ranges between $[10, \, 460] \, \rm yr^{-1}$.
Moreover, at lower frequencies, LISA can individually detect these binaries up to thousands of years from coalescence. The number of multiband events, i.e. merging in less than four years, is expected to be in the range $[1, \, 20]$. While ET will detect all these events, LIGO/Virgo is expected to detect $< 50\%$ of them.
Finally the undetected systems are expected to contribute to the stochastic background in the LISA band. We estimate that this background may be in principle detected with a signal-to-noise ratio between $\simeq 2.5$ and $\simeq 80$.
Our work has been accepted for publication: \href{https://iopscience.iop.org/article/10.3847/2041-8213/ab3f33/meta}{A. Mangiagli et al., ApJL 883, L27}

Speaker: Mr Alberto Mangiagli (University of Milan - Bicocca)
• 10:30
Detection and parameter estimation for accreting stellar-origin black-hole binaries and their electromagnetic counterpart 15m

We study the impact of mass accretion in the evolution of LIGO-like black hole binaries. Based on simulated catalogues of binary populations, we estimate that a fraction of the events will have a detectable imprint of Eddington-level accretion, when detected by LISA or by LISA and ground-based detectors (multiband). Accretion can also induce bias in the binary parameters, such as the masses and the coalescence time. For these events, the sky location is well determined and allows for targeted searches for electromagnetic counterparts, e.g. with the ATHENA mission or SKA.

Speaker: Laura Sberna (Perimeter Institute)
• 10:45
Tidal Deformability of Black Holes Immersed in Matter 15m

The tidal deformability of compact objects by an external field has a detectable imprint in the gravitational waves emitted by a binary system, which is encoded in the so-called Tidal Love Numbers (TLNs). For a particular theory of gravity, the TLNs depend solely on the object's internal structure and, remarkably, they vanish for black holes in general relativity. This fact has gathered attention recently since a non-zero measurement of the TLNs for a would-be black hole could provide evidence of new physics in the strong-field regime. However, in realistic astrophysical scenarios, a compact object will be surrounded by a non-vacuum environment. It is, therefore, crucial to evaluate if the effect on tidal deformability due to this environmental matter is comparable to the ones characterizing deviations to GR or exotic compact objects (ECOs), such as boson stars, gravastars, and wormholes. In this work, we compute the TLNs for model configurations of black holes immersed in matter and apply our results to astrophysically motivated black hole + accretion disk systems. Our results pose some questions regarding the possibility of testing strong-field gravity using TLNs.

Speaker: Francisco Duque (GRiT/CENTRA, Instituto Superior Técnico, Universidade de Lisboa)
• 11:00 11:30
Coffee break 30m
• 11:30 12:15
Morning session
• 11:30
BMS with applications 45m

This will be an overview talk about the Bondi-Metzner-Sachs group, which is the symmetry group of asymptotically flat spacetimes. After having reviewed its main properties, I will discuss some applications such as the memory effect. Next, I will discuss the BMS algebra in other contexts such as higher dimensions and black hole horizons. The latter is conjectured to be key in solving the information loss paradox. Finally, I will venture beyond asymptotic flatness to include superrotations or a positive cosmological constant.

• 12:15 14:15
Lunch 2h
• 14:15 15:45
Afternoon session
• 14:15
LISA Data Challenges: Status and future prospects 45m

The LISA Data Challenges (LDC) where established as a common ground with the aim of engaging the community to the open LISA data analysis questions. Since the first LDC, a lot of experience has been gained, and significant progress has been achieved. In this talk I will review this progress, and I will present the purpose and individual goals of the current LDCs. The status and future prospects will be discussed, and a small tutorial on the software usage will be given as well.

Speaker: Nikos Karnesis
• 15:00
Force-free electrodynamics near rotation axis of a Kerr black hole 15m

Despite their potential importance for understanding astrophysical jets, physically realistic exact solutions for magnetospheres around Kerr black holes have not been found, even in the force-free approximation. Instead approximate analytical solutions such as the Blandford-Znajek (split-)monopole, as well as numerical solutions, have been constructed. In this talk we consider a new approach to the analysis and construction of such magnetospheres. We consider force-free electrodynamics close to the rotation axis of a magnetosphere surrounding a Kerr black hole assuming axisymmetry. This is the region where the force-free approximation should work the best, and where the jets are located. We perform a systematic study of the asymptotic region with (split-)monopole, paraboloidal and vertical asymptotic behaviors. Imposing asymptotics similar to a (split-)monopole, we find that demanding regularity at the rotation axis and the event horizon restricts solutions of the stream equation so much that it is not possible for a solution to be continuously connected to the static (split-)monopole around the Schwarzschild black hole in the limit where the rotation goes to zero. This provides independent evidence to the issues discovered with the asymptotics of the Blandford-Znajek (split-)monopole in Ref. [1] from which it follows that its perturbative construction is inconsistent.

Speaker: Troels Harmark (Niels Bohr Institute)
• 15:15
Constraints on an Effective Field Theory extension to gravity using gravitational-wave observations 15m

Gravitational-wave observations of coalescing binary black holes allow for novel tests of the strong-field regime of gravity. Using the detections of the LIGO and Virgo collaborations, we place the first constraints on higher-order curvature corrections that arise in the effective-field-theory extension of general relativity where higher-order powers in the Riemann tensor are included in the gravitational-field action. We construct gravitational-wave templates that describe the quasi-circular inspiral stage in this modified theory of gravity, and use Bayesian-selection methods to constrain this theory with respect to General Relativity. We focus on the two lowest-mass gravitational-wave events observed to date (GW151226 and GW170608), but describe a general strategy for updating constraints with more events.

Speaker: Dr Richard Brito (Sapienza University of Rome)
• 15:30
Rotating black hole in a higher order scalar tensor theory 15m

We will discuss an analytic hairy black hole in a subclass of scalar tensor theories

Speaker: Christos Charmoussis (LPT-Orsay)
• 15:45 16:15
Coffee break 30m
• 16:15 17:00
Afternoon session
• 16:15
The sound of DHOST 15m

In generic higher-order scalar-tensor theories which avoid the Ostrogradsky instability, the presence of a scalar field significantly modifies the propagation of matter perturbations, even in weakly curved backgrounds. This affects notably the speed of sound in the atmosphere of the Earth. It can also generate instabilities in homogeneous media. I will use this to constrain the viable higher-order scalar-tensor models.

Speaker: Dr Antoine Lehébel (University of Nottingham)
• 16:30
Hearing the strength of gravity (with the Sun) 15m

Generic extensions of General Relativity aiming to explain dark energy typically introduce fifth forces of gravitational origin. In this talk, I will explain how helioseismic observations can provide a powerful and novel tool towards precision constraints of fifth forces, as predicted by general theories for dark energy, and I will discuss the implications for cosmology.

Speaker: Dr Ippocratis Saltas (CEICO - Czech Academy of Sciences)
• 16:45
The IR limit of Horava Gravity 15m

Horava Gravity is a renormalizable theory of Quantum Gravity which is expected to flow to GR in the low energy limit. This naive expectation is obstructed by a strongly coupled interaction when the parameters of the Lagrangian flow to the general relativistic values. However, when closely studied, only self-interactions of the extra scalar mode of the theory are strongly coupled. When matter is coupled to HG, scattering amplitudes naturally flow to the results given by GR. In other words, matter remains weakly coupled and interacting only through Lorentz invariant operators. I will discuss the implications of this behavior for HG as a realistic model of gravitational interactions.

Speaker: Dr Mario Herrero-Valea (SISSA)
• 17:00 18:00
Discussion session: WG1 (Astrophysics)
• Wednesday, 15 January
• 09:15 11:00
Morning session
• 09:15
Multi-messenger signals from merging neutron stars 45m

The first detection of a binary neutron star star system through gravitational waves and electromagnetic signals (gamma-ray burst, kilonova, radio) recently demonstrated the feasibility and usefulness of multi-messenger astronomy. In this talk, I will provide an overview of the physics of neutron star-neutron star and black hole-neutron star mergers, and of what we can learn from gravitational waves and electromagnetic signals powered by these events. I will also discuss uncertainties in existing models of merging neutron stars, and how these uncertainties still place important limits on our ability to reliably extract information from the observation of merging compact objects.

Speaker: Francois Foucart
• 10:00
BMS flux-balance equations as constraints on the gravitational radiation 15m

Asymptotically flat spacetimes admit infinite dimensional BMS symmetries which complete the Poincare symmetry algebra with super-translation and super-Lorentz generators. We show that each of these symmetries lead to a flux-balance equation at null infinity, which we compute to all orders in the post-Minkowskian expansion in terms of radiative multipole moments. The ten Poincare flux-balance laws generalize the previously known balance equations to all orders in the post-Minkowskian expansion. The rest of BMS balance laws are novel in the literature and impose infinite number of constraints on the gravitational waveforms. We show that the balance equations for quadrupolar super-translation and super-Lorentz generators give non-trivial constraints at 3PN and 2.5PN order, respectively. Our analysis also confirms the surface charge expression for the angular momentum at null infinity derived previously from the sub-leading soft graviton theorem.

Speaker: Dr Ali Seraj (ULB)
• 10:15
Gauge-invariant approach to the parameterized post-Newtonian formalism 15m

The parameterized post-Newtonian (PPN) formalism is an invaluable tool to assess the viability of gravity theories using a number of constant parameters. These parameters form a bridge between theory and experiment, as they have been measured in various solar system experiments and can be calculated for any given theory of gravity. The practical calculation, however, can become rather cumbersome, if the field equations involve couplings to additional fields. In addition, the PPN formalism relies on the choice of a particular gauge (or coordinate system), which is determined only after solving the field equations. These difficulties can be overcome by applying a gauge invariant formalism, which is conventionally used in cosmological perturbation theory. The particular nature of the PPN formalism requires perturbations of at least quadratic order to be considered, as well as a different treatment of space and time directions. In my talk I show how to develop such kind of formalism for gravity theories in metric and tetrad formulation and give prospects on how to generalize this treatment to higher perturbation orders necessary for calculating high precision orbital motion.

Speaker: Manuel Hohmann (University of Tartu)
• 10:30
Modelling black hole binaries in the intermediate-mass-ratio regime. 15m

We are working to provide accurate modelling of the dynamics and gravitational-wave signatures of black hole inspirals in the intermediate-mass-ratio regime (IMIRIs) (1:100-1:1000). In doing so we hope to bridge the gap between the accurate modelling of extreme-mass-ratio inspirals achieved through black hole perturbation theory, and that of comparable-mass inspirals using numerical relativity. Neither approach works well for IMRIs due to the inability to treat the smaller black hole as a perturbation of the larger and yet disparate length scales remain, preventing computational efficiency in numerical relativity. IMRIs remain an important open problem in the field as such binary systems are not unlikely sources for Advanced LIGO and LISA and their observation would provide us with fundamental insight into black hole formation and astrophysical populations.

Our team at Southampton will work with the numerical relativity group at the Albert Einstein Institute to tackle the problem through a combination of black-hole perturbation and numerical relativistic techniques. This talk will give an overview of a new approach to IMRI modelling: matching an approximate analytic solution near the small black hole to a fully nonlinear numerical solution in the bulk of the spacetime. Preliminary results will be presented from a scalar toy model which tests the implementation of such a matching procedure.

Speaker: Ms Mekhi Dhesi (University of Southampton)
• 10:45
Well-posedness of characteristic formulations of GR 15m

Characteristic formulations of General Relativity (GR) have advantages over more standard spacelike foliations in a number of situations. For instance, the Bondi-Sachs formalism is at the base of codes that aim to produce gravitational waveforms of high accuracy, exploiting the fact that null hypersurfaces reach future null infinity and hence avoid systematic errors of extrapolation techniques. Furthermore, characteristic formulations in asymptotically anti-de Sitter spacetimes are widely used in the field of numerical holography, which can provide insights for the behavior of strongly coupled matter. Well-posedness of the resulting PDE systems, however, remains an open question. The answer to this question affects the accuracy of the results and the reliability of the conclusions drawn from numerical studies based on such formulations. A numerical solution can converge to the continuous one only for well-posed PDE systems. The well-posedness of the initial value problem of such systems is characterized by strong hyperbolicity. We find that the PDE systems arising from the aforementioned formulations are only weakly hyperbolic, due to a shared pathological structure in both the asymptotically flat and anti de-Sitter cases. We present numerical tests that demonstrate this problem at the practical level.

Speaker: Mr Thanasis Giannakopoulos (Instituto Superior Técnico)
• 11:00 11:30
Coffee break 30m
• 11:30 12:15
Morning session
• 11:30
Scalarized black holes 45m

Spontaneous scalarization is a very interesting mechanism endowing the compact object with nontrivial scalar field. This mechanism is designed to work only in the strong gravity regime while remaining the weak field regime practically unaltered. While scalarization was discussed mainly for neutrons stars in the last few decades, it was recently discovered that black holes in Gauss-Bonnet gravity can be scalarized as well with the scalar field sourced by the curvature of the spacetime itself that attracted a lot of attention in the field. In the present talk we will review the main achievements related to scalarized black holes starting with the first papers on the subject and paying particular attention to the recent developments in Gauss-Bonnet gravity and beyond.

Speaker: Daniela Doneva (University of Tuebingen)
• 12:15 14:15
Lunch 2h
• 14:15 15:45
Afternoon session
• 14:15
The Black Hole Perturbation Toolkit 45m

As we face the task of modelling small mass-ratio binaries for LISA we, as a community, need to spend more time developing waveform models and less time writing and re-writing codes. Currently there exist multiple, scattered black hole perturbation codes developed by a wide array of individuals or groups over a number of decades. This project brings together some of the core elements of these codes into a Toolkit that can be used by anyone. The Black Hole Perturbation Toolkit, hosted at https://bhptoolkit.org, is a collection of open-source software and data for black hole perturbation theory calculations. In this talk I will overview the motivation for the Toolkit, give examples of the current code and data, show how you can contribute, and discuss where we plan to take the BHPToolkit in the near future.

Speaker: Niels Warburton
• 15:00
Teukolsky formalism for nonlinear Kerr perturbations 15m

We develop a formalism to treat higher order (nonlinear) metric perturbations of the Kerr spacetime in a Teukolsky framework. We first show that solutions to the linearized Einstein equation with nonvanishing stress tensor can be decomposed into a pure gauge part plus a zero mode (infinitesimal perturbation of the mass and spin) plus a perturbation arising from a certain scalar ("Debye-Hertz") potential, plus a so-called "corrector tensor." The scalar potential is a solution to the spin −2 Teukolsky equation with a source. This source, as well as the tetrad components of the corrector tensor, are obtained by solving certain decoupled ordinary differential equations involving the stress tensor. As we show, solving these ordinary differential equations reduces simply to integrations in the coordinate r in outgoing Kerr-Newman coordinates, so in this sense, the problem is reduced to the Teukolsky equation with source, which can be treated by a separation of variables ansatz. Since higher order perturbations are subject to a linearized Einstein equation with a stress tensor obtained from the lower order perturbations, our method also applies iteratively to the higher order metric perturbations, and could thus be used to analyze the nonlinear coupling of perturbations in the near-extremal Kerr spacetime, where weakly turbulent behavior has been conjectured to occur. Our method could also be applied to the study of perturbations generated by a pointlike body traveling on a timelike geodesic in Kerr, which is relevant to the extreme mass ratio inspiral problem.

Speaker: Dr Stephen Green (Albert Einstein Institute Potsdam)
• 15:15
Eikonal QNMs of black holes beyond GR 15m

In this talk we study the quasi-normal modes of spherically symmetric black holes in modified theories of gravity, allowing for couplings between the tensorial and scalar field degrees of freedom. Using the eikonal approximation and a largely theory-agnostic approach, we obtain analytical results for the fundamental mode of such black holes.

Speaker: Dr Kostas Glampedakis
• 15:30
On black hole spectroscopy using overtones 15m

Validating the no-hair theorem with a gravitational wave observation from a compact binary coalescence presents a compelling argument that the remnant object is indeed a black hole described by the classical general theory of relativity. Validating this theorem relies on performing a spectroscopic analysis of the post-merger signal and recovering the frequencies of either different angular modes or overtones (of the same angular mode). For an equal mass binary black hole systems, the angular modes apart from $l=m=2$ are not adequately excited but the overtones provide a prospect to perform this test. We discuss some challenges associated with performing as well as interpreting the results of the tests performed using black hole overtones. We investigate the robustness of modelling the post-merger signal of binary black hole coalescence as a superposition of overtones as well as study the bias expected in recovered frequencies as a function of the start time of the analysis. We provide a computationally cheap procedure to pick an optimal time to start the spectroscopic analysis of post-merger signal. Further, we find that resolving the frequencies of the overtones can be particularly challenging and requires high ringdown SNRs; for instance, the Rayleigh resolvability criterion suggests that for an event like GW150914, an SNR $\sim 200$ is necessary to resolve the overtone frequencies.

Speaker: Dr Swetha Bhagwat (La Sapienza)
• 15:45 16:15
Coffee break 30m
• 16:15 17:00
Afternoon session
• 16:15
Spontaneous scalarization in generalised scalar-tensor theory 15m

Spontaneous scalarization is a mechanism that endows relativistic stars and black holes with a nontrivial configuration only when their spacetime curvature exceeds some threshold. The standard way to trigger spontaneous scalarization is via a tachyonic instability at the linear level, which is eventually quenched due to the effect of non-linear terms. At this work (Phys. Rev. D 99, 124022 (2019) and arXiv:1904.06365) we identify all of the terms in the Horndeski action that contribute to the (effective) mass term in the linearized equations and, hence, can cause or contribute to the tachyonic instability that triggers scalarization. We acknowledge networking support by the COST Action GWverse Grant No. CA16104.

Speaker: Nikolas Andreou (University of Nottingham)
• 16:30
Numerical investigation of superradiant instabilities 15m

We present a numerical investigation of the superradiant instability in spinning black holes surrounded by a plasma with density increasing when moving closer to the black hole. We try to understand whether superradiant instabilities are relevant or not for astrophysical black-holes surrounded by matter.

Speaker: Mr Alexandru Dima (SISSA)
• 16:45
Causal structure of black holes in generalized scalar-tensor theories 15m

A modified causal structure of black holes in theories beyond general relativity might have implications for the stability of such solutions. In this talk, we explore the horizon structure of black holes as perceived by scalar fields for generalized scalar-tensor theories, which exhibit derivative self-interactions. This means that the propagation of perturbations on nontrivial field configurations can be superluminal and that the matter fields and gravitational perturbations do not necessarily experience the same causal structure.
Upon linearization, and imposing stationarity of the metric and of the scalar field, we prove that Killing horizons of the background metric are always Killing horizons of the effective metric as well.

Speaker: Mr Nicola Franchini (Sissa)
• 17:00 17:45
Discussion session: WG2 (Source Modelling)
• Thursday, 16 January
• 09:15 11:00
Morning session
• 09:15
Testing the no-hair theorem with LIGO and Virgo 45m

Gravitational waves may allow us to experimentally probe the structure of black holes, with important implications for fundamental physics. One of the most promising ways to do so is by studying the spectrum of quasinormal modes emitted by the remnant from a binary black hole merger. This program, known as black hole spectroscopy, could allow us to test general relativity and the nature of black holes, including the no-hair theorem---the statement that astrophysical black holes are fully described by their mass and spin according to the Kerr metric. I will discuss the prospects for carrying this out with existing ground-based detectors by relying on the shortest-lived tones of the dominant quadrupolar mode (aka 'overtones'), as well as our recent results from the analysis of GW150914.

Speaker: Maximiliano Isi
• 10:00
Coalescence of Exotic Compact Objects 15m

The direct detection of gravitational waves (GWs) by the LIGO and VIRGO interferometric detectors has begun a new era of GW astronomy, allowing us to study the strong regime of gravity through GW signals produced by coalescence of compact objects. In this talk, I will present our numerical studies on coalescence of binary Exotic Compact Objects (ECOs) performed by solving the Einstein equations with different types of exotic matter: boson stars, dark boson stars and Neutron Stars that contain a small fraction of dark matter particles clustered inside. These binaries lead to different dynamics and gravitational waves emission during their coalescence, which might be crucial to distinguish them with current/future LIGO and Virgo observations.

Speaker: Miguel Bezares (SISSA)
• 10:15
Quantum gravity predictions for black hole interior geometry 15m

In this talk I will show how to derive an effective Hamiltonian constraint for the Schwarzschild geometry starting from the full loop quantum gravity Hamiltonian constraint and computing its expectation value on coherent states sharply peaked around a spherically symmetric geometry. I then use this effective Hamiltonian to study the interior region of a Schwarzschild black hole, where a homogeneous foliation is available. I show how, for several geometrically and physically well motivated choices of coherent states, the classical black hole singularity is replaced by a homogeneous expanding Universe. The resultant geometries have no significant deviations from the classical Schwarzschild geometry in the pre-bounce sub-Planckian curvature regime, evidencing the fact that large quantum effects are avoided in these models. In all cases, we find no evidence of a white hole horizon formation. However, various aspects of the post-bounce effective geometry depend on the choice of quantum states and, in particular, to the numerical value of the Immirzi parameter.

Speaker: Daniele Pranzetti (Perimeter Institute)
• 10:30
Post merger signal from black hole mimickers 15m

Black holes mimickers, e.g. neutron stars or boson stars, are compact objects with similar properties to black holes.
The gravitational wave signal emitted by a binary of such putative objects during the inspiral phase is difficult to
distinguish from the one emitted by a black hole binary. Nevertheless, significant differences might appear in the
post merger signal. Inspired by the known behavior of black holes, neutron stars and boson stars we
propose a toy model that captures potential characteristics of such systems composed by such mimickers.
This model can be exploited to assess how well such signal could be recovered with gravitational waves observations from
earth based detectors using standard templates. By analyzing the residuals, i.e. the difference between the injected
signal and the best fit template, one can also develop strategies to extract the new physics described by these new signals.

Speaker: Mr Alexandre Toubiana (APC/IAP)
• 10:45
Importance of the tidal heating in binary coalescence 15m

With the observation of the multiple binary inspirals, we begin to question whether the components of the binary are black holes or some exotic compact objects (ECO). The black holeness or the deviation from it can be tested in several ways. The distinguishing feature of a black hole from other exotic compact objects is the presence of the horizon. This surface acts as a one-way membrane, that absorbs energy. Due to this different behavior from ECOs in the late stages of an inspiral black holes exchange energy, these backreact on the orbit, transferring energy and angular momentum from their spin into the orbit. This effect is called tidal heating. In Phys.Rev. D99 (2019) no.8, 084001 we argued that the tidal heating can be used as a test for the presence of the horizon, and for that, we introduced horizon parameter (H). Using H we showed that in LISA, presence or absence of the horizon can be tested accurately as well as precisely. In arXiv:1910.07841 we compute the orbital dephasing and the gravitational-wave signal emitted by a point particle in circular, equatorial motion around a spinning supermassive object to the leading order in the mass ratio. We showed that the absence of absorption by the central object can affect the gravitational-wave signal dramatically, especially at high spin. As result it allows us to put an unparalleled upper bound on the reflectivity of exotic compact objects, at the level of O(0.01)%. This can be used even in near
equal mass binaries to search for the horizon.

Speaker: Sayak Datta (IUCAA)
• 11:00 11:30
Coffee break 30m
• 11:30 12:15
Morning session
• 11:30
The first image of a black hole 45m

I will briefly discuss how the first image of a black hole was obtained
by the EHT collaboration. In particular, I will describe the theoretical
aspects that have allowed us to model the dynamics of the plasma
accreting onto the black hole and how such dynamics was used to generate
synthetic black-hole images. I will also illustrate how the comparison
between the theoretical images and the observations has allowed us to
deduce the presence of a black hole in M87 and to extract information
about its properties. Finally, I will describe the lessons we have
learned about strong-field gravity and alternatives to black holes.

Speaker: Luciano Rezzolla
• 12:15 14:15
Lunch 2h
• 14:15 15:00
Discussion session: WG3 (Black holes and fundamental physics)
• 15:00 16:15
Coffee+Posters session
• 16:15 17:15
Afternoon session
• 16:15
A VISual approach to science communication 45m
Speaker: Marcos Valdes