ONLINE APRIL 21-28, 2021 3-7 PM (CET) Registration is now closed
SCHEDULE
*Live stream here http://www.youtube.com/channel/UCEzvfKbk0kNf3tjohP97FnA/live or check your email for Zoom links
DAY 1 - April 21
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15:00-15:40 Alessandra Buonanno
Tests of General Relativity with Gravitational-Wave Observations
Abstract: In this talk, I will provide an overview of the tests of General Relativity enabled by observations of gravitational waves from binary black holes and binary neutron stars.
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15:40-16:20 Alessandra Silvestri
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16:20-17:00 Paulo Freire
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New constraints on gravity theories from pulsar timing
Abstract: In this talk, I will show how, over the last few years, pulsar timing programs have derived fundamentally improved constraints on the nature of gravitational waves and more generally on any violations of the strong equivalence principle, introducing much more stringent constraints on alternative theories of gravity. These experiments have also ruled out the phenomenon of ``spontaneous scalarization'' for neutron stars, showing that gravity has no strong, non-linear deviations from its predicted GR behaviour.
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17:00-17:30 BREAK
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17:30-18:10 Sourabh Nampalliwar
eXtreme Gravity with X-rays
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18:10-18:50 Avery Broderick
DAY 2 - April 22
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15:00-15:40 Antonio Padilla
Vainshtein in the UV: some random thoughts
Abstract: I offer some random thoughts on the Vainshtein mechanism and the impact of ultra-violet physics. I will argue that physics inside the Vainshtein radius is generically sensitive to ultra-violet effects, known and unknown. I give explicit examples where we have a UV completion of a low energy effective theory exhibiting Vainshtein screening, and show how the UV effects completely destroy the screening mechanism.
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15:40-16:20 Kazuya Koyama
Dark energy and gravitational waves
Abstract: The almost simultaneous detection of gravitational waves and a short gamma-ray burst from a neutron star merger has put a tight constraint on various modified gravity theories as a dark energy model. I will review the impact of the constraints from gravitational wave observations on screening mechanisms that are required in many modified gravity models to restore general relativity on small scales while modifying gravity at large distances.
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16:20-16:40 BREAK
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16:40-17:20 Discussion moderator Claudia de Rham "Modified gravity and screening mechanisms"
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17:20-18:00 David Langlois
Black hole perturbations in modified gravity
Abstract: This talk will present some recent work on the perturbations of non-rotating black holes in the context of Degenerate Higher-Order Scalar-Tensor (DHOST) theories. These results use a systematic approach that extracts the asymptotic behaviour of perturbations at spatial infinity and near the black hole horizon. This method, which has been applied to ``stealth'' Schwarzschild black holes and to a family of non-stealth black holes, can also be used to compute the quasi-normal modes of black holes.
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18:00-18:40 Jeremy Sakstein
Black Hole Archeology
Abstract: The LIGO/Virgo GWTC-2 catalog has provided us with the largest sample of binary black hole merger events to date. In this talk, I will demonstrate how the ever-increasing number of events allows us to become black hole archeologists. By studying the population statistics of astrophysical black holes, we can learn about the various processes governing how they formed from their massive star progenitors. New physics such as dark matter and dark energy could alter these processes and lead to novel features in the black hole mass function that can be constrained with upcoming LIGO/Virgo/KAGRA data. Thus, black hole population statistics is poised to become a powerful new probe of fundamental physics.
DAY 3 - April 23
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15:00-15:40 Lasma Alberte
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Positivity bounds in the presence of gravity
Abstract: Positivity bounds for theories with massless spin-2 fields could have deep implications for UV physics and the Weak Gravity Conjecture. However, the presence of a singularity in the t-channel of the scattering amplitudes precludes the application of the standard positivity bounds. I will discuss the recent progress in working around this issue. I will show in particular how for simple renormalizable field theories coupled to gravity applying the positivity bounds to amplitudes with the t-pole subtracted leads to constraints on the low-energy coupling constants in a way which is incompatible with their actual values, implying new physics at a scale parametrically smaller than expected. I will pay special attention to the case of QED minimally coupled to gravity.
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15:40-16:20 Katy Clough
Searching for light dark matter in strong gravity environments
Abstract: With no detection of WIMPs to date, attention is turning towards lighter mass (sub eV) candidates for dark matter such as axion-like-particles. I will discuss their wave-like behaviour, and highlight the challenges in detecting signatures from such light particles in strong gravity environments. In particular, I will discuss accretion of such dark matter onto black holes, and its role in dynamical friction. Whilst challenging, such observations may be the only way to confirm the nature of dark matter, in the absence of any couplings to standard model particles.
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16:20-16:40 BREAK
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16:40-17:20 Discussion moderator Thomas Sotiriou "Beyond GR: theoretical and experimental prospects"
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17:20-18:00 Astrid Eichhorn
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Connecting quantum gravity to observations: Three paths
Abstract: The arguably most critical challenge for quantum gravity theories is the connection to observations. This challenge is addressed in developments along distinct different paths. Along three of these paths, I will shine a spotlight on selected novel results.
First, I will highlight how experimental searches for the violation of symmetries in the gravity-matter interplay may constrain quantum gravity theory.
Second, I will show how experimental data on particle physics may be used to test the observational viability of quantum gravity theories.
Third, I will discuss how images of black holes may carry imprints of quantum gravity under the assumption that a naive estimation of the quantum-gravity scale does not hold for black holes.
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18:00-18:40 Toby Wiseman
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Spinning black holes in Einstein-aether theory
Abstract: We develop novel numerical methods to find stationary rotating black hole solutions in theories with multiple wave modes speeds, such that each has a different horizon. Einstein-aether is the canonical example of such a theory, and we numerically construct rotating black holes in that setting. This theory is strongly constrained by observations, yet still has an interesting allowed parameter space where various couplings remain large. One might then expect that black holes behave differently to Kerr in these phenomenologically allowed regions of parameter space. However we explain how in these phenomenological limits solutions exist which have the usual GR behaviour with a non-trivial aether ‘painted’ on top. This is precisely the behaviour we observe in the numerical solutions.
DAY 4 - April 26
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15:00-15:40 Shinji Mukohyama
Minimalism in modified gravity
Abstract: It is generally believed that modification of general relativity inevitably introduces extra physical degree(s) of freedom. In this talk I argue that this is not the case by constructing modified gravity theories with two local physical degrees of freedom. After classifying such theories into two types, I show explicit examples and discuss their cosmology and phenomenology.
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15:40-16:20 Lavinia Heisenberg
Playing with geometry
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16:20-16:40 BREAK
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16:40-17:20 Discussion moderator Diego Blas "Modified gravity in cosmology"
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17:20-18:00 Alex Bentley Nielsen
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Searching for black hole echoes in gravitational wave data
Abstract: Evidence for echo signals from black holes would be a phenomenal indication of new physics beyond standard gravitational models. I will discuss some of the wide range of tests that have been performed to date on gravitational wave data and discuss some of the theoretical and observational challenges as we go forwards.
18:00-18:40 Matt Lewandowski
Dark Energy: Nonlinear Effects and Precision Cosmology
Abstract: Upcoming precision cosmological observations will allow us to learn a tremendous amount about our universe, particularly in the nonlinear regime of large-scale structure and dark energy. In this talk, I discuss how we can use precision large-scale structure measurements and computations to constrain dark-energy theories, focusing on DHOST theories which violate the consistency conditions of large-scale structure. I also discuss other nonlinear effects, such as the decay of gravitational waves into scalar particles, the Vainshtein mechanism, and positivity bounds.
DAY 5 - April 27
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15:00-15:40 Diana López Nacir
Using pulsars and gravitational wave interferometers to probe ultralight dark matter models
Abstract: In this talk I will consider some examples of Dark Matter (DM) models where the DM can be described by a collection of oscillating (scalar, vector or tensor) waves. I will present the basic phenomenology and some particular estimates to illustrate that both data from pulsar timing and gravitational wave interferometers are useful to probe models of ultra-light DM. I will show that if the DM only interact through gravity the effects are estimated to be too small, so any measurement would be challenging. However, I will also show that it is possible to set interesting bounds on direct couplings of the DM to Standard Model fields, even with current data.
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15:40-16:20 Kostas Glampedakis
Testing GR with black hole shadows and quasinormal modes
Abstract: In the first part of this talk we summarize some recent progress in modelling quasinormal modes of non-GR black holes. In the second part we address the issue of testing GR with supermassive black hole shadows.
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16:20-16:40 BREAK
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16:40-17:20 Discussion moderator Kostas Kokkotas "GW phenomenology and post-merger tests"
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17:20-18:00 Paolo Pani
New physics on the horizon? Recent developments and challenges in tests of dark compact objects
Abstract: Gravitational-wave astronomy and new electromagnetic facilities allow us for unprecedented tests of the nature of dark compact objects and provide a novel way to search for new physics. I will give an overview of the many recent results in this area (including shadows, constraints on the multipolar structure, ringdown tests, gravitational-wave echoes, and tidal effects in binaries) and discuss the outstanding challenges ahead.
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18:00-18:40 Brando Bellazzini
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Positivity Bounds and the Galileon
Abstract: What does define the space of theories that are found at the end points of the renormalization group evolution subject to the fundamental principles of quantum mechanics and relativity?
While this question is still too hard to be addressed in generality, I will present in this talk the complete set of positivity conditions that 2-to-2 scattering amplitudes have to satisfy at low energy in order to descend from a unitary and causal microscopic theory. Positivity of the amplitudes translates into bounds of the low-energy Wilson coefficients that go beyond the implementation of symmetries, ruling out candidate effective theories that scale very quickly with energy in the infrared, such as Galileon-like theories.
DAY 6 - April 28
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15:00-15:40 Leo Stein
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An approach to taming secular instabilities in numerical simulations of beyond-GR theories
Abstract: Today the need has arrived to simulate black hole mergers in theories beyond GR, spurred by detector improvements, and the knowledge that GR can only be a low-energy effective theory (EFT). The well-posedness of a handful of theories is understood. Yet most theories' well-posedness is either unstudied, or known to be ill-posed as "exact" equations of motion. Thus we rely on numerical perturbation theory about a nonlinear GR background, which controls well-posedness (this approach is justified by the EFT point of view). However, naive perturbation theory breaks down in the presence of secular effects (for example, the different inspiral rate in a beyond-GR theory, which generates large GW dephasing). To get long GW predictions for beyond-GR theories, we need to tame these secular instabilities.
I will discuss an approach to controlling these instabilities based on the "dynamical renormalization group". The key insight is that the
rate of secular divergence encodes a beta function for a flow in parameter space, which can be used to renormalize a family of numerical solutions. By knowing the structure of the divergence, we can build the beta functions from secularly-diverging numerical
simulations. We demonstrate this approach with a toy model, treating a deformation to the KdV equation both exactly and perturbatively.
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15:40-16:20 Helvi Witek
Black hole binaries in quadratic gravity: dynamical (de-)scalarization
Abstract: With the advent of gravitational wave astronomy we are now in a perfect position to confront the multitude of beyond-GR theories, typically invoked to connect to quantum gravity paradigms, with actual observations in the strong-field regime of gravity. A necessary ingredient to do so, either via theory-specific tests or to inform parametrized approaches, are theoretical predictions of the (gravitational) radiation emitted by compact binaries in extensions of general relativity.
In this talk I will present recent results on modelling black hole binaries in scalar Gauss-Bonnet gravity, a well-motivated theory that involves higher-curvature corrections to Einstein's equations. As black holes in this theory can acquire scalar hair or spontaneously scalarize, binaries thereof yield new phenomena such as additional scalar radiation, dephasing of the gravitational wave signal and, possibly most excitingly, dynamical de-scalarization of these scalarized black holes.
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16:20-16:40 BREAK
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16:40-17:20 Discussion moderator Eugene Lim "Numerical Relativity beyond GR"
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17:20-18:00 Pau Figueras
Binary black hole mergers in cubic Horndeski theory
Abstract: In this talk I will discuss binary black hole mergers in cubic Horndeski theories of gravity, which are known to have a well-posed initial value problem in the standard CCZ4 formulation of the Einstein equations. In our work we consider black hole binary mergers in this class of theories treating Horndeski terms fully non-linearly whilst ensuring that the theory remains in the regime of validity of EFT. As we shall see, even if the Horndeski terms are suitably small at all times, over long enough times (e.g., a few orbits of the binary), small effects accumulate giving rise to significant differences compared to standard GR. Time permitting, towards the end of the talk I will discuss how one can study black hole binaries in higher derivative theories of gravity.
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18:00-18:40 Jose Antonio Font
Stability of spinning bosonic stars
Abstract: This talk will discuss recent results regarding the stability of spinning bosonic stars. Using numerical-relativity simulations we will show that scalar and vector bosonic stars have different stability properties. For all unstable models the energy density displays a toroidal morphology. These transient models develop a dynamical bar-mode instability which closely resembles what has long been found for rotating neutron stars. The instability triggers the emission of gravitational waves which might be within reach of future experiments. Recent proposals to quench the instability will also be discussed.
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