## Poster Session

In order to stimulate discussions, we will organize a poster session on the first day of the conference (Monday, September 27) where participants will be able to present the results of their works. The deadline for poster submission is Friday, September 10. The authors of the selected posters will be notified by Sunday, September 12.

The presenters of the three best posters will have the opportunity to give a plenary talk. Please note that, while we encourage everyone to submit an abstract and present a poster, the contest will be limited to young researchers (students and early postdocs).

Posters of young researchers will be evaluated by the organizers and the advisory board and, after casting a vote, three poster presenters will be selected. The decision will be communicated to the poster presenters shortly after the poster session (taking place on Monday 27, September), and the three talks will be scheduled for the fourth day of the conference (Thursday, September 30).

# Selected Posters

Masaya Amo

Asymptotic behavior of null geodesics near future null infinity: Significance of gravitational waves
We analyze the asymptotic behavior of null geodesics near future null infinity in asymptotically flat spacetimes. In particular, we focus on null geodesics which correspond to worldlines of photons initially emitted in the directions tangential to the constant radial surfaces in the Bondi coordinates. In four dimensions, several assumptions are required to guarantee the null geodesics to reach future null infinity. With the vacuum Einstein equation, these assumptions correspond to the condition that the gravitational waves are weak enough. In higher dimensions, by contrast, such assumptions are not necessary and gravitational waves do not affect the asymptotics of null geodesics.

Poster

Presentation

Georgios Antoniou

The importance of a Ricci-scalar coupling in black hole scalarization
Spontaneous scalarization is a gravitational phenomenon in which deviations from general relativity arise once a certain threshold in curvature is exceeded. For black holes, scalarization is known to be triggered by a coupling between a scalar field and the Gauss-Bonnet invariant. In this poster we present the importance of including a Ricci-scalar coupling in the theory, which turns out to be crucial for the stability and cosmological consistency of the solutions.

Poster

Presentation

Benjamin Berczi

Gravitational Collapse with Quantum Fields

Semiclassical black holes have been in the forefront of research for decades, in various shapes and forms. We have developed a formalism using which a genuine 4D quantum scalar field can be numerically simulated to collapse into a black hole. The new formalism utilises the notion of coherent states to relate the semiclassical simulation to the well-studied classical one. Thus the classical scenarios are easily replicated and also the new quantum effects can be separated straightforwardly.

Poster

Presentation

Reginald Christian Bernardo

Gravitational wave signatures from dark sector interactions

Working within Horndeski theory, we show that in a general relativistic black hole, dark sector interactions effectively reduce to an interaction charge which influences the scalar and tensor waveforms. We exploit a conservation law for the total dark matter field to setup the Regge-Wheeler equation and the coupled Zerilli and scalar wave equations for a Schwarzschild-(anti) de Sitter black hole. We present numerical results for the case of a dark matter particle falling straight into a Schwarzschild black hole.

Poster

Presentation

Alexandra Brosius

Fractional transformations in the complex plane to map spacetime curvature near supermassive black holes

Fractional transformations have been a maritime navigation tool for centuries. This presentation explores the geometries of supermassive black holes as manipulations of complex parametric curves and surfaces. We compare curve families for various EHT image simulations in order to identify similarities and differences in underlying physics assumptions, such as deviations from spherical symmetry, stationarity, or thin disks. For each simulation, the resulting curves are embedded in a variety of surfaces, including a plane, sphere, and a cone. This exploratory technique may provide a starting point for future tests of general relativity.

Poster

Presentation

Alejandro Cardenas Avendano

Searching for a needle in a haystack: Chaos in EMRIs

While the test particle’s geodesic motion around a Kerr black hole is fully integrable, it may not be in modified theories of gravity. What are the observational consequences of observing an EMRI system that is not integrable through gravitational waves? In this talk, I will show how a truncated integrable system leads to chaotic behavior imprinted in the motion’s fundamental frequencies’ temporal evolution, and discuss its detectability with the future space-based detector LISA. To show how chaos affects the gravitational waves emitted, we have systematically analysed the Fourier transform of approximate gravitational waveforms computed in the semi- relativistic approximation, augmented with gravitational wave dissipation, on a slowly-rotating Kerr background. Since signatures of chaotic dynamics in gravitational waves have been suggested to test general relativity in the strong field, if these effects are not correctly modeled and understood, they may undermine present proposals to verify the no-hair theorem’s assumptions.

Poster

Presentation

Srija Chakraborty

Probing high redshift massive black hole seeds with LISA and possibility of EM counterparts

We study hydrodynamical simulations of galaxy formation, based on the GADGET-3 code, and investigate supermassive black hole binaries coalescence at $5.5<z<14$ and the expected gravitational waves emitted from the binary mergers for different AGN feedback models. A fraction of the accreted rest-mass energy is radiated away by each black hole. A fraction of this radiated energy is coupled to the surrounding gas as feedback energy. We consider the cases of $\rm AGN_fid$ feedback where the feedback energy is thermal, as well as kinetic feedback,which includes AGNcone and AGNsphere,where in the former case the kinetic black hole feedback is distributed inside bi-cone (45\textdegree half opening angle) and in latter the kinetic feedback is distributed in sphere (90\textdegree half opening angle). We further consider the case in which no AGN feedback is implemented in the simulation. We find the merger rate for the kinetic feedback of the order between 100 to 1000 mergers for the chirpmass range less than $10^6~M_{\odot}$ and for the thermal feedback model to be between 10 to 100 in the same chirp mass range. We stress the comparisons to be made between simulations of same resolution: kinetic with $R_{smooth}$= 1ckpc/h and thermal with $R_{smooth}$=0.5 ckpc/h. For each model, we estimate the expected characteristic strain of gravitational waves emitted by supermassive black hole binary mergers, the time to coalesce, and the expected number of resolved events and compare our predictions with the LISA sensitivity and resolution. We further investigate the host galaxy properties for the events detectable by LISA and make predictions of the electromagnetic counterparts expected events to be detected by other electromagnetic instruments operating along the proposed operational time of LISA and present a panoramic view of merger events through different detectors.

Poster

Presentation

Che-Yu Chen

Non-singular rotating black hole inspired by loop quantum gravity and its observational consequences

The lack of rotating black hole models, which are typically found in nature, in loop quantum gravity (LQG) substantially hinders the progress of testing LQG from observations. Starting with a non-rotating LQG black hole as a seed metric, we construct a rotating spacetime using the revised Newman-Janis algorithm. The rotating solution is non-singular everywhere and it reduces to the Kerr black hole asymptotically. In different regions of the parameter space, the solution describes i) a wormhole without event horizon (which, we show, is almost ruled out by observations), ii) a black hole with a spacelike transition surface inside the event horizon, or iii) a black hole with a timelike transition region inside the inner horizon. It is shown how fundamental parameters of LQG can be constrained by the observational implications of the shadow cast by this object. The causal structure of our solution depends crucially only on the spacelike transition surface of the non-rotating seed metric, while being agnostic about specific details of the latter, and therefore captures universal features of an effective rotating, non-singular black hole in LQG.

Poster

Presentation

Andrew Coates

Area Quantization and its astrophysical (ir)relevance

Recent works have suggested that the area quantization of Bekenstein and Mukhanov leads to gravitational wave echoes. We argued that the quantum spectrum of a black hole should be washed out during and after black hole mergers, and hence one should not expect echoes in this scenario. Furthermore this area quantization is incompatible with semi-classical Hawking radiation calculations.

Poster

Presentation

Pietro Conzinu

Primordial Black Holes from Pre-Big Bang Inflation

We discuss the possibility of producing a significant fraction of dark matter in the form of primordial black holes in the context of the pre-big bang inflationary scenario. We take into account, to this purpose, the enhancement of curvature perturbations possibly induced by a variation of the sound-speed parameter c_s during the string phase of high- curvature inflation. After imposing all relevant observational constraints, we find that the considered class of models is compatible with the production of a large amount of primordial black holes in the mass range relevant to dark matter, provided the sound-speed parameter is confined in a rather narrow range of values.

Poster

Presentation

Sofia Di Gennaro

Hawking Evaporation and Mutual Information Optimization: Implications for Cosmic Censorship and Weak Gravity Conjecture

Black holes in general relativity are commonly believed to evolve towards a Schwarzschild state as they gradually lose angular momentum and electrical charge under Hawking evaporation. However, when Kim and Wen applied quantum information theory to Hawking evaporation and argued that Hawking particles with maximum mutual information could dominate the emission process, they found that charged black holes tend towards extremality. In view of some evidence pointing towards extremal black holes being effectively singular, this would violate the cosmic censorship conjecture. In this work, we clarified the difference between the two models -- they pertain to two different regimes. By taking into account the discreteness of the charge-to-mass ratio of finite species of charged particles, we also obtained a clearer picture of the end state of Hawking evaporation. We found that in agreement with the weak gravity conjecture, if there is no particle with charge-to-mass ratio q/m>1, stable remnant states are formed, though they are non-extremal. Furthermore, and more surprisingly, we show that extremality can be reached if there exists a particle with q/m<1, even in the presence of other particles with q/m>1. This may explain why there is no charged particle with q/m<1 in the Standard Model of particle physics. Cosmic censorship could thus play a bigger role in fundamental physics than previously thought.

Poster

Presentation

Shailesh Kumar

Signatures of asymptotic symmetries in gravitational memory

Gravitational memory is a non-oscillatory part to the gravitational wave amplitude which generates a permanent displacement for freely falling test particles or test detectors. I shall highlight the theoretical aspects of gravitational memory, and its connection with asymptotic symmetries (BMS symmetries) emerging near the horizon of black holes. I would discuss a model scenario where asymptotic symmetries appear as a soldering freedom in the context of stitching of two black hole spacetimes, and examine the impact of interaction between test detectors and horizon shells generated through the gluing formalism. Further, we show a more realistic approach of computing displacement memory in terms of BMS parameters which is analogous to the conventional memory obtained at asymptotic null infinity. We also touch upon the possibility of investigating the astrophysical signatures of asymptotic symmetries in other possible directions such as gravitational lensing and quasinormal modes which might play a significant role in understanding the BMS memory not only from theoretical perspectives but also from observational standpoints.

Poster

Presentation

Yu-Hsien Kung

Modify Hawking temperature by a flow-induced supertranslation

One interesting proposal to solve the black hole information loss paradox without modifying either general relativity or quantum field theory, is the soft hair, a diffeomorphism charge that records the anisotropic radiation in the asymptotic region. This proposal, however, has been challenged, given that away from the source the soft hair behaves as a coordinate transformation that forms an Abelian group, thus unable to store any information. To maintain the spirit of the soft hair but circumvent these obstacles, we consider Hawking radiation as a probe sensitive to the entire history of the black hole evaporation, where the soft hairs on the horizon are induced by the absorption of a null anisotropic flow. To do so we introduce two different time-dependent extensions of the diffeomorphism associated with the soft hair, where one is the backreaction of the anisotropic null flow, and the other is a coordinate transformation that produces the Unruh effect and a Doppler shift to the Hawking spectrum. Together, they form an exact BMS charge generator on the entire manifold that allows the nonperturbative analysis of the black hole horizon, whose surface gravity, i.e. the Hawking temperature, is found to be modified. The modification depends on an exponential average of the anisotropy of the null flow with a decay rate of 4M, suggesting the emergence of a new 2-D degree of freedom on the horizon, which could be a way out of the information loss paradox.

Poster

Presentation

Guillermo Lara

Inverse problem in black hole imaging

Recent observations by the Event Horizon Telescope of the image of the supermassive Black Hole at the center of M87 and SagA* provide a window into the strong-field regime of gravity. Several authors have studied the constraints that black hole shadow measurements cast on specific gravitational solutions or on the coefficients of more general parametrized frameworks. When multiple parameters are present, these constraints are usually degenerate. In this work, we tackle the inverse problem of gravitational imaging, i.e. we aim to reconstruct the metric and accretion disk profile from the image seen by a far-away observer. We allow the metric and disk profile to be described by an arbitrary frame or basis in functional space. By performing a Fisher Matrix analysis, and identifying the principal components of the covariance matrix, we extract the most prominent features in the metric and disk profile that produce the image. We test this framework in a simplified model by injecting mock deviations of the Schwarzchild metric surrounded by a spherical accretion disk. Since the problem is highly degenerate, we are able to reconstruct the metric deviations and disk profile provided that we use strong enough theoretical priors on the metric and disk profile.

Poster

Presentation

Johannes Münch

Effective Quantum Black Hole Collapse via Surface Matching

The fate of matter forming a black hole is still an open problem, although models of quantum gravity corrected black holes are available. In loop quantum gravity (LQG) models were presented, which resolve the classical singularity in the centre of the black hole by means of a black-to-white hole transition, but neglect the collapse process. The situation is similar in other quantum gravity approaches, where eternal non-singular models are available. A strategy is presented to generalise eternal models to dynamical collapse models by surface matching. Assuming 1) the validity of a static quantum black hole spacetime outside the collapsing matter, 2) homogeneity of the collapsing matter, and 3) differentiability at the surface of the matter fixes the dynamics of the spacetime uniquely. It is argued that these assumptions resemble a collapse of pressure-less dust and thus generalises the Oppenheimer-Snyder-Datt model. The junction conditions and the spacetime dynamics are discussed generically for bouncing black hole spacetimes, as proposed by LQG, although the scheme is approach independent. A global spacetime picture of the collapse for a specific LQG inspired model is discussed.

Poster

Presentation

Ángel Jesús Murcia Gil

Exact electromagnetic duality with nonminimal couplings

We study nonminimal extensions of Einstein-Maxwell theory with exact electromagnetic duality invariance. Any such theory involves an infinite tower of higher-derivative terms whose computation and summation usually represents a challenging problem. Despite that, we manage to obtain a closed form of the action for all the theories with a quadratic dependence on the vector field strength. In these theories we find that the Maxwell field couples to gravity through a curvature-dependent susceptibility tensor that takes a peculiar form, reminiscent of that of Born-Infeld Lagrangians. We study the static and spherically symmetric black hole solutions of the simplest of these models, showing that the corresponding equations of motion are invariant under rotations of the electric and magnetic charges. We compute the perturbative corrections to the Reissner-Nordström solution in this theory, and in the case of extremal black holes we determine exactly the near-horizon geometry as well as the entropy. Remarkably, the entropy only possesses a constant correction despite the action containing an infinite number of terms. In addition, we find there is a lower bound for the charge and the mass of extremal black holes. When the sign of the coupling is such that the weak gravity conjecture is satisfied, the area and the entropy of extremal black holes vanish at the minimal charge. The poster would be mainly based on arXiv:2105.09868.

Poster

Presentation

Naritaka Oshita

Black hole ringing and overtones -Quantification of the importance of overtones by the excitation factors-

The excitation factors of black hole quasinormal modes quantify the ease of excitation of the quasinormal modes and are independent of the source of perturbation. We compute the excitation factors of Kerr black holes up to the 20th overtone and show the 4th, 5th, and 6th overtones have the first three highest excitation factors for intermediate and high spin parameters. This provides an independent confirmation of the importance of overtones that has been confirmed by the fitting data analysis of numerical relativity waveforms beginning around the strain peak amplitude.

Poster

Presentation

Costantino Pacilio

Merger-ringdown consistency: A new test of strong gravity using deep learning

The gravitational waves emitted during the coalescence of binary black holes are an excellent probe to test the behaviour of strong gravity. In this paper, we propose a new test called the merger-ringdown consistency test that focuses on probing the imprints of the dynamics in strong-gravity around the black-holes during the plunge-merger and ringdown phase. Furthermore, we present a scheme that allows us to efficiently combine information across multiple ringdown observations to perform a statistical null test of GR using the detected BH population. We present a proof-of-concept study for this test using simulated binary black hole ringdowns embedded in the next-generation ground-based detector noise. We demonstrate the feasibility of our test using a deep learning framework.

Poster

Presentation

Zachary Picker

Cosmological PBHs as dark matter

In the early universe, primordial black holes (PBHs) can no longer be described by the simple Schwarzschild metric-- we need a metric which is locally surrounded by the cosmological fluid and asymptotically FLRW. It turns out that the phenomenology of PBHs is very sensitive to the choice of such a metric. In particular, the Thakurta metric stands out as perhaps the most justifiable metric for the radiation-dominated universe. In this description, PBHs have an effective mass proportional to the cosmological scale factor. We demonstrate two very significant effects of this choice of metric for the phenomenology of PBHs as dark matter (DM) candidates. Firstly, the binary abundance bounds which tightly constrain LIGO-size PBHs as DM candidates are entirely evaded. Secondly, these PBHs are significantly hotter and so evaporate very rapidly-- we show that the smallest black hole which actually survives until today is of order 10^21 g, which fully closes the asteroid-mass window for DM candidates, which was previously totally unconstrained.

Poster

Presentation

Ultracompact Schwarzschild stars without Love

One of the macroscopically measurable effects of gravity is the tidal deformability of astrophysical objects, which can be quantified by their tidal Love numbers (TLNs). For planets and stars, these numbers measure the resistance of their material against the tidal forces, and the resulting contribution to their gravitational multipole moments. According to GR the TLNs of nonrotating black holes (BHs) are precisely zero. Here we explore different configurations of nonrotating compact and ultracompact stars to bridge the compactness gap between BHs and neutron stars and calculate their Love number k2. We consider, for the first time, uniform density ultracompact stars (Schwarzschild stars) with compactness M/R beyond the Buchdahl limit and we find that k2 approaches smoothly to zero as M/R approaches the Schwarzschild BH limit. Our results provide insight on the zero tidal deformability limit.

Poster

Presentation

Javier Relancio

Non-extremal rotating black holes as particle accelerators

The possibility that rotating black holes could be natural particle accelerators has been subject of intense debate. While it appears that for extremal Kerr black holes arbitrarily high center of mass energies could be achieved, several works pointed out that both theoretical as well as astrophysical arguments would severely dampen the attainable energies. In this work we study particle collisions near Kerr--Newman black holes, by reviewing and extending previously proposed scenarios and by suggesting a new one. In all cases, we take the hoop conjecture into account and we discuss astrophysical relevance of these collisional Penrose processes. The outcome of this investigation is that scenarios involving near-horizon target particles are in principle able to attain, sub-Planckian, but still ultra high, center of mass energies of the order of $10^{21}-10^{23}$ eV. Thus, these target particle collisional Penrose processes could contribute to the observed spectrum of ultra high-energy cosmic rays and as such deserve further scrutiny in realistic settings.

Poster

Presentation

João Luís Rosa

Testing the hypothesis of a bosonic star at the galactic center

The GRAVITY collaboration has recently a detected continuous circular relativistic motion during infrared flares of Sgr A*, which has been interpreted as orbital motion near the event horizon of a black-hole. In this work, we use the ray-tracing code GYOTO to analyze the possibility of these observations being consistent with a central bosonic star instead of a black-hole. Our model consists of an isotropically emitting hot-spot orbiting a central boson or Proca star. Images of the orbit at different times and the integrated flux were obtained for both models and compared with the case of a Schwarzschild black-hole. Although the overall qualitative picture is comparable, the bosonic star models present an extra image when the emitting hot-spot passes behind the central object caused by photons travelling through the interior of the star. Furthermore, there are also measurable differences in the angles of deflection, orbital periods, and centroid of the flux, which can potentially be detected.

Poster

Presentation

K Saurabh

Theory-agnostic tests of gravity with black hole shadows

Observations of black hole shadows with the Event Horizon Telescope have paved the way for a novel approach to testing Einstein's theory of general relativity. Early analyses of the measured shadow put constraints on theory-agnostic parameters typically used to study deviations from Einstein's theory, but the robustness of these constraints was called into question. In this letter, we use a generic theory-agnostic metric to study the robustness of parameter estimation with BH shadows, taking into consideration current measurements made with the Event Horizon Telescope and future measurements expected with the Event Horizon Imager. We find that the robustness issue is highly nuanced, and parameter constraints can be highly misleading if parameter degeneracy is not handled carefully. We find that a certain kind of deviation is particularly well suited for the shadow-based analysis, and can be recovered robustly with shadow measurements in the future.

Reference: https://arxiv.org/abs/2108.01190

Poster

Presentation

Marc Schneider

Classical versus Quantum Completeness

We adapt the notion of quantum-mechanical completeness to situations where the only adequate description is in terms of quantum field theory in curved space-times. Explicitly, we analyse the behaviour of quantum probes on geodesically incomplete (dynamical) space-times, such as the interior Schwarzschild black hole.

Poster

Presentation

Andrew Spiers

Solving the second-order Teukolsky equation for self-force applications

Precise parameter extraction from EMRI signals with LISA offers the most precise probe of black hole spacetime of any planned experiment. Extracting precise parameters requires, among other things, the dissipative piece of the second-order self-force in a Kerr background. Calculating the second-order self-force involves the extension of black hole perturbation theory to second order. We have shown how a new form of the second-order Teukolsky equation has a well-defined source in a highly regular gauge, and how its solutions can be used to construct gauge invariant second-order quantities. We present the first source of the second-order Teukolsky equation for quasi-circular orbits in Schwarzschild, and show how we have ameliorated the divergence of the source near future null infinity by transforming towards the Bondi–Sachs gauge.

Poster

Presentation

Silvia Trabucco

Hawking temperature and phonon emission by acoustic holes

We present a novel interpretation of the Hawking emission of acoustic holes, the hydrodynamic analogue of standard black holes, by connecting the geometrical properties of the acoustic horizon to the — effectively one dimensional — distribution function of the spontaneously generated phonons.

We derive the Hawking temperature of phonons by fully exploiting the analogy between black and acoustic holes, within a covariant kinetic theory approach for the phonon gas.

Our derivation does not require a microscopical treatment. Indeed, we only need to associate to the acoustic horizon the Bekenstein entropy. Then the fluctuations of the acoustic horizon result in an increase of the entropy and energy of the phonon gas.

Since our method only depends on the geometrical properties of the acoustic horizon and on the statistical properties of the phonon gas, it is well suited to be extended to standard black holes and to out-of-equilibrium systems.

Poster

Presentation

Valeri Vardanyan

Finding the origin of gravitational wave binaries

Gravitational waves emitted from Binary Black Hole (BBH) and Black Hole-Neutron Star (BH-NS) sources have the promise to teach us about the origin of such objects. One compelling possibility is that the detected black holes are of primordial origin. In this poster I will demonstrate that while the possibility of all the BBHs being of primordial origin is still wide open, the companion black holes in BH-NS mergers cannot all be of primordial origin. I will discuss the implications for the recent BH-NS detections made by LIGO-Virgo-KAGRA.

Poster

Presentation