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

Europe/Berlin
ECAP Laboratory Building

ECAP Laboratory Building

Nikolaus-Fiebiger-Straße 2 91058 Erlangen
Description

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.

 

Current not complete 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 

   

Registration
Participants
Participants
    • 11:00 12:00
      Registration
    • 13:30 16:00
      Session 1
      • 13:30
        Asymptotically safe quantum gravity on foliated spacetimes 40m

        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
        TBA 40m
        Speaker: Thomas Thiemann
      • 14:50
        The universal swampland 40m

        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 30m
        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 40m

        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
      • 17:10
        Quantum Nature of the Big Bang with Polymer Matter 40m

        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 40m

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

        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
      • 19:00
        Preserving Gauge Symmetries in Cosmology via Loop Quantum Gravity 30m

        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)
    • 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
    • 12:00 13:00
      Physics Colloquium on Quantum Gravity Physics Department

      Physics Department

      Staudtstrasse 7
    • 14:00 16:20
      Session 3
      • 14:00
        Dynamical models of regular black holes and singularity resolution from effective quantum spacetimes 40m

        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)
      • 14:40
        TBA 40m
        Speaker: Hongguang Liu
      • 15:20
        New results for the shock wave model 30m

        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)
      • 15:50
        Causal structure of nonhomogeneous dust collapse in effective loop quantum gravity 30m

        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)
    • 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 40m
        Speaker: Johanna Erdmenger
      • 17:30
        Lorentzian cosmological path integrals from effective spin foams 40m

        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
      • 18:10
        Extracting effective continuum physics from quantum gravity 40m

        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 30m

        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)
    • 19:20 20:05
      Closing remarks: Final discussion with all speakers of the day
    • 20:05 23:05
      Workshop Dinner Barbecue: Workshop Dinner
    • 13:30 16:50
      Session 5
      • 13:30
        Gravitational algebras, entropies and quantum reference frames 40m
        Speaker: Philipp Höhn
      • 14:10
        Effective actions and observables 40m

        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 30m
        Speaker: Melissa Rodriguez Zarate
      • 15:20
        What triangulations at criticality in TGFT? 30m
        Speaker: Johannes Thürigen
      • 15:50
        Dynamics of semiclassical states in the single-vertex model of quantum-reduced loop gravity 30m
        Speaker: Ilkka Mäkinen
      • 16:20
        Computing the graph-changing dynamics of loop quantum gravity 30m
        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 40m

        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)
      • 18:00
        Hawking evaporation in Loop Quantum Gravity 30m

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

        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