**Jahed Abedi**, Institute for Research in Fundamental Sciences

*Echoes from the Abyss: Tentative Evidence for Planck-Scale Structure at Black Hole Horizons*

In classical General Relativity (GR), an observer falling into an astrophysical black hole is not expected to experience anything dramatic as she crosses the event horizon. However, tentative resolutions to problems in quantum gravity, such as the cosmological constant problem, or the black hole information paradox, invoke significant departures from classicality in the vicinity of the horizon. It was recently pointed out that such near-horizon structures can lead to late-time echoes in the black hole merger gravitational wave signals that are otherwise indistinguishable from GR. We search for observational signatures of these echoes in the gravitational wave data released by advanced Laser Interferometer Gravitational-Wave Observatory (LIGO), following the three black hole merger events GW150914, GW151226, and LVT151012. In particular, we look for repeating damped echoes with time-delays of 8MlogM (+spin corrections, in Planck units), corresponding to Planck-scale departures from GR near their respective horizons. Accounting for the "look elsewhere" effect due to uncertainty in the echo template, we find tentative evidence for Planck-scale structure near black hole horizons at false detection probability of 1% (corresponding to 2.5σ significance level). We also report the results of same search for echoes in the new black hole merger event GW170104. Future observations from interferometric detectors at higher sensitivity, along with more physical echo templates, will be able to confirm (or rule out) this finding, providing possible empirical evidence for alternatives to classical black holes, such as in firewall or fuzzball paradigms.

**Valentina Baccetti**, Macquarie University

*Effects of black hole radiation: horizon avoidance*

Event horizons are the defining feature of classical black holes. They are the key ingredient of the information loss paradox which, as paradoxes in quantum foundations, is built on a combination of predictions of quantum theory and counterfactual classical features. Within the semi-classical theory we investigate the possibility that black hole radiation still does not allow for a finite time crossing of the Schwarzschild radius of collapsing matter as seen by distant observers. The exact form of the pre-Hawking radiation is not yet settled, and we make only minimal assumptions about its nature.

**Carlos Barcelo**, Institute of Astrophysics of Andalusia

*Probing Extreme Gravity in Stellar Collapse*

The standard way to understand quantum corrected black holes leads to the information loss paradox and the lifetime dilemma. A radical way out of this situation is to give up a hypothesis which is tacitly assumed in the vast majority of works on the subject: that the classical singularity is substituted by something effectively acting as a sink for a long period of time, as seen by asymptotic observers.

Eliminating this characteristic changes drastically much of the physics now associated to black holes. A nice feature of the new hypothesis it that it offers a

clear possibility of experimental falsifiability with upcoming gravitational waves observations. In this talk I will discuss these possibilities.

**Iosif Bena**, CEA Saclay

*Thou Shalt Not Put (Ordinary) Stuff at The Horizon*

Black holes appear to lead to information loss, thus violating one of the fundamental tenets of Quantum Mechanics. Recent Information-Theory-based arguments imply that information loss can only be avoided if at the scale of the black hole horizon there exists a structure (commonly called fuzzball or

firewall) that allows information to escape. I will discuss the highly-unusual properties that this structure must have and how these properties emerge in the realization of this structure in String Theory via branes, fluxes and topology.

**Ofek Birnholtz**, Albert Einstien Institute

*Testing pseudo-complex general relativity with gravitational waves*

We show how the model of pseudo-complex general relativity can be tested using gravitational wave signals from coalescing compact objects. The Model, which agrees with Einstein gravity in the weak-field limit, diverges dramatically in the near-horizon regime, with certain parameter ranges excluding the existence of black holes. We show that simple limits can be placed on the model in both the inspiral and ringdown phase of coalescing compact objects.

We discuss further how these limits relate to current observational bounds.

In particular, for minimal scenarios previously considered in the literature, gravitational wave observations are able to constrain pseudo-complex general relativity parameters to values that require the existence of black hole horizons.

**Avery Broderick**, Perimeter Institute

*What does a quantum black hole look like to the Event Horizon Telescope?*

**Ramy Brustein**, Ben Gurion University

*What’s inside a BH?*

We propose that a large Schwarzschild black hole (BH) is a bound state of highly excited, long, closed strings just above the Hagedorn temperature. The effective free-energy density is expressed as a function of its entropy density and contains only linear and quadratic terms, in analogy with that of collapsed living polymers. Classically, the horizon of such BH’s is completely opaque, hiding any clues about the state and very existence of its interior. Quantum mechanically and in equilibrium, the situation is not much different: Hawking radiation will now be emitted, but it carries a minimal amount of information. The situation is significantly different when such a quantum BH is out of equilibrium. The BH can then emit ``supersized" Hawking radiation with a much larger amplitude than that emitted in equilibrium. The result is a new type of quantum hair that can reveal the state and composition of the BH interior to an external observer.

**Katerina Chatziioannou**, Canadian Institute for Theoretical Astrophysics

*Measuring the polarization content of gravitational waves with LIGO*

I will give a brief overview of LIGO’s efforts to test general relativity with gravitational waves. My main focus will be on tests of alternative polarizations.

**Roberto Emparan**, University of Barcelona

*Remarks on Cosmic Censorship and Its Possible Violations*

**Archisman Ghosh**, International Centre for Theoretical Sciences

*A model-independent search for gravitational-wave echoes*

Exotic compact objects (e.g. boson stars, dark matter stars, gravastars), and certain quantum modifications to black holes (e.g. firewalls) are speculated to give out ``echoes'' or bursts of radiation appearing at regular time intervals due to a perturbation by any infalling matter or field. In particular, these echoes are also expected to appear soon after their formation. The presence (or absence) of gravitational-wave echoes following observations of coalescences of compact binaries by detectors like Advanced LIGO and Virgo, might be able to directly probe (or constrain) the nature of the remnant compact object. For a large class of these objects, the echoes are expected to appear at time scales that are amenable to a search. However, there can be a substantial variation in the detailed waveform models, and this might make a template-based search inffective. We propose and demonstrate a model-independent search method that relies only on the constancy of the time difference between subsequent echoes.

**Steven Giddings**, University of California, Santa Barbara

*Rescuing Quantum Mechanics with Soft Gravitational Structure: Postulates to Observational Prospects*

Postulates are given for a quantum-gravitational description of black holes, that include correspondence with a quantum field theory description for freely falling observers crossing the horizon. These lead to “soft gravitational structure,” which can transfer information to outgoing radiation either with or *without* large metric perturbations. Prospects for observing such departures from the standard field-theoretic description of black holes will be briefly discussed.

**Shaun Hampton**, Ohio State University

*Can we observe firewalls or fuzzballs?*

**Chad Hanna**, Pennsylvania State University

*Could LIGO's black holes be primordial and what is an easy way to figure that out?*

**James Hartle**, University of California, Santa Barbara

*Classical Spacetime and Quantum Black Holes*

A quantum system behaves classically when quantum probabilities are high for coarse-grained histories correlated in time by deterministic laws. That is as true for the flight of a tennis ball as for the behavior of spacetime geometry in gravitational collapse. Classical spacetime may be available only in patches of configuration space with quantum transitions between them. Global structures of general relativity. such as event horizons may not be available.

We consider the quantum dynamics of gravitational collapse in a model in which classical spacetime breaks down because the wave function spreads over a large ensemble of classical end states as envisioned in the fuzzball proposal. Probabilities of coarse-grained observables are highly peaked around the classical black hole values. By contrast, probabilities for finer-grained observables probing the near horizon region are broadly distributed, and no notion of `averaging' applies. This means that the formation of fuzzballs may result significant observational features including a novel type of gravitational wave burst associated with tunneling between classical solutions.

**Thomas Hertog**, University of Leuven

*Emergent State-dependence in Holographic Models of Black Holes*

**Yinzhe Ma**, University of KwaZulu-Natal

*Black Hole Mining Effect*

**Emil Martinec**, University of Chicago

*String Theory of Supertubes*

The internal structure of extremal and near-extremal black holes in string theory involves a variety of ingredients — strings and branes — that lie beyond supergravity, yet it is often difficult to achieve quantitative control over these ingredients in a regime where the state being described approximates a black hole. The supertube is a brane bound state that has been proposed as a paradigm for how string theory resolves black hole horizon structure. This talk will describe how the worldsheet dynamics of strings can be solved exactly in a wide variety of supertube backgrounds, opening up the study of stringy effects in states near the black hole transition.

**Samir Mathur**, Ohio State University

*The fuzzball paradigm*

The black hole information paradox poses a serious difficulty for theoretical physics. Over the last two decades there has emerged a resolution to this paradox in string theory, based on the discovery that heavy states in string theory swell up into horizon sized "fuzzballs". The talk will review the fuzzball construction and how the traditional semiclassical expectation of a vacuum horizon gets violated. It has been recently argued that objects with a surface like a fuzzball should behave like a "firewall" for infalling objects; we show that this firewall argument is inconsistent because its assumptions violate causality.

**Emil Mottola**, Los Alamos National Laboratory

*Schwarzschild's Interior Solution, Gravastars and Echoes*

**Alex Nielsen**, Albert Einstein Institute

*An Alternative Significance Estimation for the Evidence for Echoes*

The noise dominated nature of the gravitational wave detectors requires an assessment of the noise background in the search for astrophysical signals. Starting with a frequentist approach, the original analysis used about 16 seconds of data after the merger signal to find how frequently random noise mimics the expected signal. We present the results of extending the background estimation to 4096 seconds of public LIGO data and discuss the concerns arising from subtleties in the analysis for the long and self-similar echo templates.

**Don Page**, University of Alberta

*Qubit Model for Black Hole Evaporation without Firewalls*

Kento Osuga and I give an explicit toy qubit transport model for transferring information from the gravitational field of a black hole to the Hawking radiation by a continuous unitary transformation of the outgoing radiation and the black hole gravitational field. The model has no firewalls or other drama at the event horizon and fits the set of six physical constraints that Giddings has proposed for models of black hole evaporation. It does utilize nonlocal qubits for the gravitational field but assumes that the radiation interacts locally with these nonlocal qubits, so in some sense the nonlocality is confined to the gravitational sector. Although the qubit model is too crude to be quantitively correct for the detailed spectrum of Hawking radiation, it fits qualitatively with what is expected.

**Paolo Pani**, Sapienza University of Rome

*Quantifying the evidence for black holes with GW and EM probes*

**Jing Ren**, University of Toronto

*From Quadratic Gravity to Observation*

Astrophysical black hole candidates might be horizonless ultra-compact objects. Of particular interest is the plausible fundamental connection with quantum gravity. The puzzle is then why we shall expect Planck scale corrections around the horizon of a macroscopic black hole.

Taking asymptotically free quadratic gravity as a possible candidate of UV completion of general relativity, I will show how the would-be horizon can be naturally replaced by a tiny interior as dictated by the dynamics. The new horizonless 2-2-hole, as a quite generic static solution sourced by sufficiently dense matter, may then be the nearly black endpoint of gravitational collapse. In the era of gravitational wave astronomy, echoes in the post-merger phase provide a great opportunity to probe such scenario.

Given the uncertainties associated with the waveform of echoes, I will discuss some model-independent search strategies, where the primary task is to find the time delay between echoes. The search range is then motivated by the Planck scale deviation outside the would-be horizon.

**Ulrich Sperhake**, University of Cambridge

*Long-Lived Inverse Chirp Signals from Core-Collapse in Massive Scalar-Tensor Gravity*

We model stellar core collapse in massive scalar-tensor theories of gravity.

The presence of a mass term for the scalar field allows for dramatic increases in the radiated gravitational wave signal and may stretch out the signal to last for years or even centuries. There are several potential smoking gun signatures of a departure from general relativity associated with this process. These signatures could show up within existing LIGO-Virgo searches.

**David Turton**, University of Southhampton

*Black Hole Microstates in String Theory*

I will give an overview of recent work in the study of black hole microstates in string theory. I will describe constructions of smooth horizonless supergravity solutions, both supersymmetric and non-supersymmetric, and where applicable, their holographic description. I will also comment on the physics of an infalling observer.

**Julian Westerweck**, Albert Einstein Institute

*Improvements on the Methods for Searching Echoes*

The recent detections of merging black holes allow for observational tests of the nature of these objects, such as searching for the GW echo signals proposed in some models. Tentative evidence for these was presented, found in an analysis based upon methods for GW data analysis as demonstrated on the Ligo Open Science Center. We present the results of characterising these method's behaviour when applied to the specific form of the echo signals, and address problems and improvements based on our findings.

**Helvi Witek**, University of Barcelona

*Growing Black-Hole Hair in Extensions of General Relativity*

**Kent Yagi**, University of Virginia

*Inspiral Tests of Strong-field Gravity and Ringdown Tests of Quantum Black Holes*

The binary black hole merger events recently discovered by the LIGO and Virgo Collaboration offer us excellent testbeds for exploring extreme (strong and dynamical-field) gravity that was previously inaccessible. In this talk, I will first explain the current status of probing fundamental pillars of General Relativity using the inspiral part of the gravitational waveform. I will next describe how well one can constrain one type of quantum black holes, collapsed polymers, with the GW150914 ringdown. I will conclude with a list of important open problems.

**Yuki Yokokura**, RIKEN

*A Self-consistent Model of Evaporating Black Holes*

We analyze the time evolution of a spherically-symmetric collapsing matter from the point of view that black holes evaporate by nature. We obtain a self-consistent solution of the semi-classical Einstein equation. The solution indicates that the collapsing matter forms a dense object and evaporates without horizon or singularity, and it has a surface but looks like an ordinary black hole from the outside. Any object we recognize as a black hole should be such an object. In the case of stationary black holes that are formed adiabatically in the heat bath, the area law is reproduced by integrating the entropy density over the interior volume. This result implies that the information is stored inside the object.

**Aaron Zimmerman**, Canadian Institute for Theoretical Astrophysics

*A Recipe for Echoes*