FAU^2 Workshop on quantum gravity across scales: from physics at the Planck scale to effective theories

20 May 2025, 11:00 → 22 May 2025, 23:00 Europe/Berlin

ECAP Laboratory Building

Quantum gravity seeks to reconcile general relativity with quantum field theory, providing a consistent description of spacetime at all energy scales. This workshop aims to explore the interplay between different approaches of quantum gravity at the Planck scale, such as for instance  canonical and covariant loop quantum gravity, group field theory and asymptotically safe gravity, and their corresponding low-energy effective descriptions. We aim to discuss how characteristic properties at the Planck scale might leave an imprint in the effective description of gravitational phenomena at macroscopic scales. Main topics include the application in cosmology and the gravitational collapse, modified gravity and the role of physical (quantum) reference frames in the effective description. By bringing together experts from different areas of quantum gravity, one of the main focuses of the workshop is on comparing different methods for accessing the low-energy sector and analysing possible physical implications of quantum gravity across different scales.

 

List of confirmed invited speakers:

Ana Alonso-Serrano (HU Berlin & AEI MPI for Graviational Physics Golm)

Alfio Bonnano (INAF & INFN Catania)

Astrid Eichhorn (Universität Heidelberg)

Johanna Erdmenger (Julius-Maximilians-Universität Würzburg)

Kevin Falls* (Universidad de la Republica Montevideo)

Markus Fröb (Universität Leipzig)

Philipp Höhn* (Okinawa Institute of Science and Technology)

Hongguang Liu* (Westlake University, Hangzhou)

Guillermo Mena Marugan (IEM Madrid, CSIC)

Daniele Oriti* (Universitdad Complutense de Madrid)

Frank Saueressig* (Radboud University Nijmegen)

Parrampreet Singh* (Louisiana State University)

Sebastian Steinhaus (Universität Jena)

Thomas Thiemann (FAU Erlangen-Nürnberg)

*remotely. 

Organizers: Renata Ferrero, Kristina Giesel and Muxin Han 

   

Tuesday 20 May

11:00 → 12:00 Registration

13:30 → 16:00 Session 1

13:30 Asymptotically safe quantum gravity on foliated spacetimes

The gravitational asymptotic safety program is a promising candidate for a quantum theory of the gravitational interactions. Traditionally, many investigations have been carried out in an Euclidean setting and the transition to Lorentzian signature computations has started only recently. In this talk, I will give a pedagogical introduction to the key elements of asymptotic safety before reviewing the various routes taken towards extending the program to Lorentzian signature. I will then zoom in on asymptotic safety based on the ADM-formalism, which introduces a natural time-direction and provides natural connections to Cosmology and Causal Dynamical Triangulations.

Speaker: Frank Saueressig

14:10 Asymptotically safe - canonical quantum gravity junction

We show in a concrete model that asymptotically safe and canonical quantum gravity
methods can be brought into harmonic interaction despite the fact that the two
formulations employ different metric signatures. Focus is laid on concepts rather
than technical details to make this contribution hopefully useful for researchers of both communities.

Speaker: Thomas Thiemann

14:50 The universal swampland

The swampland is the space of those effective field theories that cannot be ultraviolet completed in quantum gravity. Understanding the swampland is relevant for phenomenological model-building and for observational tests of quantum gravity.
This talk will have three parts:
First, I will introduce the notion relative swamplands, to distinguish the swamplands of different quantum-gravity approaches. Their intersection forms the absolute swampland.
Second, I will discuss a subset of swampland conjectures in the light of asymptotically safe gravity.
Third, I will explain how asymptotic safety can provide a mechanism to generate universality, when it is realized within an intermediate regime between a non-quantum-field-theoretic quantum regime of gravity and the standard effective field theory regime below the Planck scale.

Speaker: Astrid Eichhorn

15:30 Probing asymptotically safe quantum gravity with matter with functional and lattice methods

Speaker: Marc Schiffer

16:00 → 16:30 Coffee break

16:30 → 19:30 Session 2

16:30 Invariant observables in quantum gravity and quantum corrections to the Hubble rate and the Newton potential

I report on recent progress in the computation of gauge-invariant relational observables around highly symmetric backgrounds, to arbitrary orders in perturbative quantum gravity and without introducing extra fields which change the dynamics. I then explain how one can compute quantum gravitational corrections to the Hubble rate (the local expansion rate of the universe) and the Newtonian gravitational potential of a point particle using these observables. For the Hubble rate, the one-loop result confirms the physical picture of the mutual attraction of low-energy gravitons which are created during inflation. The result for the Newtonian potential consist of two parts: the first stems from graviton loops and agrees with the correction derived by other methods, while the second one is sourced by the quantum fluctuations of the particle's position and energy-momentum, and may be viewed as an analog of a "Zitterbewegung". Based on arXiv:2303.16218, arXiv:2109.09753, arXiv:1806.11124 and references therein.

Speaker: Markus B. Fröb

talk-froeb.pdf

17:10 Quantum Nature of the Big Bang with Polymer Matter

Quantum geometric modifications to general relativistic dynamics have successfully resulted in the resolution of various cosmological singularities in loop quantized models. The polymer nature of gravity in these models dictates the evolution in the Planck regime, replacing big bang by a (swift) big bounce. But studies so far treat matter on an unequal footing by considering it only in the Schrodinger representation or unpolymerized. We show that if the matter is also polymerized, it dictates singularity resolution and changes the Planck scale physics as understood in loop quantum cosmology in a significant way. We discuss some phenomenological consequences of this model in particular implications for the inflationary paradigm.

Speaker: Parampreet Singh

17:50 Effective quantum corrections to Bianchi models and its connection to BKL scenarios

In this talk I present an analysis of Bianchi I and Bianchi II universes as solutions to an effective quantum-gravity dynamics. We have found modified Bianchi solutions with different matter fields and studied their dynamics to connect it with the classical BKL conjecture.

Speaker: Ana Alonso-Serrano (Humboldt University of Berlin and AEI-MPI)

Bianchi-pheno.pdf

effective-bianchi.pdf

18:30 Operator dressing & proto-gauge theory from quantum mereology

Instead of quantizing a classical phase space, the program of quantum mereology takes abstract Hamiltonian operators defined in some Hilbert space as its starting point, and investigates under which conditions such a setting induces semi-classical dynamics. We advance this program by studying the emergence of entire sets of degrees-of-freedom from random Hamiltonians. We show that these emergent degrees-of-freedom can be interpreted as the modes of a proto-gauge theory. And we demonstrate that these modes are overlapping, i.e. they obey non-trivial commutation relations and are reminiscent of (e.g. gravitationally) dressed operators and of the framework of holographic QFT.

Speaker: Oliver Friedrich

presentation_FAU^2.pdf

19:00 Preserving Gauge Symmetries in Cosmology via Loop Quantum Gravity

This talk presents a novel symmetry-reduction approach that preserves local gauge degrees of freedom. We apply this procedure to identify a classical cosmological sector of General Relativity formulated in Ashtekar variables. The quantization follows the techniques of Loop Quantum Gravity and yields spin-network states with remarkable properties, closely resembling those of standard LQC states. We will show that the implementation of the kinematical constraints can be achieved via a suitable rigging map, and we will discuss the quantum dynamics.

Speaker: Matteo Bruno (Sapienza University of Rome)

BrunoFAU.pdf

19:30 → 20:15 Final Discussion with all speakers: Final discussion with all speakers of the day

20:15 → 23:15 Poster Session: Welcome get together with drinks and snacks

Wednesday 21 May

12:00 → 13:00 Physics Colloquium on Quantum Gravity

14:00 → 16:20 Session 3

14:00 Dynamical models of regular black holes and singularity resolution from effective quantum spacetimes

The study of regular black holes has recently garnered significant attention, driven by the need to extend general relativity to accurately describe black hole interiors and by advances in observational technologies. Despite substantial progress in this field, significant challenges remain in identifying and characterizing physically well-motivated classes of regular black holes. In this talk, I will discuss recent results in Quantum Einstein Gravity within the framework of Asymptotic Safety, offering a new perspective on this problem and, more broadly, on the resolution of singularities.

Speaker: Alfio Bonanno (INAF - INFN Catania)

ifpu.pdf

14:40 From Cosmology to Black Holes: Effective Dynamics via LTB and Scalar-Tensor Reconstruction

Regular black holes play a pivotal role in understanding effective dynamics arising from quantum gravity. In this talk, I will present a framework that maps cosmological effective dynamics onto spherically symmetric systems via Lemaitre-Tolman-Bondi (LTB) embeddings. This method allows for the reconstruction of the 1+1d effective dynamics and a 4D covariant scalar-tensor Lagrangian, derived directly from cosmological or spherically symmetric effective metrics. I will explain how this approach resolves ambiguities in interpreting quantum-gravity-inspired dynamics and clarify the relationship between cosmological and black hole solutions under symmetry reduction with examples. Finally, I will discuss its connections to canonical formulations of quantum gravity, particularly in the reconciling with covariant effective theories.

Speaker: Hongguang Liu

15:20 New results for the shock wave model

Effective stellar collapse inspired by loop quantum cosmology predicts a a bounce of the stellar core when the energy density becomes planckian, and a post-bounce shell-crossing singularity/shock wave formation. I will show recent developments of the model, highlighting some of its strengths and limitations.

Speaker: Francesco Fazzini (University of New Brunswick)

Fazzini.pdf

15:50 Causal structure of nonhomogeneous dust collapse in effective loop quantum gravity

I will discuss aspects of the Husain-Kelly-Santacruz-Wilson-Ewing dust collapse model in effective loop quantum gravity. I will focus on rigorous causal structure analysis, in particular, I will present numerically computed Penrose-Carter diagrams for both homogeneous Oppenheimer-Snyder collapse scenario and nonhomogeneous one with a Gaussian initial dust profile. I will argue that a significant region of spacetime remains inaccessible to the model's dynamics due to the formation of the shell-crossing singularities. The question of whether a timelike singularity, similar to that one in the homogeneous collapse scenario, arises in the nonhomogeneous case remains unresolved. Furthermore, I will prove that phenomena such as black hole explosions or gravitational shock waves cannot be witnessed by an external observer who does not cross any horizon. Indeed, the collapse cannot take place within single asymptotic region.

Speaker: Michał Bobula (University of Wrocław)

FAU_2_mbobula1.pdf

16:20 → 16:50 Coffee break

16:50 → 19:20 Session 4

16:50 Berry phases and von Neumann algebras for wormholes in AdS/CFT

Within the AdS/CFT correspondence, two manifestations of the black hole information paradox are given by the non-isometric nature of the bulk-boundary map and by the factorisation puzzle. By considering time-shifted microstates of the eternal black hole, we demonstrate that both these puzzles may be simultaneously resolved by taking into account non-local quantum corrections that correspond to wormholes arising from state averaging. This is achieved by showing, using a resolvent technique, that the resulting Hilbert space for an eternal black hole in Anti-de Sitter space is finite-dimensional with a discrete energy spectrum. The latter gives rise to a transition to a type I von Neumann algebra. Based on 2411.09616.

Speaker: Johanna Erdmenger

17:30 Lorentzian cosmological path integrals from effective spin foams

In this talk, I will present a spatially flat Lorentzian cosmological subsector of (effective) spin foams and show first steps towards computing its path integral and a continuous time limit.
I will begin by introducing the classical theory in Lorentzian Regge calculus, where we discuss the classical discrete equations of motion and their solutions, coupling (massless) scalar fields, causality violations, symmetries of the action and a continuum limit.
We go to the path integral in two steps: first we review lessons learned from a (2+1)d model, whose amplitudes are motivated by the asymptotic analysis of a Lorentzian spin foam model. Then we go to (3+1)d effective spin foams and present how to efficiently evaluate the path integral using acceleration operators. Furthermore, we show under which conditions classical physics are recovered and show first indications for bouncing cosmologies.
We close with an overview of open questions and the relation to the full spin foam theory.

This talk is based on works with Alexander Jercher and José Simão.

Speaker: Sebastian Steinhaus

Steinhaus_Lorentzian_cosmo_pi.pdf

18:10 Extracting effective continuum physics from quantum gravity

We outline key steps, assumptions and approximations taken to extract an effective continuum dynamics for both cosmological backgrounds and perturbations around them (thus effective quantum field theory) from quantum gravity models in the tensorial group field theory (thus spin foam and LQG) formalism. We also discuss in more detail recent results, showing an emergent cosmological acceleration at both late- and early-times produced by quantum gravity interactions. If time allows, we emphasize how the approximate framework in which these results are obtained, a non-linear extension of (loop) quantum cosmology, can be obtained and motivated from a more general perspective than a specific quantum gravity formalism.

Speaker: Daniele Oriti (Dept Fisica Teorica, Universidad Complutense de Madrid)

18:50 Emergent early and late acceleration from Quantum Gravity

Recent cosmological observations are increasingly challenging the standard understanding of the physics underlying cosmic acceleration. In this talk, I present a new mechanism for acceleration, emerging from quantum gravity interactions as described by Group Field Theory (GFT) models. For a broad class of physically motivated GFT interactions, the resulting cosmological dynamics exhibit a late-time fixed point corresponding to a dynamical dark energy–dominated phase. At earlier times, similar quantum gravitational effects can give rise to a slow-roll inflationary epoch, effectively captured by the dynamics of a fictitious single-inflaton model. This emergent inflation can sustain the required expansion, naturally avoids the graceful exit problem, and appears to transition into a persistent, non-accelerating phase consistent with classical expectations.

Speaker: Dr Luca Marchetti (Okinawa Institute of Science and Technology)

25_05_21_FAU2.pdf

19:20 → 20:05 Closing remarks: Final discussion with all speakers of the day

20:05 → 23:05 Workshop Dinner Barbecue: Workshop Dinner

Thursday 22 May

13:30 → 16:50 Session 5

13:30 Quantum reference frames and their application in quantum gravity

Quantum reference frames are a universal tool for dealing with symmetries in quantum systems. They are the means to describe a quantum system "from the inside" and key for completing the relational paradigm. As such, they naturally arise in quantum gravity whenever we build invariant observables. In this talk, I will review quantum reference frames and some of their applications to questions in quantum gravity. This will include gravitational von Neumann algebras, relational entanglement entropies, and (hopefully) a relational perspective on renormalization.

Speaker: Philipp Höhn

14:10 Effective actions and observables

In quantum gravity, there is a huge freedom in constructing effective actions since they depend on a choice of gauge and field parameterisation. In this talk, I will discuss an approach to constructing effective actions for gauge-invariant fields. Moreover, I will discuss recent progress in understanding when two effective actions are physically equivalent. Some consequences for quantum gravity will be sketched.

Speaker: Kevin Falls

14:50 Hamiltonian renormalisation of the U(1)^3 model for 3+1 quantum gravity

Speaker: Melissa Rodriguez Zarate

15:20 What triangulations at criticality in TGFT?

One way to obtain a continuum limit of Spin Foams is to use a field-theory partitition function to generate triangulations as Feynman diagrams and tune to criticality. This philosophy works well for Matrix-Models with a limit to 2D Quantum Gravity and these have a well established generalization to Tensorial Group Field Theory (TGFT) generating 4D triangulations. However, finding a 4D continuum geometry at criticality remains an open challenge.
In this contribution we report on recent progress with Leonardo Juliano in the systematic exploration of the TGFT phase space. We show how already in the simplest (so-called melonic) regime of TGFT renormalization group flows to non-perturbative fixed points presenting various types of triangulation ensemble occur, and discuss strategies to approach the question of a 4D continuum limit.

Speaker: Johannes Thürigen

15:50 Dynamics of semiclassical states in the single-vertex model of quantum-reduced loop gravity

Speaker: Ilkka Mäkinen

slides_Makinen.pdf

16:20 Computing the graph-changing dynamics of loop quantum gravity

Speaker: Thiago Lucena de Macedo Guedes

16:50 → 17:20 Coffee break

17:20 → 19:00 Session 6

17:20 Darboux transformations in the interior of nonrotating black holes

There is a growing interest in investigating Kantowski-Sachs cosmologies owing to their relation with the interior of nonrotating, uncharged black holes. In particular, we recently developed a Hamiltonian formulation of axial perturbations for a massless scalar field minimally coupled to these cosmologies, describing physical perturbations by gauge invariants. Adopting this formulation, we offer a clear geometric interpretation of Darboux transformations between (axial) gauge invariants, understanding them as generalized canonical transformations that preserve the Hamiltonian structure of the perturbations in the black hole interior.

Speaker: Guillermo A. Mena Marugán (IEM, CSIC)

FAU2025.pdf

18:00 Hawking evaporation in Loop Quantum Gravity

A recent covariant formulation, which includes non-perturbative effects from loop quantum gravity (LQG) as self-consistent effective models, has revealed the possibility of non-singular black hole solutions. The new framework makes it possible to couple scalar matter to such LQG black holes and derive Hawking radiation in the presence of quantum space-time effects while respecting general covariance. Standard methods to derive particle production both within the geometric optics approximation and the Parikh-Wilczek tunneling approach are therefore available and confirm the thermal nature of Hawking radiation. The covariant description of scale-dependent decreasing holonomy corrections maintains Hawking temperature as well as universality of the low-energy transmission coefficients, stating that the absorption rates are proportional to the horizon area at leading order. Quantum-geometry effects enter the thermal distribution only through sub-leading corrections in the greybody factors. Nevertheless, they do impact energy emission of the black hole and its final state in a crucial way regarding one of the main questions of black-hole evaporation: whether a black-to-white-hole transition, or a stable remnant, is preferred. For the first time, a first-principles derivation, based on a discussion of backreaction, finds evidence that points to the former outcome.

Speaker: Idrus Husin Belfaqih

slides_Idrus_Husin_Belfaqih.pdf

18:30 Across the Horizon: Singularities in Quantum Black Hole Spacetimes

Black holes serve as key testing grounds for quantum gravity due to their singular nature and have been extensively studied in various quantum gravity approaches. In this talk, I apply the Henneaux-Teitelboim formulation of unimodular gravity to the symmetry-reduced Schwarzschild-(Anti-)de Sitter model. We perform a canonical quantization, leading to a Wheeler-DeWitt equation that takes the form of a Schrödinger equation in unimodular time. By enforcing unitary evolution in this time coordinate, we naturally treat the cosmological constant as an observable. We find a family of quantum theories in each of which the classical singularity is resolved, and we derive an analytical expression for the quantum-corrected Schwarzschild-(Anti-)de Sitter metric. Furthermore, we show that each quantum theory permits only semi-classical states corresponding to either positive, negative, or zero-mass black holes. Therefore we avoid problems that would otherwise occur in singularity free theories with arbitrarily large negative energy states. Our approach allows us to be agnostic about what the full quantum gravity theory looks like, assuming it leads to unitary evolution in unimodular time, at least in the semiclassical limit.

Speaker: Sofie Ried (Univesity of Sheffield)

19:00 → 19:45 Final Discussion with all speakers: Final discussion with all speakers of the day

19:45 → 20:00 Closing remarks