**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.

**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.

**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.

**Thomas Hertog**, University of Leuven

*Holographic Toy Model of AdS 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.

**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.

**Helvi Witek**, University of Barcelona

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

**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*