London-Oldenburg Relativity Seminar

(Online) London-Oldenburg Relativity Seminar

Organizers: Betti Hartmann (UCL) and Jutta Kunz (University of Oldenburg, Germany)

The aim of the London-Oldenburg Relativity Seminar is to discuss recent and important new research results in General Relativity, Quantum gravity and physics in curved space-time in general. It is a joint seminar between University College London and the University of Oldenburg, Germany. This seminar is completely online. To obtain the link to join the seminars, please contact one of the organizers.

Upcoming talks

18 October 2023, 16:15 BST // 17:15 CEST

Zenia Zuraiq (Indian Institute of Science)

Title: t.b.a.

Abstract: t.b.a.

25 October 2023, 16:15 BST // 17:15 CEST

Nicoleta Voicu (Transilvania University of Brasov, Romania)

Title: t.b.a.

Abstract: t.b.a.

15 November 2023, 16:15 BST // 17:15 CEST

Zoe Wyatt (King's College London, UK)

Title: t.b.a.

Abstract: t.b.a.

22 November 2023, 16:15 BST // 17:15 CEST

Mayusree Das (Indian Institute of Science)

Plausible detection of rotating magnetized neutron stars by their continuous gravitational waves

Abstract: t.b.a.

29 November 2023, 16:15 BST // 17:15 CEST

Huanchen Hun (Max-Planck-Institute for Radiastronomy Bonn, Germany)

Title: t.b.a.

Abstract: t.b.a.

6 December 2023, 16:15 BST // 17:15 CEST

Eleonora Di Valentino (University of Sheffield, UK)

Title: t.b.a.

Abstract: t.b.a.

24 January 2024, 16:15 BST // 17:15 CEST

Laura Bernard (Laboratoire Univers et Théories (LUTH), Meudon, France)

Title: t.b.a.

Abstract: t.b.a.

Previous talks - Spring/Summer 2023

12 July 2023, 16:15 BST // 17:15 CEST

Alessia Platania (Perimeter Institute, Canada)

Asymptotic constraints on quantum black holes

We tackle the question of whether regular black holes or other alternatives to the Schwarzschild solution can arise from an action principle in quantum gravity. Focusing on an asymptotic expansion of such solutions and inspecting the corresponding field equations, we demonstrate that their realization within a principle of stationary action would require either fine-tuning, or strong infrared non-localities in the gravitational effective action. We will also show that the black hole entropy of theories displaying such infrared non-localities diverge. The principle of least action and the consistency of Wald entropy thus yield non-trivial asymptotic constraints on the metric of quantum black holes.

5 July 2023, 16:15 BST // 17:15 CEST

Elena Giorgi (Columbia University, USA)

Black Hole Stability Problems in GR

In this talk, I will give an overview of the stability problems for black hole solutions, starting with the mode stability results in black hole perturbation theory in the 80’s to more recent mathematical proofs, as the linear and the fully non-linear stability of black hole solutions require new mathematical techniques. Finally, I will present some aspects of our recent proof with Klainerman and Szeftel of the non-linear stability of the slowly rotating Kerr black hole.

28 June 2023, 16:15 BST // 17:15 CEST

Katy Clough (Queen Mary University London, UK)

Using numerical simulations to probe fundamental physics in strong gravity regimes

Numerical relativity simulations have allowed us to isolate and characterise the gravitational wave signals from merging black holes and neutron stars, giving us information about their parameters and astrophysical origins. The same simulations provide an opportunity to probe new effects beyond the standard model + GR paradigm, including new particles and fields, modifications to gravity, and strong gravity effects in the early universe. I will give an overview of the state of the art in such simulations, discuss their main challenges and highlight their potential to shed light on fundamental questions.

21 June 2023, 16:15 BST // 17:15 CEST

Ludovic Ducobu (Transilvania University of Brasov, Brasov, Romania)

Black holes with scalar hair beyond Riemannian geometry: the case of teleparallel gravity

Despite its consequential successes, the necessity to question the framework of General Relativity [GR] is clear at both the experimental (origin and composition of dark matter and dark energy) and theoretical (search for quantum gravity) levels. Since not all these puzzles can be purely reduced to quantum correction problems, this motivates the study of alternative theories of gravitation already at the classical level. To this purpose, one can follow two distinct but complementary routes. On the one hand, one can try to modify Einstein's equations by introducing new fields while maintaining the assumptions related to the geometrical structure of spacetime. On the other hand, one can precisely question the framework of pseudo-Riemannian geometry as being adapted to the full description of spacetime (typically, at cosmological scales). In the first case, scalar-tensor theories of gravity (and especially Horndeski gravity), where the new degrees of freedom are encoded by means of a scalar field, have been extensively investigated in the literature. In the later case, a minimal generalisation of GR's framework consists in relaxing the hypothesis linking the connection to the metric, leading to the framework of metric-affine gravity. This reveals new starting points to generalise GR through, for instance, an examination of the so-called geometrical trinity of gravity. The aim of this talk is to present the main motivations and features of modified theories of gravity involving scalar fields in the metric-affine approach. I will first propose a quick overview of Horndeski gravity and of the metric-affine approach of GR; focussing on the so-called Teleparallel theories of gravity and on the Teleparallel equivalent to GR. Then, "unifying" the two approaches, I will advocate for the interest of modified theories of gravity involving scalar fields in the teleparallel framework. To this purpose, I will present recent results from my own research and discuss the first study of spontaneous scalarization of asymptotically flat black holes in a teleparallel generalisation of Horndeski gravity. This study gives an explicit demonstration of how scalar-torsion gravity (and possibly other non-Riemaniann models) provides a richer canvas for the study of phenomenology beyond GR and open the way for novel studies regarding scalarized black hole solutions in non-Riemannian theories of gravity. This presentation is based on the work done in [arXiv:2212.07653 [gr-qc]] ( in collaboration with Sebastian Bahamonde, Daniela Doneva, Christian Pfeifer and Stoytcho Yazadjiev.

24 May 2023, 16:15 BST // 17:15 CEST

Clare Burrage (University of Nottingham, UK)

Testing dark energy models with atom interferometry

The accelerated expansion of the universe motivates a wide class of scalar field theories that modify gravity on large scales. In regions where the General Relativity has been confirmed by experiment, such theories need a screening mechanism to suppress the new force. I will describe how theories with screening mechanisms can be tested in the laboratory, in particular with atom-interferometry experiments. I will describe the results of a recent experiment in which we measured the acceleration of an atom toward a macroscopic test mass inside a high vacuum chamber, where the new force is unscreened in some theories. Our measurement shows that the attraction between atoms and the test mass does not differ appreciably from Newtonian gravity. This result places stringent limits on the free parameters in chameleon and symmetron theories of modified gravity.

17 May 2023, 16:15 BST // 17:15 CEST

Petra Sukova (Astronomical Institute of the Czech Academy of Sciences, Prague, Czech Republic)

Quasiperiodic ultrafast outflows from galactic nuclei pointing towards EMRI systems

Based on the high cadence X-ray monitoring of a flare coming from the nucleus of a galaxy located cca 250 Mpc away and discovered by ASASSN in December 2020, the existence of ultra-fast outflows in the source was revealed. The outflows were quasiperiodically launched from near the supermassive black hole (SMBH) with variable column density and ionization, but constant velocity, for more than three months. I will present our model interpreting the observed phenomenon as outflows induced by an object periodically transiting through the accretion flow onto the SMBH. The secondary object is much less massive than the primary, making the system the location for an (future) Extreme Mass Ratio Inspiral (EMRI). Such scenario is supported by our GRMHD simulations of stellar transits through accretion disc. The strength of the outflow suggests large influence radius of the perturber, hence indicating it to be intermediate-mass black hole rather than a usual star. Tweaking the parameters of the model, which include the semi-major axis, elipticity and inclination of the orbit of the secondary and its influence radius, the system exhibits different observational signs, including the ultrafast outflows.

10 May 2023, 16:15 BST // 17:15 CEST

Silvia Pla Garcia (King's College London)

How can quantum tunnelling induce a cosmological bounce?

In this talk, I will present a novel mechanism to induce a cosmological bounce that is purely generated by quantum fluctuations without the need for exotic matter or modified gravity. In the first part of the talk, I will give a gentle introduction to quantum field theory in curved spacetimes. In the second part, I will explain how this quantum-induced bounce emerges in this context when we allow a scalar field on a flat FLRW universe to tunnel between two degenerate minima.

19 April 2023, 16:15 BST // 17:15 CEST

Merce Guerrero Roman (University of Madrid, Spain)

From the properties of regular compact objects in Palatini formalism to their phenomenology

The Kerr (exterior) solution of General Relativity (GR) is one of the most relevant results of such theory since it is compatible with the current observational results. But GR has some shortcomings as, for example, unavoidable singularities in the inner part of the Kerr solution which entail a lack of predictability of our theory. This suggests extending GR in order to get rid of such shortcomings. In particular, in this seminar we are using the so-called Ricci-Based Gravities formulated "à la" Palatini. We will see that we can find non-singular static solutions within these theories. Additionally, we will also analyse some phenomenology of such a solution, like the optical appearance, or some possible tests of the theory using Low-Mass Stars.

26 April 2023, 16:15 BST // 17:15 CEST

Maria Chivers (University College London, UK)

Geometries of Black Hole horizons

The geometry of every black hole is very different and complicated. The aim of this talk is to introduce three black holes in order to compare their symmetries, horizons and other important features. I will try to show the importance of studying the near horizon geometry of black holes, where Killing vectors will be used to generate the symmetries of their horizons. For each black hole, Killing vector fields will be found in order to find the symmetries and isometries of their individual metric. Using this, special type of horizons called the Killing horizon will be derived.

12 April 2023, 16:15 BST // 17:15 CEST - HYBRID EVENT (in person at University Oldenburg)

Kamal Hajian (Department of Physics, Middle East Technical University, Ankara, Turkey)

Gauge Invariant First Law of Black Hole Thermodynamics

In gauge invariant theories, like Einstein-Maxwell theory, physical observables should be gauge invariant. In particular, mass, entropy, angular momentum, electric charge and their respective chemical potentials, temperature, horizon angular velocity and electric potential which appear in the laws of black hole thermodynamics should be gauge invariant. In the usual construction of the laws of black hole thermodynamics, gauge invariance of the intensive quantities is subtle; I will present our recent work (in collaboration with B. Tekin and M.M. Sheikh-Jabbari) in which we have tried to resolve this issue and provide a gauge invariant derivation and the proof of the first law of black hole thermodynamics.

Previous talks - Autumn/Winter 2022/2023

7 June 2023, 16:15 BST // 17:15 CEST

Jennie Traschen (University of Massachusetts Amherst, USA)

Thermodynamics of KNdS Black Holes and Spin Systems Or, How I came to love negative temperature

A system of non-interacting spins in a magnetic field provides a simple model of a paramagnet. The entropy of the system as a function of energy is symmetric about its maximal value since the counting of states is symmetric upon interchanging up and down. The energy of states does depend on how many spins are up, and increases monotonically from the state with all spins up to the state with all down. As a result, the temperature of the system is positive on the lower E side of the S(E) curve and negative on the other. The thermodynamic properties of rotating and/or charged black holes with positive cosmological constant (KNdS) have many of the same features as the spin system, due to the presence of the black hole, cosmological and inner horizons. The mass is bounded between minimal and maximal possible values. The black hole surface gravity is zero at each limit, and positive for intermediate masses. The cosmological horizon surface gravity is negative, with magnitude decreasing to zero at the maximal mass. In a portion of the black hole parameter space, the total area A of the black hole plus cosmological horizons has a symmetry that maps between two distinct spacetimes with the same total area. Hence A(M) has a maximum in the middle of the mass range. The A(M) curve defines a `` surface gravity of the system" which is positive on the lower M side of the curve and negative on the large M side. We divide the space of spin states into two constrained subsystems, such that their temperatures parallel the behavior of the black hole and cosmological surface gravities, and for which the system temperature of the combined set has the same qualitative behavior as the KNdS system surface gravity. We speculate on what the spin fractions of up or down corresponds to in the black hole spacetimes.

3 May 2023, 16:15 BST // 17:15 CEST

Saikat Das (Indian Institute of Technology Madras, India)

Chaotic dynamics of orbits around Pseudo-Newtonian compact objects

General Relativistic calculations can be intensive and computationally demanding while studying orbital or accretion dynamics around compact objects. The mimicking potentials come in handy, simplifying the computation significantly and maintaining high accuracy. The phenomenon of chaos comes naturally into the picture while studying the nature of the orbits arising from these pseudo-potentials. In this seminar, I will present the general notion of chaos in the field of orbital dynamics around a central, pseudo-Newtonian compact object primary. We will specifically consider the Schwarzschild and Kerr-like compact objects with an artificial exterior dipolar potential representing an asymmetrically placed halo around the central body. We will look at the dependence of chaos on different orbital parameters with the help of chaotic indicators. We will begin with analysing equatorial orbits and proceed to the off-equatorial orbits inclined at a certain angle with the equatorial plane.

25 Feburary 2023, 8:00 GMT // 9:00 CET (SPECIAL TIME)

Renate Meyer (University of Auckland, New Zealand)

Spectral analysis of gravitational wave data using Bayesian nonparametrics

The new era of gravitational wave astronomy truly began on September 14, 2015, with the sensational first direct observation of gravitational waves, when LIGO recorded the signature of the merger of two black holes. In the subsequent three observing runs of the LIGO/Virgo network, gravitational waves from 90 compact binary mergers have been announced. Improvements to the sensitivity of the current ground-based detectors are being made and the next generation detectors such as the Einstein Telescope and the Cosmic Explorer planned. Moreover, the future space-based observatory LISA will open the low-frequency window on gravitational waves and will be sensitive to a vast range of sources including the white dwarf binaries in our Milky Way and mergers of supermassive black holes at the centre of galaxies. A careful statistical analysis of the measurements is crucial to realize the full potential for new science, in particular characterizing the instrumental noise and correctly modelling its spectrum. A review of the basics of gravitational wave data analysis will be given. With a focus on LISA, the main new challenges will be highlighted and some novel Bayesian nonparametric approaches to spectral density estimation of multivariate, non-Gaussian and locally stationary time series proposed.

18 January 2023, 16:15 GMT // 17:15 CET

Antonio Dalfonso Del Sordo (University College London)

Dynamical systems in cosmology: an introduction

From something as simple as the swinging of a pendulum to the random motion of particles in air and the spread of infectious diseases, dynamical system techniques are widely employed to analyse physical phenomena. They also turn out to be a convenient tool for the study of cosmology and, in particular, the evolution of the universe itself. In this talk, I will provide a concise, gentle, pedagogical introduction to both cosmology and dynamical systems, and explore models of dark energy that have been studied using this approach.

11 January 2023, 16:15 GMT // 17:15 CET

Bahareh Azad (University of Oldenburg, Germany)

Quasi normal modes of Ellis wormhole for polar perturbations

Currently, gravitational wave astronomy is giving us a new potential approach to observe wormholes through their damping modes called quasi-normal modes. Quasi-normal modes are characteristic modes of a freely oscillating space-time. In this talk I will explain how we calculate the Quasi-normal modes in perturbation theory considering polar perturbations of static Ellis–Bronnikov wormholes. I will discuss wormholes with a finite mass, moreover, the interesting results on Isospectrality and also coupled set of perturbation equations which are agree with the master equation for the axial modes for massless wormholes. Finally I will show the quasi-normal mode frequencies and decay rates we calculated for l = 2 , 3 and 4.

14 Dcember 2022, 16:15 GMT // 17:15 CET

Deanna Hooper (University of Helsinki, Finland)

Lyman-alpha constraints on non-standard dark matter

Despite its remarkable success, the standard LCDM paradigm has been challenged lately by significant tensions between different datasets. This has fueled interest in beyond-LCDM models, such as dark matter models with interactions or non-negligible velocities, referred to with the umbrella term 'non-standard dark matter'. These models induce a suppression of the matter power spectrum on small scales, making them an ideal target to be constrained with Lyman-alpha data. In this talk I will discuss a method to use Lyman-alpha data without needing new computationally-expensive hydrodynamical simulations. I will present recent competitive bounds for warm dark matter, mixed warm+cold models, and dark matter interactions, highlighting the broad range of applicability of this method.

9 December 2022, 13:30 GMT // 14:30 CET (SPECIAL TIME AND DAY)

Sheila Kannappan (University of North Carolina, USA)

Toward a z=0 census of active galactic nuclei in dwarf galaxies

We recently announced the discovery of a major overlooked population of massive black holes in dwarf galaxies in the local universe (Polimera et al. 2022). The new population consists of galaxies that are classified as star-forming (SF) by the most widely used optical emission-line diagnostic plot but as active galactic nuclei (AGN) by alternative diagnostic plots. We have confirmed both their SF and AGN nature, so we call them SF-AGN. In hindsight, the inconsistent signatures of SF-AGN should have been expected in dwarf galaxies, but the biases inherent in standard galaxy surveys made these AGN easy to overlook. The unusually deep mass- and volume-limited design of our redshift z=0 surveys, RESOLVE and ECO, both led us to SF-AGN and enabled a census of their distribution in the galaxy population. This work sets the most robust lower limit on the dwarf AGN occupation fraction currently available, several times higher than previous reliable estimates. I will describe the challenges and opportunities ahead as we seek to build on this census.

30 November 2022, 16:15 GMT // 17:15 CET

Hyat Huang (University of Oldenburg, Germany and Jiangxi Normal University, China)

How well are trapping horizons for describing an evolving wormhole

Black holes and wormholes are two fascinating objects in General Relativity. In stationary metrics, the event horizon and the static wormhole throat are two important hypersurfaces for black hole and wormhole respectively. However, things become complicated when we go to the dynamic cases. In this talk, we are going to discuss the definitions of dynamic wormhole throat and black hole horizon. By comparing several definitions, one can find the trapping horizon may serve as a proper definition for both wormhole throat and black hole horizon. Based on this assumption, we provide an analytical solution in the Einstein-Maxwell-scalar theory, which can be treated as a toy model to describe the black hole/wormhole transition.

23 November 2022, 16:15 GMT // 17:15 CET

Tessa Baker (Queen Mary University London)

Beyond standard sirens: Gravitational wave cosmology without electromagnetic counterparts

The first seven years of direct gravitational wave detection have had a huge impact not only on astrophysics, but also on cosmology and fundamental physics. Central to these developments have been `Standard Sirens’, gravitational wave sources with electromagnetic counterparts. However, these represent only a tiny fraction of gravitational wave events; the rest happen entirely without any detectable electromagnetic emission. I’ll start by explaining why Standard Sirens are such powerful tools for cosmology, and how they enable us to test the laws of gravity on the largest distance scales in the universe. We’ll look at the results obtained from our one and only Standard Siren to date, binary neutron star merger GW170817. Realising that GW170817 was a very rare event, we’ll then ask: can we do any cosmology at all with ordinary, non-counterpart gravitational wave events? I’ll explain the `Dark Sirens' technique developed for this purpose, and the challenges to be overcome along the way.

10 November 2022 (SPECIAL DAY), 16:15 GMT // 17:15 CET

Alexander Jenkins (University College London)

From the tabletop to the Big Bang - Quantum simulators of false vacuum decay

False vacuum decay (FVD) plays a vital role in many models of the early Universe, with important implications for inflation, the multiverse, and gravitational waves. However, we still lack a satisfying theoretical understanding of this process, with existing approaches working only in imaginary (Euclidean) time, and relying on numerous assumptions that have yet to be empirically tested. An exciting route forward is to use laboratory experiments which undergo transitions analogous to FVD, allowing nature to simulate all of the non perturbative quantum effects for us. In this talk, I will discuss ongoing work to develop such analogue FVD experiments within the Quantum Simulators for Fundamental Physics (QSimFP) consortium. In particular, I will present numerical lattice simulations of ultracold atom systems undergoing FVD, and discuss outstanding challenges in using upcoming experimental data to understand the early Universe.

16 November 2022, 16:15 GMT // 17:15 CET

Michael Wondrak (Radbound University Nijmegen, The Netherlands)

Quadratic Gravity in the Light of the Event Horizon Telescope

Despite the success over a large range of length scales, general relativity is known to lose predictability in the quantum regime. On the contrary, quadratic gravity (QG), a theory containing quadratic terms in curvature, is known as a prototypical example of a perturbatively renormalizable quantum theory of gravity. The focus of this talk is on the imprints of QG on the images of compact objects as observed by the Event Horizon Telescope. We construct the phase space of Schwarzschild-like objects (static, spherically symmetric, asymptotically flat) in QG and find it dominated by naked singularities and wormholes. Despite the fact that these solution resemble the Schwarzschild metric outside of the would-be horizon, there is a variety in the reduction of the photon absorption cross section. Actually, the interior metric profile allows us to identify topologically inequivalent classes of solutions, which leave a direct imprint on EHT images in terms of the shadow radius and intensity profile for an optically thin accretion model.

2 November 2022, 16:15 GMT // 17:15 CET

Joanes Lizarraga (University of the Basque Country (UPV/EHU), Bilbao, Spain)

Non-linear gauge excitation in Axion Inflation

The axion inflation model is expected to be very efficient generating GWs and scalar density perturbations at the end of the inflationary period due to the exponential growth of gauge modes. In this talk I will show new results for the non-linear dynamics of the axion-inflation model, where we study how the gauge fields backreact on the inflaton dynamics by using lattice simulation, simulating all degrees of freedom simultaneously. We will also compare the results from our lattice simulations with those that use analytical approximations and where inhomogeneous effects are neglected. Our simulations were performed using a new lattice formulation based on CosmoLattice, which is a package for lattice simulations of field dynamics in expanding universes.

26 October 2022, 16:15 BST // 17:15 CEST

Nele Callebaut (University of Cologne, Germany)

Reconstructing bulk equation of motion using CFT modular Hamiltonians

n the context of AdS/CFT, the program of bulk reconstruction aims to reconstruct observables in AdS gravity (or 'bulk' observables) in terms of CFT data. A free scalar field in AdS for example can be reconstructed in terms of smeared primary operators of the CFT in a process called HKLL bulk reconstruction. It is significantly harder to reconstruct interacting fields in AdS, in particular gravitationally interacting fields. In the highly symmetric case of 3-dimensional AdS however, bulk reconstruction of a gravitationally dressed scalar field is possible. In this talk I will discuss how these bulk objects are special from the point of view of CFT entanglement data, in particular what the action of the modular Hamiltonian on the (respectively free or gravitationally interacting) bulk field is. The other way around, the action of the modular Hamiltonian on the CFT representation of the bulk field constrains the field's bulk equation of motion to be consistent with diffeomorphism invariance. The talk will be based on 2205.15244.

19 October 2022, 16:15 BST // 17:15 CEST

Katherine Mack (North Carolina State University, USA and Perimeter Institute, Canada)

Dark Matter, First Light

Dark matter forms the foundation for all cosmic structure, and its fundamental nature is one of science's most pressing enigmas. As we search for the most distant galaxies in the universe with radio and infrared observations, we are in a position to explore the particle physics of dark matter — the possibility of annihilation, decay, or other particle interactions — through its effects on early stars and galaxies. I will give an update on the quest to identify dark matter both in the lab and in the sky, major unsolved problems in dark matter theory, and how upcoming observations of the epoch of the first cosmic structures can be used to open a new window on the dark universe.

Previous talks - Spring/Summer 2022

19 July 2022, 15:15 BST // 16:15 CEST

Stefanie Walch-Gassner (University of Cologne, Germany)

Simulations of star formation and feedback in the multi-phase interstellar medium

Star formation takes place in the densest and coldest parts of the interstellar medium (ISM), in molecular clouds. These are swept up by multiple supernova explosions on scales of several hundred parsec. While condensing out of the warm ISM, the clouds are continuously fed with fresh gas. Thus, the turbulent substructure and magnetic field properties are imprinted during cloud formation. The formation of dense clouds from the multi-phase ISM, the onset of star formation, and the evolution of the molecular clouds under the impact of stellar feedback from massive stars is studied in high-resolution simulations within the SILCC-Zoom project. We use adaptive-mesh-refinement, ideal magneto-hydrodynamics simulations with the FLASH code, which we have heavily developed to include multiple physics modules such as radiation transport and a live chemical network. I will present some of our latest results from these simulations and give an overview on the numerical model. Due to our live chemical network, we are also able to reconcile simulations and observations using molecular lines and dust continuum emission. These so-called synthetic observations help us to better understand observational data.

12 July 2022, 15:15 BST // 16:15 CEST

Alexander Zhuk (Odessa I.I.Mechnikov National University, Ukraine & Center for Advanced Systems Understanding (CASUS), Görlitz, Germany)

Relativistic approach to the large-scale structure formation: cosmic screening vs. gevolution

Due to the modern telescopes, we found that the Universe is filled with a cosmic web which is composed of interconnected filaments of galaxies separated by giant voids. The emergence of this large-scale structure is one of the major challenges of modern cosmology. We study this phenomenon with the help of relativistic N-body cosmological simulation based on General Relativity. It is well known that gravity is the main force responsible for the structure formation in the Universe. In the first part of my talk, I demonstrate that in the cosmological setting gravitational interaction undergoes an exponential cutoff at large cosmological scales. This effect is called cosmic screening. It arises due to the interaction of the gravitational field with the background matter. Then, I compare two competing relativistic approaches to the N-body simulation of the Universe large-scale structure: “gevolution” vs. “screening”. To this end, employing the corresponding alternative computer codes, I demonstrate that the corresponding power spectra are in very good agreement between the compared schemes. However, since the perturbed Einstein equations have much simpler form in the “screening” approach, the simulation with this code consumes less computational time, saving almost 40% of CPU (central processing unit) hours.

5 July 2022, 15:15 BST // 16:15 CEST

Olivera Mišković (Pontifical Catholic University of Valparaiso, Chile)

Spontanous scalarization of extremal black hole

We study the entropy of an electrically charged extremal black hole coupled to a scalar field. We show that the scalar field condensates at the horizon for some critical charge, triggering an instability of the Reissner-Nordström (A)dS extremal black hole. A near-critical analysis reveals that, on one side of the critical point, the hairy black hole has larger entropy than the non-hairy one, thus giving rise to a zero temperature phase transition. Our results are analytical and based on the laws of thermodynamics.

28 June 2022, 15:15 BST // 16:15 CEST

Jon Urrestilla (University of the Basque Country (UPV/EHU), Bilbao, Spain)

Topological defects in Cosmology: Introduction and some current research

In this talk I will give a pedagogical introduction about topological defects for an audience who has not encountered them before. Then, I will briefly discuss two current lines of research in this area: First, I will discuss the discrepancies that different approaches predict for the Gravitational Waves coming from strings, and how we are trying to tackle this issue. Second, I will present how numerical simulations of defects are involved in predictions related to axionic dark matter particles.

13 June 2022, 16:15 BST // 17:15 CEST (SPECIAL DAY and TIME)

Lara Sousa (Centro de Astrofísica da Universidade do Porto & Instituto de Astrofísica e Ciências do Espaço Porto, Portugal)

Probing cosmic (super)strings with gravitational waves

In this talk, I discuss the stochastic gravitational wave generated by cosmic superstring networks and compare it to that generated by ordinary cosmic strings, by using current semi-analytical models to describe their dynamics. I show that approximating this spectrum using that of ordinary strings with reduced intercommuting probability (as is often done in the literature) leads, in general, to an under-estimation of the amplitude of this spectrum. I also show that heavier string types may leave distinctive signatures on the stochastic gravitational wave background that may be within the reach of current and upcoming gravitational wave backgrounds, which may allow us, in principle, to distinguish this spectrum from that of ordinary strings.

21 June 2022, 15:15 BST // 16:15 CEST

Erik Jensko (Department of Mathematics, University College London)

Cosmological dynamical systems in modified gravity

In this talk, I will apply dynamical systems theory to cosmological models in modified theories of gravity. In particular, I will focus on the "second-order" geometric modifications, i.e., geometries that are non-Riemannian but lead to second-order equations of motion. The dynamical systems formulation is presented in a model independent way and is equally applicable to the so-called teleparallel f(T) gravity and symmetric teleparallel f(Q) gravity theories. The equivalence of these theories is also briefly discussed. Lastly, I analyse the existence and stability criteria for the fixed points and show that stable de Sitter points are quite generic.

7 June 2022, 16:15 BST // 17:15 CEST (SPECIAL TIME)

Kamal Hajian (University of Ankara, Turkey & HWK Delmenhorst, Germany)

Light speed memory as a local observable for soft hairs

In this talk, first I will review the Asymptotic Symmetry Group (ASG) and its special case of the BMS group in flat asymptotics. Then, the soft charges and memory effect will be explained. At the end, I will present the relation of the speed of light with the memory effect and soft charges.

31 May 2022, 15:15 BST // 16:15 CEST

Antonina Zinhailo (Silesian University in Opava, Czech Republic)

Higher order WKB formula for quasinormal modes and grey-body factors: recipes for quick and accurate calculations

I will consider one of the most used methods for the calculation of quasinormal modes and grey-body factors. It is called the WKB-method. We review various situations in which the WKB formula can or cannot bring us to reliable conclusions. We show that averaging of the Padé approximations, suggested first by J. Matyjasek and M. Opala, leads to much higher accuracy of the WKB approach, estimate the error and present the automatic code which computes quasinormal modes. After that, we present results for the simplest case of the Schwarzschild metric and compare them with the exact values obtained by numerical methods. In conclusion, we will show that for calculations in most cases, the WKB expansion up to 6-7th order is optimal.

24 May 2022, 15:15 BST // 16:15 CEST

Céline Zwikel (Perimeter Institute, Canada)

The partial Bondi gauge: Further enlarging the asymptotic structure of gravity

In this talk, I will introduce the partial Bondi gauge in 4 spacetime dimensions where only 3 conditions are fixed on the metric (g_{rr}=0=g_{rA}). This gauge is designed to approach asymptotic boundaries along null rays. I will derive the solution space and the symmetries preserving such boundary conditions. This has the advantage to treat simultaneously the Bondi-Sach gauge (where the determinant of the transverse metric is fixed) and the Newman-Unti gauge (where the radial coordinate is affinely parametrized). Arbitrary time dependent boundary sources and polyhomogeneous expansion are allowed. Most importantly a new symmetry generator is uncovered in the partial Bondi gauge associated to radial translations where the translation parameters are time and angle dependent. The symmetry algebra is then spanned by time-dependent supertranslations, superrotations, Weyl transformations and the new radial translations.

17 May 2022, 15:15 BST // 16:15 CEST

Alessandra Gnecchi (Max Planck Institute for Physics Munich, Germany)

Large and Small Non-extremal Black Holes, Thermodynamics Dualities and the Swampland

I will consider charged black holes at finite temperature and investigate infinite distance limits of these configurations in the temperature (T) vs entropy (S) parameter space. Due to the coupling of the geometry to scalar fields, I will show how the large/small T limit, as well as the small/large S limit, are often associated to the appearance of a tower of massless states, pushing these solutions outside of the quantum gravity landscape and into the swampland. I will discuss how this can be associated to a temperature distance conjecture. I will consider general couplings of charged, dilatonic black holes, and show that the same considerations apply to multi-charge solutions of N=2 supergravity.

10 May 2022, 15:30 BST // 16:30 CEST (Special time)

Victoria Yankelevich (University of Liverpool, UK)

The halo bispectrum as a sensitive probe of massive neutrinos and baryon physics

The power spectrum has been a workhorse for cosmological studies of large-scale structure. However, the present-day matter distribution is highly non-Gaussian and significant cosmological information is also contained in higher-order correlation functions. Meanwhile, baryon physics (particularly AGN feedback) has previously been shown to strongly affect the two-point statistics but there has been limited exploration of its effects on higher-order functions to date. Here we use the BAHAMAS suite of cosmological hydrodynamical simulations to explore the effects of baryon physics and massive neutrinos on the halo bispectrum. In contrast to matter clustering which is suppressed by baryon physics, we find that the halo clustering is typically enhanced. The strength of the effect and the scale over which it extends depends on how haloes are selected. On small scales k > 1 h/Mpc, dominated by satellites of groups/clusters), we find that the bispectrum is highly sensitive to the efficiency of star formation and feedback, making it an excellent testing ground for galaxy formation models. We show that the effects of feedback and the effects of massive neutrinos are largely separable (independent of each other) and that massive neutrinos strongly suppress the halo bispectrum on virtually all scales up to the free-streaming length (apart from the smallest scales, where baryon physics dominates). The strong sensitivity of the bispectrum to neutrinos on the largest scales and galaxy formation physics on the smallest scales bodes well for upcoming precision measurements from the next generation of wide-field surveys.

3 May 2022, 15:15 BST // 16:15 CEST

Marika Taylor (University of Southampton, UK)

Charges and fluxes in cosmological backgrounds

Following the detection of gravitational waves by the ground based LIGO detectors, space based experiments are planned to detect gravitational waves at different frequencies, both those associated with supermassive black holes and those produced in the early Universe. This talk will explore the definition of fluxes and charges in cosmological backgrounds, which is relevant to the theoretical understanding of gravitational waves in our Universe. We will begin by explaining the subtleties in defining charges in general relativity, and how these are addressed by an approach developed by Wald and collaborators. We will then extend the Wald approach to de Sitter like cosmologies and explain how charges/fluxes can be defined in this context.

26 April 2022, 15:15 BST // 16:15 CEST

Archana Pai (Indian Institute of Technology Mumbai, India)

Colliding black holes in gravitational-wave window

Thanks to the continuous improvement in the detectors and the search algorithms, the advanced LIGO-Virgo gravitational wave detector network has observed many compact binary merger events during the past three observational runs. The source properties of these observed black holes have provided inputs on the black hole population and posed new questions. This talk will summarize the current status of gravitational wave astronomy, with an emphasis on special events including the significant intermediate mass black hole event.

19 April 2022, 15:15 BST // 16:15 CEST

Betti Hartmann (Department of Mathematics, University College London)

Black holes and boson stars with new scalar hair

Static, spherically symmetric black holes can carry scalar hair when coupling standard Einstein gravity minimally to a self-interacting complex scalar field and a U(1) gauge field. For this scalar hair to exist, the frequency of the scalar field needs to be fine-tuned. In this talk, I will discuss these solutions and point out that for sufficiently large gravitational coupling, the space-time splits into two distinct parts: (a) a inflating interior and (b) an exterior which is described by the extremal Reissner-Nordström solution. Moreover, for a specific range of parameters, the scalar hair develops spatial oscillations, i.e. both black holes as well as boson stars can carry so-called wavy scalar hair.

Previous talks - Autumn/Winter 2021/2022

19 October 2021, 13:15 - 14:45 BST

Gianmassimo Tasinato (University Of Swansea, UK)

Probing the Physics of Inflation with Gravitational Wave Experiments

Cosmological inflation predicts the existence of a stochastic background of gravitational waves (GW), whose features depend on the model of inflation under consideration. There exist well motivated frameworks leading to an enhancement of the primordial GW spectrum at frequency scales testable with GW experiments, with specific features as parity violation, anisotropies, and non-Gaussianity. I will explain the properties of such scenarios, and their distinctive predictions for what respect GW observables. I will then discuss perspectives for testing these predictions with future GW experiments.

26 October 2021, 13:15 - 14:45 GMT

Dennis Stock (University of Geneva, Switzerland)

The Hawking energy of the observable universe

Addressing cosmological questions exclusively based on observations requires a formulation on the past lightcone of a cosmic observer. In this talk, I will discuss the question of gravitational energy associated with the past lightcone by introducing Hawking’s quasi-local energy as a tentative measure of energy of the observable universe. The Hawking energy phenomenologically quantifies energy in terms of the amount of light bending. I will first discuss general properties such as well-definedness, positivity, and monotonicity of the Hawking energy on the past lightcone of a cosmic observer, before addressing the question of how it can be related to cosmic observables within linearly perturbed FLRW spacetimes.

2 November 2021, 13:15 - 14:45 GMT

FechScen Khoo (University of Oldenburg, Germany)

Quasinormal modes of neutron stars in GR and in massive scalar-tensor theories

The gravitational waves during the ringdown phase of a binary neutron star merger are emitted at characteristic frequencies that can be calculated using the quasinormal modes. We will review the computations of quasinormal modes of the neutron stars in general relativity, and discuss some recent results for a class of massive scalar-tensor gravity theory which can be reformulated as R-squared gravity. We observe how the results deviate from GR, and where some of the properties in GR are broken.

16 November 2021, 13:15 - 14:45 GMT

Nathalia Pio Aprile (University of Sao Paulo, Brazil)

Holographic Superconductors

The most prominent and best understood example of gauge/duality is the AdS/CFT conjecture was established by the physicist Juan Maldacena in 1997. Here, AdS stands for Anti-Sitter spacetime, while CFT stands for conformal field theory. This conjecture states that there is a physical equivalence between both theories. States, observables, correlation functions and dynamics are equivalent in both theories. Due to the strong/weak coupling between the parameters of this conjecture, it has been applied to study strongly correlated systems in condensed matter. This work aims at the application of AdS/CFT to the phase transition diagrams for systems of kind conductor/superconductor using three different models.

23 November 2021, 13:15 - 14:45 GMT

Eugeny Babichev (University of Paris-Saclay, France)

Disformed Kerr metric

I will talk about disformal versions of the Kerr spacetime in higher order scalar tensor theories. Properties of the constricted solutions are rather non-trivial and in many aspects differ from those of the Kerr solution. Although the disformed metric has only a ring singularity and asymptotically is quite similar to Kerr, it is found to be neither Ricci flat nor circular. Non-circularity has far reaching consequences on the structure of the solution. The horizon for the disformed metric does not coincide with the stationary limit of infalling observers, unlike the Kerr case. I will also discuss constraints using the recent measurement of the pericenter precession of the star S2 by the GRAVITY Collaboration, and a possibility to probe these Kerr deformations in future experiments.

30 November 2021, 13:15 - 14:45 GMT

Raissa Fernandes Pessoa Mendes (Federal University Fluminense, Brazil)

Neutron star oscillations as a probe of modified theories of gravitation

Due to their extreme compactness, neutron stars provide an interesting testbed for general relativity and modified theories of gravity. On the other hand, the persisting uncertainties in the nuclear equation of state for ultra-dense matter inevitably introduce difficulties in attempts to constrain alternatives to general relativity with neutron star observations. In this talk, I will discuss some prospects for disentangling gravity/equation-of-state effects by exploring the richer oscillation spectrum of neutron stars in modified theories of gravity. Additionally, I will discuss how the picture drawn from the analysis of linear perturbations is enriched by investigations of the nonlinear dynamics of oscillating neutron stars. (Based on PRL 120, 201104 (2018) & PRD 104, 104036 (2021))

14 December 2021, 13:15 - 14:45 GMT

Yosef Verbin (Open University, Israel)

Scalarized electric and magnetic black holes in the vector-tensor Horndeski gravity

In this talk I will describe the main features of the Horndeski-Reissner-Nordstrom black hole (HRNBH) solutions. The electric and magnetic types of these BHs differ from each other, since there is no duality symmetry between electric and magnetic fields in the vector-tensor Horndeski gravity. The magnetic black hole solutions are especially simple and can be written explicitly in terms of hypergeometric functions. It is natural to expect that these black holes undergo spontaneous scalarization and it is indeed the case, if we add a massless real scalar field which is coupled to the vector-tensor Horndeski term. The single coupling constant of the Horndeski term is promoted now to a scalar field-dependent coupling function. This system contains (ordinary) Reissner-Nordstrom black holes with a vanishing scalar field, but on a certain “bifurcation surface” in parameter space these solutions become unstable and scalarized HRNBHs appear. The “double-faced” solutions of dyonic scalarized BHs exist in a certain domain of parameter space which is bounded by a critical surface on which the nature of the limiting solutions is determined by the proximity to the lines of vanishing electric or magnetic charges.

20 December 2021 (MONDAY!), 13:15 - 14:45 GMT

Georgios Antoniou (University of Nottingham, UK)

Scalar hair and compact objects

Gravitational wave observations might reveal new physics associated with the strong-gravity regime in the coming years. Could there be a (classical) gravitational theory describing deviations from General Relativity (GR) in the strong-field regime while being consistent with GR elsewhere? Spontaneous scalarization of compact objects has been known to do that. The model describing the process should, in principle, yield stable solutions and describe a cosmic attractor at late times. In this talk, I will discuss the general principles of black hole scalarization, and how o proper effective model can be built.

11 January 2022, 16:15-17:45 GMT (Special time!!)

Henriette Elvang (University of Michigan, USA)

A new view of the double-copy

The double-copy is often described as a map that takes a product of tree-level gluon amplitudes and outputs the tree-level graviton amplitudes. This in itself is rather remarkable since the Lagrangian descriptions for Yang-Mills theory and general relativity are so different and there is no hint of the double-copy in the Feynman rules. The double-copy is more broadly a map that allows certain field theory amplitudes to be mapped to those of certain other theories. The question then is what is the selection principle for this map? Or what is the most general self-consistent double-copy map that can be defined? In this talk I'll review the double-copy and explain new ideas to generalize it.

18 January 2022, 13:00 - 14:30 GMT (SPECIAL TIME)

Elizabeth Winstanley (University of Sheffield, UK)

Superradiance and quantum states on black hole space-times

We consider the definition of the Boulware and Hartle-Hawking states for quantum fields on black hole space-times. The properties of these states on a Schwarzschild black hole have been understood for many years, but neither of these states has a direct analogue on a Kerr black hole. We show how superradiant modes play an important role in the definition of quantum states on Kerr. Superradiance is also possible on static black hole space-times, in particular for a charged scalar field on a Reissner-Nordstrom black hole. We explore whether analogues of the Boulware and Hartle-Hawking states exist in this situation.

25 January 2022, 13:00 - 14:30 GMT (SPECIAL TIME)

Saskia Grunau (University of Oldenburg, Germany)

The ISCO of charged particles

The innermost stable circular orbit (ISCO) is defined by the smallest radius at which a test particle can move on a circular trajectory around a black hole. In accretion disk theory the ISCO is an important property of black holes, since it marks the inner edge of the accretion disk. In the Schwarzschild spacetime the ISCO is located at 6M, while in the Kerr spacetime the ISCO depends on the angular momentum of the black hole as well as on the direction of the angular momentum of the test particle. Furthermore, the charge of a particle will also influence the ISCO in the vicinity of a charged black hole. Here we will explore the ISCO of electrically charged particles in Reissner-Nordström, Kerr-Newman and Kerr-Sen spacetime.

1 Feburary 2022, 16:00 - 17:45 GMT (SPECIAL TIME)

Jose-Juan Blanco-Pillado (University of the Basque Country, Bilbao, Spain)

Cosmic Strings and the NANOGrav 12.5 yr data

Many cosmological scenarios beyond the Standard Model lead to the formation of a network of cosmic strings. In this talk, I will review how these models lead in a natural way to the production of a stochastic gravitational wave background and how this signal could account for the recently reported results from the NANOGrav collaboration. Finally, we will explain how future observations could allow us to confirm this interpretation of the NANOGrav data.

Previous talks - Spring/Summer 2021

13 April 2021, 15:15 - 16:45 BST

Daniela Doneva (University of Tuebingen, Germany)

Black holes with scalar hair

20 April 2021, 15:15 - 16:45 BST

Sayantani Lahiri (ZARM Bremen, Germany)

A tale of viscous accretion torus in the Schwarzschild space-time

27 April 2021, 15:15 - 16:45 BST

Celia Escamilla Rivera (UNAM Mexico)

Modified gravity theories in the light of multi-messenger gravitational-wave astronomy

4 May 2021, 15:15 - 16:45 BST

Carla Cederbaum (University of Tuebingen, Germany)

On the center of mass in General Relativity

11 May 2021, 15:15 - 16:45 BST

Ruth Gregory (King's College London)

Primordial Black holes and Higgs vacuum decay

18 May 2021, 15:15 - 16:45 BST

Helvi Witek (University of Illinois, USA)

The adventures of black holes: The case of quadratic gravity

25 May 2021, 15:15 - 16:45 BST

Renate Loll (Radbound University Nijmegen, Netherlands)

Quantum Gravity and Spacetime from Causal Dynamical Triangulations

1 June 2021, 15:15 - 16:45 BST

Yakov Shnir (BLTP Dubna, Russia)

Multicomponent boson stars and other soliton stars

8 June 2021, 15:15 - 16:45 BST

Panagiota Kanti (University of Ioannina, Greece)

From Black Holes to Wormholes in GR and Beyond

15 June 2021, 15:15 - 16:45 BST

Raul Vera (University of the Basque Country Bilbao, Spain)

Existence and uniqueness of rigidly rotating stars in GR in second order perturbation theory

29 June 2021, 15:15 - 16:45 BST

Petya Nedkova (University of Sofia, Bulgaria)

Shadows and accretion disk images of compact objects

1 July 2021 (Thursday), 15:15 - 16:45 BST

Daya Kulshresththa (Delhi University, India)

Introducton to Supergravity

13 July 2021, 15:15 - 16:45 BST

Vincent Vennin (APC Paris, France)

Can we prove that cosmic structures are of quantum mechanical origin?


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