FAU^2 Focus Workshop on quantum black holes and the relation to asymptotic infinity

25 Jun 2024, 11:30 → 27 Jun 2024, 20:20 Europe/Berlin

ECAP Laboratory Building

The FAU^2 focus workshop is organized to promote scientific exchange between different quantum gravity groups world wide on recent results and open questions on quantum black holes and the relation to asymptotic infinity. It is organised  by the  Florida Atlantic University (FAU) and FAU Erlangen-Nürnberg and is organised a hybrid workshop. The workshop topics include current research on quantum black holes in both the canonical and covariant formalisms and will be held in a hybrid format.

Invited speakers:

Jonathan Engle, Florida Atlantic University, (remotely)

Laurent Freidel, Perimeter Institute for Theoretical Physics, (remotely)

Viqar Husain, University of New Brunswick, (remotely)

Wojciech Kamiński, University of Warsaw

Jerzy Lewandowski, University of Warsaw,  (remotely)

Hongguang Liu, FAU Erlangen-Nürnberg

Guillermo Mena Marugán, CSIC Madrid

Beatriz Elizaga Navascués, Louisiana State University

Javier Olmedo, University of Granda (remotely)

Alejandro Perez, Aix-Marseille University, (remotely)

Ana-Maria Raclariu, Kings College London, (remotely)

Carlo Rovelli, Western University, Aix-Marseille University, (remotely)

Hanno Sahlmann, FAU Erlangen-Nürnberg

Thomas Thiemann, FAU Erlangen-Nürnberg

Madhavan Varadarajan, Raman Research Institute, (remotely)

Francesca Vidotto, Western University

Wolfgang Wieland, FAU Erlangen-Nürnberg

Edward Wilson-Ewing, University of New Brunswick,  (remotely)

Cong Zhang, FAU Erlangen-Nürnberg

 

 

Organizers: Muxin Han and Kristina Giesel

   

Tuesday 25 June

Registration

Session 1

1 Axial perturbations in hybrid Loop Quantum Cosmology

There is a growing interest in investigating homogeneous but anisotropic spacetimes owing to their relation with nonrotating, uncharged black hole interiors. In this context, we present a description of axial perturbations in Kantowski-Sachs spacetimes with a massless scalar field. We truncate the action at the quadratic perturbative order. Perturbations are expressed in terms of perturbative gauge invariants, linear perturbative constraints, and their canonical momenta. Moreover, the entire system formed by these perturbations and the background degrees of freedom is also described by a canonical set of variables. We adopt a hybrid approach to quantize this system, combining a quantum representation of the background sector using Loop Quantum Cosmology with a more conventional Fock quantization of the perturbations. We comment on possible lines for future research and applications to black hole physics.

Speaker: Guillermo A. Mena Marugán (Instituto de Estructura de la Materia, CSIC)

FAU2Mena.pdf

2 A testable prediction from LQG? 20μg bound-states

The life of a black hole is dominated by dissipative effects, hard to model exactly. I discuss their effects on the full life of a blackhole and its while hole remnant.

Speaker: Carlo Rovelli (Zoom) (Aix-Marseille University)

FAU^2_2024_rovelli.pdf

3 Phenomenology of black hole remnants in the early universe

Understanding the properties of remnants resulting from the exhaustion of black holes can open new frontiers for quantum gravity phenomenology, both in terms of direct and indirect detection. In my talk,  I examine different possible ways in which black hole remnants may manifest their self in the early universe. I discuss possible constraints as well as possible desirable effects in relation to cosmological models with or without inflation.

Speaker: Francesca Vidotto (Western University)

4 Reissner–Nordström black holes in LQG: the improved dynamics

In this talk we will discuss the quantization of a charged black hole within the improved dynamics of loop quantum gravity and several properties of its semiclassical effective geometries. Adopting a redefined scalar constraint, that renders the algebra of constraints into a Lie algebra, we apply loop quantum gravity techniques adhered to a novel improved dynamics scheme. The model is solvable in closed form. We explicitly compute effective geometries for small charges, and show that the resulting effective space-times replace the inner horizon with a transition surface that connects trapped and antitrapped regions within the charged black hole interior. Namely, quantum effects stabilize the classical inner horizons in the limit of small charge, where the structure of these space-times remains the same as the uncharged case. We conclude discussing the properties of these effective geometries and show that they do not obey the null energy condition at high curvatures.

Speaker: Javier Olmedo (Zoom) (Universidade de Granada)

FAU^2_2024_Engle.pdf

5 Final discussion with all speakers of session 1

Coffee break

Session 2

6 Non-perturbative quantisation of impulsive radiative data

This talk gives a broad introduction into an emerging field at the interface of quantum foundations and quantum gravity. What connects these different research areas are common questions of shared interest: What is the causal structure at the quantum level? What is the role of the equivalence principle at the quantum level? Are the reference frames classical or quantum? After a brief introduction, I will explain the practical relevance of these questions for the characteristic null initial problem, in which we seek a quantization of gravity from observables on a light-cone. In four spacetime dimensions, several simplifications occur. For a specific class of impulsive gravitational null initial data (shock-waves), it is possible to find a complete and non-perturbative quantization on the light cone. One of the quantum numbers is the total luminosity carried to infinity. A transition happens when the luminosity reaches the Planck power. Below the Planck power, the spectrum of the radiated power is discrete. Above the Planck power, the spectrum is continuous. The talk is based on arXiv:2402.12578, arXiv:2401.17491, arXiv:2302.12799.

Speaker: Wolfgang Wieland (FAU Erlangen-Nürnbreg)

FAU^2_2024_wieland.pdf

7 Canonical structure, Dressing time and and quantization of null geometry.

In this talk, I will review our recent work with L. Ciambelli and R. Leigh, focusing on the non-perturbative characterization of the gravitational phase space along null boundaries.

I will present the construction of the Carrollian curved geometry, the Poisson bracket and discuss the construction of a field-dependent time called dressing time that allows the construction of a positive definite boost generator for expanding null surfaces. If time permits, I will discuss some elements of the quantization and the appearance of a crucial Quantum central charge in the algebra of null constraint. I will also discuss how having a finite central charge can be achieved through a molecular quantization. A quantum fluid description that provides a description of quantum geometry.If time permits I'll show how some of these results connects with celestial holography program and asymptotic quantization.

Speaker: Laurent Freidel (Zoom) (Perimeter Institute )

FAU2-workshop_2024_Freidelpdf.pdf

8 Spherical collapse and black hole evaporation

We consider spherically symmetric gravity coupled to a spherically symmetric scalar field with a specific coupling which depends on the Areal Radius. Suitable boundary conditions ensure the existence of an axis of symmetry and consequently a single asymptotic past and future (as opposed to a pair of left and right ones). The scalar field stress energy takes the form of null dust. Its classical collapse is described by the  Vaidya solution. From a two dimensional $(r,t)$ perspective, the scalar field is conformally coupled so that its quantum stress energy expectation value is well defined. Quantum back reaction is then incorporated through an explicit formulation of the 4d semiclassical  Einstein equations.The semiclassical solution describes black hole formation together with its subsequent evaporation along a timelike apparent horizon.  A balance law at future null infinity relates the rate of change of  a back reaction-corrected Bondi mass to a  manifestly {\em positive} flux.
The detailed form of this balance law together with a proposal for the dynamics of the true degrees of freedom underlying the putative non-perturbative quantum gravity theory is supportive of the paradigm of singularity
resolution and information recovery proposed by Ashtekar and Bojowald.  In particular all the information
including that in the collapsing matter is expected, in our proposed scenario, to emerge along a single quantum extended future null infinity.

Speaker: Madhavan Varadarajan (Zoom) (RRI)

FAU2_workshop_2024_Varadarajan.pdf

9 Final discussion with all speakers of session 2

Final Discussion with all speakers: Final discussion with all speakers of the day

Welcome get together with drinks and snacks

Wednesday 26 June

Session 3

10 Isolated horizon state-counting in the presence of matter

I will review some older results on counting states and calculating the entropy of isolated horizons in loop quantum gravity in the presence of charges/matter fields on the horizon. One question that I will answer at least partially in some examples is if the matter DOF contribute to the entropy, and if so, to what order.
I will also briefly touch on some newer results and work in progress for state counting for surfaces in supergravity.

Speaker: Hanno Sahlmann (Friedrich-Alexander-Universität Erlangen Nürnberg (FAU))

11 Entanglement, soft modes and an infrared on-shell action in asymptotically flat spacetimes

In this talk I will show that edge modes in free Maxwell theory in 3+1 dimensions lead to correlated fluctuations in the large gauge charges of a subregion and its complement, and result in a non-trivial contribution to the entanglement entropy across a cut of future null infinity. I will then outline a generalization of these ideas to gravity. In particular, I will highlight a novel relation between the on-shell action in 3+1 dimensional asymptotically flat spacetime, the 2d soft effective action and the shockwave effective action leading to an estimate of spacetime fluctuations in a causal diamond. The talk is based on 2308.12341 with Vincent Chen and Rob Myers, as well as 2305.14411 and  work in progress with Temple He and Kathryn Zurek.

Speaker: Ana-Maria Raclariu (Zoom) (Department of Mathematics, King's College London.)

12 Conformal Einstein's equations

Einstein field equations are not conformally invariant. Despite this fact, conformal geometry plays an important role in description of asymptotics of the solutions via Penrose conformal compactification. Moreover, in many cases such conformal compactification of solution is smooth. It turns out that these facts are linked to a fundamental object in conformal geometry, Fefferman-Graham obstruction tensor. I will describe this connection in may talk, together with some interesting implications.

Speaker: Wojciech Kaminski (University of Warsaw)

13 Final discussion with speakers of session 3

Coffee break

Session 4

14 Quantum Field Theory of Black Hole Perturbations

In order to increase our understanding of the black hole information loss paradox
it appears mandatory to incorporate backreaction effects into the black hole evaporation process. We propose a natural extension of the well known linear,
backreaction free, perturbation theory of black holes. The extended framework
is based on the reduced phase space approach and therefore is able to reconcile
gauge invariance with higher order perturbation theory. The resulting reduced
Hamiltonian describes the interaction between symmetric ("background") and
non-symmetric ("perturbations") observable modes and can be expanded to any desired order. A given truncation of that Hamiltonian and similar truncations of evaporation sensitive observables such as the apparent horizon area can then be
quantised.

Speaker: Thomas Thiemann (FAU Erlangen-Nuremberg)

15 Entanglement of fields in evaporating black holes

Understanding the quantum information aspects of fields in gravitational collapse scenarios remains an open issue to date. Here, we present a conservative QFT study about the entanglement structure of the Hawking effect in evaporating black holes. For this purpose, first we review the known concept of Hawking partners, as being the field modes that are entangled with, and thus purify, the thermal radiation. Then, we show how the definition of these partners can be generalized to any case where the temperature of the radiation increases over time according to the semiclassical evaporation process. Our explicit computation allows us to study the relative location of radiation and partner modes at past null infinity. These results point to the inevitable need of a quantum gravity description of the high curvature regions of spacetime in order to know the evolution and ultimate fate of the Hawking partners.

Speaker: Beatriz Elizaga Navascués (Louisiana State University)

16 Is Planckian discreteness observable in cosmology?

A Planck scale inflationary era—in a quantum gravity theory predicting discreteness of quantum geometry at the fundamental scale—produces the scale invariant spectrum of inhomogeneities with very small tensor-to-scalar ratio of perturbations and a hot big bang leading to a natural dark matter genesis scenario. In this talk I evoke the possibility that some of the major puzzles in cosmology could have an explanation rooted in quantum gravity.

Speaker: Alejandro Perez (Zoom) ( Centre de Physique Théorique )

Fau2-workshop-2024_Perez..pdf

17 Final discussion with all speakers of session 4

Closing remarks: Final discussion with all speakers of the day

Workshop Dinner Barbecue: Workshop Dinner

Thursday 27 June

Session 5

18 Semiclassical and quantum black holes

TBA

Speaker: Jerzy Lewandowski (Zoom) (University of Warsaw)

19 Spin foam amplitude of the black-to-white hole transition

It has been conjectured that quantum gravity effects may cause the black-to-white hole transition due to quantum tunneling. The transition amplitude of this process is explored within the framework of the spin foam model on a 2-complex containing 56 vertices. We develop a systematic way to construct the bulk triangulation from the boundary triangulation to obtain the 2-complex. By using Thiemann's complexifier coherent state as the boundary state to resemble the semiclassical geometry, we introduce a procedure to calculate the parameters labeling the coherent state from the continuous curved geometry. Considering that triad fields of different orientations give the same intrinsic geometry of the boundary, we creatively adopt the boundary state as a superposition of the coherent states associated with both orientations. We employ the method of complex critical point to numerically compute the transition amplitude. Despite the numerical results, it is interestingly found that the transition amplitude is dominated by the terms allowing the change in orientation. This suggests that the black-to-white hole transition should be accompanied by quantum tunneling process of a change in orientation.

Speaker: Cong Zhang (FAU Erlangen-Nürnberg)

20 Criteria for Loop Quantization of Black Hole Systems

Thus far, there is no proposal for a loop quantization -- even at the level of effective dynamics -- of Kantowski-Sachs that both is exactly covariant under residual diffeomorphisms and has the correct classical limit. The models of Chiou in 2008, Joe and Singh in 2015, and other models based on similar mu-bar schemes, are the former but not the latter. The model of Ashtekar, Olmedo, and Singh (AOS) are the latter but not the former. In this talk we will review these facts, and explore distilling the insights of AOS into a criterion for the correct classical limit which can be used to construct a model that has both of these properties simultaneously. We will also address the issue of the existence of an underlying exact quantum dynamics on (the square of) the Bohr Hilbert space arising naturally from the kinematical loop quantization of Kantowski-Sachs. The possibility of extending these insights to the more physically relevant case in which only spherical symmetry is imposed will also be considered.

Speaker: Jonathan Engle (Zoom) (Florida Atlantic University)

FAU2_workshop_2024_Engle.pdf

21 Final discussion with all speakers of session 5

Coffee break

Session 6

22 Shell-crossings and shock formation during gravitational collapse

Effective models of gravitational collapse in loop quantum gravity for the Lemaître-Tolman-Bondi spacetime predict that collapsing matter reaches a maximum finite density, bounces, and then expands outward. I will explain that in the marginally bound case, shell-crossing singularities commonly occur for inhomogeneous initial profiles of the dust energy density; this is the case in particular for all profiles that are continuous and of compact support, including configurations arbitrarily close to the Oppenheimer-Snyder model. When a shell-crossing singularity occurs, it is necessary to seek weak solutions to the dynamics; these typically contain shock waves.

Speaker: Edward Wilson-Ewing (Zoom) (University of New Brunswick)

FAU^2_2024_wilson-ewing.pdf

23 Understanding shock waves in gravitational collapse

Recent work on the gravitational collapse of dust with quantum gravity corrections provides evidence that black holes end in a shock wave. I will
provide analytical evidence and describe the numerical procedure that leads to this conclusion.

Speaker: Viqar Husain (Zoom) (University of New Brunswick)

Burgers.mov

LTB_ClassicalMovie.mov

Understanding_Shock_Waves3.pdf

24 Generalized analysis of dust collapses in polymerized theory: non-marginally bound solutions, fate of shocks and covariance

Based on modifications inspired from loop quantum gravity (LQG), spherically symmetric models have recently been explored to understand the resolution of classical singularities and the fate of the spacetime beyond. While such phenomenological studies have provided useful insights, questions remain on whether such models exhibit some of the desired properties such as consistent LTB conditions, covariance and compatibility with the improved dynamics of loop quantum cosmology in the cosmological and LTB sectors. We provide a systematic procedure to construct effective spherically symmetric models encoding LQG modifications as a 1+1d field theory models encoding these properties. Moreover, the existence of the LTB condition allows us to derive analytically the general dust collapse solution in both the bound and unbound cases. With the general solution and the help of the underlying covariant model, we can analyze the corresponding solution in the Schwarzschild-like coordinate, the existence of the Birkhoff-like theorem, and weak singularities and shock solutions. As concrete examples of our generalized strategy, we obtain and compare with several phenomenological models that have been recently studied.

Speaker: Hongguang Liu (FAU Erlangen)

25 Final discussion with all speakers of session 6

Final Discussion with all speakers: Final discussion with all speakers of the day

Closing remarks