Quantum Gravity 2023

10 Jul 2023, 08:00 → 14 Jul 2023, 17:00 Europe/Amsterdam

Collegezalencomplex CC2, CC3 & CC5 (Radboud University Nijmegen)

Quantum Gravity 2023

Quantum Gravity 2023 will take place on 10 - 14 July, 2023 at Radboud University, Nijmegen in the Netherlands. The conference aims at bringing together researchers from all approaches to quantum gravity as well as scientists working on its conceptual aspects and potential phenomenological implications. The meeting will provide a platform to discuss the open questions currently driving the research field in an open and constructive format. The goal is to work towards combining the lessons learned within various complementary approaches followed by the field. The conference follows the spirit of Quantum Gravity 2020, which was organized online by Perimeter Institute, but this time in a face to face setting.

Reliving QG2023: recordings, slides, posters

Recordings of 73 of the 81 talks are now available on the QG2023 YouTube recordings playlist.
(Unfortunately, recordings of the 7 talks of parallel session S1 as well as the last talk of S10 are missing due to technical issues.)

Slides of the talks can be found in the timetable or contribution list.

Many of the posters can be revisited on the poster sessions page.

Quantum Gravity at the cultural terrace De Kaaij

On Tuesday, 11 July 2023 the Science Café Nijmegen hosts a special outdoor event "The nature and origin of space-time - an insider's perspective on the universe" open to the general public. The event will take place at the Cultural Terrace De Kaaij at the Waal River situated at the very heart of Nijmegen. All conference participants are cordially invited to join for this very special occasion.

Invited speakers

The line-up of invited speakers:

• Jan Ambjørn (U Copenhagen & Radboud U)
• Ignatios Antoniadis (Sorbonne U)
• Seth Kurankyi Asante (U Jena)
• Markus Aspelmeyer (U Vienna)
• Jan de Boer (U Amsterdam)
• Suddhasattwa Brahma (U Edinburgh)
• David Brizuela (U Basque Country)
• Steve Carlip (UC Davis)
• John Joseph Carrasco (Northwestern U)
• Bianca Dittrich (Perimeter I)
• Job Feldbrugge (U Edinburgh)
• Laurent Freidel (Perimeter I)
• Flaminia Giacomini (ETH Zürich)
• Hal Haggard (Bard College)
• Arthur Hebecker (U Heidelberg)
• Lavinia Heisenberg (ETH Zürich)
• Thomas Hertog (KU Leuven)
• Philipp Höhn (OIST)
• Bei-Lok Hu (U Maryland)
• Claus Kiefer (U Köln)
• Benjamin Knorr (Nordita)
• João Magueijo (Imperial College)
• Thomas Mertens (Gent U)
• Daniele Oriti (LMU München)
• Jan Plefka (Humboldt-U Berlin)
• Kasia Rejzner (U York)
• Michael Ruf (UCLA)
• Sumati Surya (Raman Research I)
• Madhavan Varadarajan (Raman Research I)
• Christof Wetterich (U Heidelberg)
• Richard Woodard (U Florida)
• Kathryn Zurek (Caltech)

(Download conference poster here.)

Sponsors

Delta Institute for Theoretical Physics	Stichting Physica	Lorentz fonds
Gravity Theory Trust	European Research Council	Deutsche Forschungsgemeinschaft
Dutch Research Council (NWO)

Local Organization Committee

• Beatrice Bonga
• Timothy Budd
• Badri Krishnan
• Renate Loll
• Antonio Pereira
• Frank Saueressig

 

 

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Monday, 10 July

Registration: and Information desk

1 Welcoming Words

The event will be opened by Sijbrand de Jong (Dean of the faculty of science). Subsequently Daniele Oriti (chair of the International Society of Quantum Gravity) will welcome the participants.

Speakers: Sijbrand de Jong (Radboud University), Daniele Oriti (LMU Munich)

2 Common challenges in Quantum Gravity | Review

I will discuss some of the key challenges in quantum gravity. These will include Lorentzian signature, understanding quantum space-time, explaining our universe and finding signatures of quantum gravity.

Speaker: Bianca Dittrich

2023-07 Common challenges Bianca Dittrich.pdf

3 Common challenges in Quantum Gravity | Focus: Problem of Time

The problem of time is among the main conceptual challenges for any approach to quantum gravity. It is already present in classical general relativity, where it is called background independence, but it exhibits its full power only after quantization. I give a general introduction and point out ways
for its solution. The presentation is focused on quantum geometrodynamics, which is the canonical quantization of general relativity in metric variables. I give some examples and briefly discuss connected issues such as decoherence, semiclassical approximation, and the arrow of time.

Speaker: Claus Kiefer

nijmegen23-kiefer Claus Kiefer.pdf

10:30 Coffee/tea break

4 Recent developments in String Theory | Review

Speaker: Ignatios Antoniadis

5 Recent developments in String Theory | Focus: From String Landscape and Swampland to Cobordism and Wormholes

I will start with a pedagogical introduction to how the so-called String Landscape arises and what the best candidates for realistic vacua, including in particular cosmic acceleration, are. I will then review how this subject has evolved in view of the recent revival of the Swampland paradigm. Finally, I will comment on the related current topics of Cobordism Conjecture and euclidean wormholes, in particular the various problems associated with the latter.

Speaker: Arthur Hebecker (University of Heidelberg)

Hebecker.pdf

12:30 Lunch

6 Posters A

14:45 Coffee/tea break

Parallel sessions: S1

7 Lorentzian functional renormalization group: Hadamard property and state (in)-dependence.

While physically desirable, the extension of Functional Renormalization Group techniques to Lorentzian manifolds is technically challenging and is expected to lead to qualitatively new features. One of these features is the dependence on an underlying vacuum-like state, which I illustrate via Hadamard states in a perturbative expansion. For regulators that render the RHS of the flow equation finite, the ultraviolet aspects will by-and-large match those obtained in Euclidean signature through the standard heat kernel methodology. In contrast, the infrared aspects are state-dependent, technically difficult to extract, yet vital for the extrapolation of beta functions to small RG scales. We outline these features on cosmological spacetimes using a "spatial" regulator and so-called States of Low Energy.

Speaker: Rudrajit Banerjee (Okinawa Institute of Science and Technology)

RBanerjee_QG2023_talk Rudi Banerjee.pdf

8 Wave Function Renormalization and Flow of Couplings in Asymptotically Safe Quantum Gravity

The importance of the proper treatment of the wave function renormalization in the renormalization group analysis of quantum gravity is pointed out. The renormalization factor, sometimes called an inessential coupling, can be used to fix any one of the coupling constants, with the exception of the coupling constants that remain unchanged by the rescaling of the field. If we choose to fix the cosmological constant, the renormalization group equation tells us the flow of the Newton and $R^2$ couplings. We then find that the Newton coupling reaches a nontrivial ultraviolet fixed point and becomes small in the low energy, and the $R^2$ couplings are asymptotically free. For the asymptotically free fixed point of the higher curvature couplings, we find that both of the two independent terms are relevant operators in the high energy.

Speaker: Nobuyoshi Ohta (National Central University)

2023Nijmegen nobuyoshi ohta.pdf

9 Composite Operators in Asymptotically Safe Quantum Gravity

The renormalization of composite operators in the Asymptotic Safety scenario is instructive from two points of view, which is due to two different interpretations of their anomalous dimensions. These can be interpreted either as corrections to the actual geometric scaling dimensions of a set of operators, or as approximations of the critical exponents characterizing the Reuter fixed point(s) in theory space. The former interpretation is in particular relevant for the construction of observables, while the latter caters to the heart of the Asymptotic Safety program itself. In this talk, I will introduce the composite-operator method and present in more detail recent developments in the latter avenue of research.

Speaker: Maximilian Becker (Radboud University, Nijmegen)

20230710-Nijmegen Maximilian Becker.pdf

10 Lorentzian quantum gravity and the graviton spectral function

I will present the first direct and non-perturbative computation of the graviton spectral function in quantum gravity. This is achieved with the help of a novel Lorentzian renormalisation group approach, combined with a spectral representation of correlation functions. We find a positive graviton spectral function, showing a massless one-graviton peak and a multi-graviton continuum with an asymptotically safe scaling for large spectral values. There are no ghost or tachyonic instabilities, which is crucial for the unitarity of the theory. Our findings open a door to investigate scattering amplitudes and unitarity of fully quantised gravity.

Speaker: Manuel Reichert (University of Sussex)

Talk-Reichert Manuel Reichert.pdf

11 Decoupling limits in Quantum Gravity

In the context of perturbative quantum field theory, the addition of quadratic-curvature operators to the Einstein-Hilbert action makes it possible to achieve strict renormalizability in four dimensions. The additional terms are multiplied by dimensionless coefficients that are related to the masses of the extra gravitational degrees of freedom and to the interaction couplings. In this talk, we will study the limits in which those parameters tend to infinity and show that different types of decouplings can occur. Remarkably, these limits are found to be sensitive to a non-zero cosmological constant. In particular, we will show that the helicity-0 of the spin-2 massive ghost is the only one that survives when the coefficient of the Weyl-squared term tends to infinity in the presence of a cosmological constant, while other helicities decouple. We will discuss physical implications of our analysis in relation to the high-energy limit and the role of the cosmological constant in quantum gravity.

Speaker: Luca Buoninfante (Nordita)

talk@QG2023_Buoninfante Luca Buoninfante.pdf

12 A Phase of 4-dimensional Geometry Generated by Combinatorially Non-local Field Theory

Recovering the observed four-dimensional spacetime is a major challenge and distinguishing criterion for any quantum theory of gravity based on fundamentally discrete structures. Tensor models seem to generate random geometries, but only two-dimensional or fractal ones. I will show here how adding geometric degrees of freedom in the spirit of group field theory enables to overcome this obstacle. Examples of four-dimensional random geometries in such field theories open up the possibility that group-field quantum gravity entails a semiclassical regime of continuum spacetime in agreement with observations.

Speaker: Johannes Thürigen (University of Münster)

qg23thuerigen Johannes.pdf

13 High-energy properties of the matter-graviton scattering in quadratic gravity

We obtain the matter-graviton scattering amplitude in the gravitational theory of quadratic curvature, which has R_{μν}^{2} term in the action. Unitarity bound is not satisfied because of the existence of negative norm states, while an analog of unitarity bound for S-matrix unitarity holds due to the cancelation among the positive norm states and negative norm ones in the unitarity summation in the optical theorem. The violation of unitarity bound is a counter example of Llewellyn Smith’s conjecture on the relation between tree-level unitarity and renormalizability. We have recently proposed a new conjecture that an analog of the unitarity bound for S-matrix unitarity gives the equivalent conditions to those for renormalizability. We show that the gravitational theory of quadratic curvature is a nontrivial example consistent with our conjecture. This talk based on work in arXiv:2210.13666.

Speaker: Yugo Abe (National Institute of Technology, Miyakonojo College)

QG2023(YugoAbe) Yugo Abe.pdf

Parallel sessions: S2

14 A discrete discontinuity between the two phases of gravity

We address the problem of setting up a Hamiltonian theory of gravity when the tetrad (metric) fields are not invertible. We find that this theory exhibits three local degrees of freedom, thus revealing that the limit to a vanishing tetrad determinant is discretely discontinuous. This outcome is shown to be independent of whether the null eigenvalue of the four-metric lies along a timelike or a spacelike direction. For the particular case of vanishing lapse, the Hamiltonian constraint could be made to disappear from the canonical theory by fixing the torsional gauge freedom. Any state functional invariant under the internal gauge rotations and spatial diffeomorphisms is a formal solution of the associated quantum gravity theory. The framework here could provide a Hamiltonian basis to analyze a physical singularity when it is associated with a degenerate tetrad everywhere, as in the case of the Belinski-Khalatnikov-Lifshitz behaviour in cosmology.

Speaker: Sandipan Sengupta (Indian Institute of Technology Kharagpur)

sandipan-sengupta-QG2023 Sandipan Sengupta.pdf

15 Radiative correction for the 4D spinfoam model with a cosmological constant

Infrared divergence is a common feature of spinfoam models with a vanishing cosmological constant. The disappearance of divergence at the present of a non-zero cosmological constant was conjectured and then recently proved in a spinfoam model constructed with complex Chern-Simons theory. Based on this spinfoam model, we study how such divergence depends on the value of the cosmological constant. In this talk, I will present our recent results on the first-order divergence, also called the radiative correction, of the 4D spinfoam model with a cosmological constant. Taking the cosmological constant to approach the zero limit, our analysis gives a lower bound on the divergence, which is an inverse power law in (the absolute value of) the cosmological constant.

Speaker: Qiaoyin Pan (Florida Atlantic University)

Qiaoyin Pan copy Qiaoyin Pan.pdf

16 A new perspective in the construction of spin foam models to quantum gravity

In this talk, we will present an alternative paradigm to construct spin foam models that consists in a change in the discretization on which the theory is based. This change produces a classical theory and a corresponding spin foam model with interesting new characteristics. We hope this will help to understand better the rol of the curvature in the quantum domain, to study renormalization of spin foam models from a new perspective, and to solve an issue about the imposition of the simplicity constraints that has been previously remarked.

Speaker: Carlos Beltrán (National Autonomous University of Mexico)

Beltrán_Carlos_A new perspective in spinfoam models carlos beltran.pdf

17 Reviving Quantum Geometrodynamics: Conceptual Setup and Lattice Discretization

The Quantum Geometrodynamics program was an early effort to canonically quantize General Relativity in the most straightforward manner possible. The approach involves a formal $3+1$--split of spacetime and applies Dirac's procedure for the quantization of constraint systems to the ADM formulation of General Relativity, with the configuration variables represented by the components of the spatial metric. While DeWitt was able to write down a formal Hamiltonian constraint operator, the problems of obtaining a well-defined Hilbert space and defining the constraints thereon remained unsurmountable challenges, among others. The canonical Loop Quantum Gravity approach, the spiritual successor of Quantum Geometrodynamics, resolved these problems at the expense of introducing a few aesthetically displeasing features. These include non--separable Hilbert spaces, non--strongly continuous representations and states that are excited on very singular geometries. In this talk, we propose a novel approach to overcome the difficulties faced by Quantum Geometrodynamics, while avoiding the above-mentioned characteristics of canonical Loop Quantum Gravity. We employ a lattice discretization that adheres as closely as possible to the original formulation, and examine the lattice corrections to the theory. We discuss the steps necessary for obtaining a well--defined continuum limit and explain how to perform calculations in the effective theory. The accompanying talk, titled "Reviving Quantum Geometrodynamics: Representations of the Kinematical Observables and the Diffeomorphism Group," will delve further into the intricacies of our approach.

Speaker: Susanne Schander (Perimeter Institute for Theoretical Physics, Canada)

SCHANDER-QG-Conference-2023 Susanne Schander.pdf

18 Reviving Quantum Geometrodynamics: Representations of the Kinematical Observables and the Diffeomorphism Group

Quantum Geometrodynamics represents an early attempt at the canonical quantization of General Relativity. In a seminal paper, DeWitt proposed a formal Hamiltonian constraint operator by substituting canonical momenta with variational derivative operators. However, the rigorous interpretation of this operator has remained elusive due to the highly nonlinear nature of the constraint and the associated issues arising from the multiplication of distributions. Consequently, a well-defined Hilbert space for the theory could not be specified. In this talk, we build upon the conceptual framework introduced in the companion talk, "Reviving Quantum Geometrodynamics: Conceptual Setup and Lattice Discretization." We establish Hilbert spaces for the lattice theories that allow for well-defined lattice approximations of all continuum quantities. These Hilbert spaces exhibit a non-standard representation of the canonical commutation relations between the matrix elements of the spatial metric and the conjugate momenta. This approach ensures that states are exclusively supported on positive definite symmetric matrices. Furthermore, we construct a continuum Hilbert space and a representation of the group of spatial diffeomorphisms thereon.

Speaker: Thorsten Lang (University of Erlangen–Nuremberg)

LANG-QG-Conference-2023 Thorsten Lang.pdf

19 Towards new numerical algorithms for spin foam quantum gravity

Spin foam quantum gravity describes quantum space-time as a sum over (pre-)geometric configurations encoded in group theoretic labels. In absence of experimental data, it is therefore crucial to establish connection to classical, continuum physics. While the connection to discrete (Regge) gravity is well established for small discretisations, large triangulations are far less explored. To learn about their geometry, extraction of observables, e.g. the spectral dimension, via numerical simulations is indispensable. In this talk, I will briefly review the numerical challenges and progress made in the last years. Then I present the so-called hybrid representation of spin foams that allows us to understand spin foams as a superposition of non-matching building blocks in the semi-classical regime. This allows us to explore two directions: on the one hand, we can define an effective model utilizing semi-classical amplitudes, which will give indications which configurations dominate in the path integral. On the other hand, we use it to define probability distribution (for intertwiner labels) that can be used for Monte Carlo simulations.

Speaker: Sebastian Steinhaus (FSU Jena)

Steinhaus_new_methods_sf Sebastian Steinhaus.pdf

20 Emergence of geometry from Fluctuations

I will present recent work in which we defined a euclidean path integral of gravity and matter fields in the eigenbasis of the wave operators, using a result of Hawking and Gilkey. On one hand, working in the eigenbasis of the wave operators means working exclusively with geometric invariants and this avoids the need to mod out the diffeomorphism group and makes it possible to carry out the path integral explicitly. On the other hand, working in the eigenbasis of the wave operators does not enforce the existence of a coordinate basis. As a consequence, this path integral also describes physical setups that are pre-geometric in the sense that they do not admit a mathematical representation in terms of fields on a manifold. We focus on the regime in which a representation in terms of a manifold and matter fields emerges. We find that the geometric properties of the emergent manifold, such as its volume and number of dimensions, depend on the energy scale considered and on the balance of bosonic and fermionic species.

Speaker: Marcus Reitz (Jagiellonian University)

Presentation Marcus Reitz.pdf

17:00 Welcome Drinks

Tuesday, 11 July

Registration: Registration and Information desk

21 Quantum Gravity in Table-Top Experiments | Review: Theoretical Basis for Low Energy Quantum Gravity and Quantum Information Phenomena

General relativity and quantum field theory being the two most tested theories which explain exceedingly well the large scale structure and dynamics of spacetime on the one hand and the quantum properties of matter on the other, it is of no surprise that they form the theoretical basis for low energy perturbative quantum gravity experiments. Imposing self-consistency conditions between solutions of the Einstein equation and the field equations for quantum matter leads naturally to semiclassical gravity (SCG). However, when tackling quantum information issues, starting with as simple an entity as the entangled state between two masses (gravitational cat state), Einstein equation sourced by the expectation values of the stress energy tensor of quantum matter fields is inadequate. Correlations or fluctuations of mass densities enter as a necessity. The requisite theory is called stochastic semiclassical gravity. Introduced and constructed in the 90s, rooted in quantum field theory in curved spacetime of the 70s and SCG of the 80s, it has been applied to quantum field processes in the early universe and black holes where fluctuations phenomena are expected to play an important role. In this talk I shall summarize the key features of this theory and show how it, even in the nonrelativistic, weak field form, is needed for treating gravitational decoherence and entanglement problems (part 2), topics of rising current interest, as well as the effect of graviton noises on geodesic separations (part 3), all at experimentally accessible energies. Before that, in deference to the central theme of this conference which is (nonperturbative) quantum gravity, I want to share my view (part 1) and raise some questions of principle in the daunting quest for viable theories describing the microscopic structures of spacetime.

Speaker: Bei Lok Hu

Bei-Lok-Hu-safety-slides.pdf

22 Quantum Gravity in Table-Top Experiments | Focus: How to avoid the appearance of a classical world in gravity experiments?

No experiment today provides evidence that gravity requires a quantum description. The growing ability to achieve quantum optical control over massive solid-state objects may change that situation – by enabling experiments that directly probe the phenomenology of quantum states of gravitational source masses. This can lead to experimental outcomes that are inconsistent with the predictions of a purely classical field theory of gravity. Such 'Quantum Cavendish' experiments will rely on delocalized motional quantum states of sufficiently massive objects and gravity experiments on the micrometer scale. I review the current status in the lab and the challenges to be overcome for future experiments.

Speaker: Hans Hepach (U.of Vienna)

2023_Quantum_Gravity_Hepach Hans Hepach.pdf

10:30 Coffee/tea break

23 Gravity as Quantum Field Theory | Review

Speaker: J.J. Carrasco

Carrasco_QG2023.pdf

24 Gravity as Quantum Field Theory | Focus: The road towards non-perturbative gravitational scattering amplitudes in Asymptotic Safety

Scattering amplitudes are a key tool of modern quantum field theories, them being used for a wide range of applications, from collider experiments all the way to gravitational waves. In a gravitational context, they showcase the perturbative non-renormalisability of General Relativity: they grow unboundedly at high energies, violating standard unitarity bounds. Any putative quantum gravity theory has to resolve this issue.

After a discussion of some general aspects of scattering amplitudes in QFT, I will present some recent advances within Asymptotic Safety to derive non-perturbative gravitational scattering amplitudes from first principles.

Speaker: Benjamin Knorr

QG2023_Knorr Benjamin Knorr.pdf

12:30 Lunch

25 Quantum Gravity for Bachelor Students

26 Posters B

14:45 Coffee/tea break

Parallel sessions: S3 - Observables in Quantum Gravity

27 Observables in Quantum Gravity: Observables and dynamical frames in gravity

I will discuss how the notion of dynamical frames helps in the construction (and interpretation) of diffeomorphism-invariant observables, encompassing and unifying the relational and dressed observables approaches. This also extends the concept of general covariance to a relational (and thus arguably more physical) form, associated with the covariance of gauge-invariant descriptions of physical laws under changes of dynamical frame.

Speaker: Philipp Hoehn (OIST)

Nijmegen2023_Hoehn Philipp Hoehn.pdf

28 Observables in Quantum Gravity: Classical observables from quantum scattering amplitudes

We explain how a quantum viewpoint benefits the computation of observables in classical general relativity. We give details on the amplitudes-based approach, highlight recent results and outline challenges faced in pushing the program to higher orders in perturbation theory.

Speaker: Michael Ruf (UCLA)

Talk_QG Michael Ruf (1).pdf

29 Algebras and states for quantum fields on causal sets

Geometric quantization is a natural way to construct a quantization map over classical field data that is given as a symplectic manifold with an inner product (a Riemannian metic). This yields a (non-commutative) quantum algebra that can be equipped with a state determined by a map dual to the quantization. We investigated this technique for a free scalar field on a causal set (locally finite, partially ordered set) and showed that the associated state coincides with the Sorkin-Johnston state in causal set theory. Our mathematical construction suggests a natural generalization to less linear examples, such as interacting fields. This is based on joint work with Eli Hawkins and Kasia Rejzner (arXiv:2207.05667).

Speaker: Christoph Minz (Universität Leipzig, ITP)

20230711Talk_Minz_Nijmegen Christoph Minz.pdf

30 Discussion Block

Parallel sessions: S4 - Lorentzian structures in Quantum Gravity

31 Lorentzian Structures in Quantum Gravity: Exploring causality violations in Lorentzian path integrals for discrete gravity

Computing the gravitational path integral over Lorentzian geometries presents two key challenges. Firstly, determining the appropriate configurations to include in the path integral, in particular whether to allow for causality-violating configurations. Secondly, the path integrals exhibit high oscillatory behaviour and are defined over unbounded domains, posing difficulties for analytical and numerical computations. In this talk, we explore how these challenges can be effectively addressed employing Picard-Leftschetz methods applied to Regge calculus, a discrete approach to gravity. We introduce the Regge action for complexified variables and examine its analytical continuation, resulting in a unique complex action, except for causality-violating configurations, which manifest as branch cuts. We also highlight on implications and applications to cosmology and effective spinfoam models.

Speaker: Seth Asante (FSU Jena)

SethAsante-QG2023 Seth Asante.pdf

32 Lorentzian Structures in Quantum Gravity: Calibrating the continuum approximation of discrete quantum gravity

A central question in discrete quantum gravity is how, and in what sense, the continuum can be recovered in the classical limit of the theory. I will address this question in the context of causal set theory, with an emphasis on recent work. In particular, I will discuss how recent developments in Lorentzian geometry can be used to study the convergence properties of causal sets in the large n limit, and may be of broader interest to Lorentzian quantum gravity.

Speaker: Sumati Surya (Raman Research Institution)

QG2023-SSurya Sumati Surya.pdf

33 Lorentzian Structures in Quantum Gravity: On the existence of real-time path integrals

Interference is among the most universal phenomena in physics, as exemplified by the famous Feynman path integral describing quantum physics as a sum over histories. However, conditionally convergent oscillatory integrals are often delicate to define and even more difficult to evaluate. Moreover, the real-time Feynman path integral is still lacking a mathematically rigorous definition. In this talk, I will show steps towards defining the Feynman's path integral non-perturbatively, without a Wick rotation to imaginary time. I start by introducing a class of smooth regulators rendering the interference integral absolutely convergent. Subsequently, I continue the integration domain to its Lefschetz thimble associated with its relevant instantons. The resulting set of integrals can be formally defined in terms of the infinite-dimensional Wiener measure developed for the theory of Brownian motion. As I will show, the resulting path integral is completely governed by caustics as classified by catastrophe theory. The real-time propagator does not share this property with the Euclidean path integral. I will illustrate this approach by studying the real-time path integral of the one-dimensional Teller well and barrier and the quartic oscillator in quantum mechanics.

Speaker: Job Feldbrugge (University of Edinburgh)

Job_Feldbrugge Job Feldbrugge.pdf

34 Discussion Block

19:30 Science Café Nijmegen - Outreach Event

The Science Café Nijmegen (organized Eric Stoffels) will run a special edition of the "Science Café Nijmegen". In this open-air Science Café at the border of the Waal river we want to fascinate, inform and engage the general audience on the concepts and ideas in the field of Quantum Gravity. It will feature three talks (~20 minutes each) focusing on ideas related to the structure and emergence of spacetime up to philosophical challenges in quantum gravity. The event, with live music and drinks, will have an informal and lively atmosphere and offers lots of interaction with an enthusiastic audience.

Wednesday, 12 July

Registration: Registration and Information desk

35 Quantum Gravity Phenomenology I | Review: Are we now really at the dawn of quantum-gravity phenomenology?

Quantum-gravity phenomenology is entering a more mature phase, especially thanks to the advent of multimessenger astrophysics. For in-vacuo dispersion by photons the CTA and multisatellite gamma-ray telescopes will soon provide data probing higher values of the magnitude of the effects and also suitable for discriminating between different models
of the redshift dependence of the effects. IceCube has opened a window on in-vacuo dispersion by neutrinos and tangible improvements in sensitivity will be achieved with KM3NeT, Baikal-GVD and IceCube-Gen2. For the study of Planck-sale-induced threshold anomalies telescopes such as LHAASO should ensure significant progress, by investigating indirectly the inconsistencies presently found among different estimates of the cosmic far-infrared background.The coming years should also bring about meaningful tests of phenomenological models of IR-UV mixing, thanks to improved astrophysics data and even higher accuracy of cold-atom interferometers.

This session is shared with the 4th Annual Conference COST CA18108 : Quantum gravity phenomenology in the multi-messenger approach.

Speaker: Giovanni Amelino-Camelia

36 Quantum Gravity Phenomenology I | Focus: High precision classical gravitational wave physics from a worldline quantum field theory

This session is shared with the 4th Annual Conference COST CA18108 : Quantum gravity phenomenology in the multi-messenger approach.

Speaker: Jan Plefka

qg2023_Jan_Plefka.pdf

10:30 Coffee/tea break

37 Quantum Gravity Phenomenology II | Review: Quantum gravity predictions for particle physics and cosmology

Contrary to general belief, the metric fluctuations in quantum gravity can have important consequences for present day experiments in particle physics. The quantum scale symmetry at an ultraviolet fixed point can predict some couplings of the standard model of particle physics. Quantum fluctuations of the metric determine the quartic self-interaction of the Higgs-scalar at an energy scale close to the Planck mass. Extrapolating the running couplings to the electroweak scale, the mass of the Higgs boson has been predicted in the range found later by experiment. Further predictions may concern the quark masses or beyond standard model particles.

For cosmology the scaling solution of quantum gravity for scalar potentials is found to have a form that predicts both a very early inflationary epoch and late dynamical dark energy. The spectrum of primordial cosmic fluctuations is, in principle, computable. The beginning of the Universe is typically found to be a scale invariant state that may be called „great emptiness".

This session is shared with the 4th Annual Conference COST CA18108 : Quantum gravity phenomenology in the multi-messenger approach.

Speaker: Christof Wetterich

Quantum gravity predictions for particle physics and cosmology_Nijmegen_0723 Christof Wetterich.pdf

38 Quantum Gravity Phenomenology II | Focus: CA18108 network: new challenges and opportunities for research in quantum gravity phenomenology

The COST Action CA18108 was established with the aim of fostering the collaboration and synergies among diverse communities interested in the development and astrophysical testing of quantum gravity models. By combining expertises in theoretical models for quantum gravity effects and in the detection of the various cosmic messengers (gamma rays, neutrinos, cosmic rays, and gravitational waves), this network has started to explore exciting challenges in the field of quantum gravity phenomenology.

Noteworthy opportunities have arisen since the beginning of the Action, such as the first detection of Gamma Ray Bursts by Imaging Atmospheric Cherenkov Telescopes (IACTs), the discovery of PeVatrons within our galaxy, or the detection of a high-energy neutrino event compatible with the Glashow resonance. These achievements have showcased the potential of the ongoing collaboration between these communities. Together with the upgrades of various instruments, the future holds promising prospects for the exploration of new paths for discovering quantum gravity effects in the physics of the cosmic messengers.

This session is shared with the 4th Annual Conference COST CA18108 : Quantum gravity phenomenology in the multi-messenger approach.

Speaker: Jose Manuel Carmona

12:30 Lunch

39 Quantum Gravity for Master Students

In this talk Mercedes and Laurent will take you through their personal experience with Quantum Gravity, how they themselves came to this topic and what challenges they all face now in their research.

Speakers: Laurent Freidel, Mercedes Martinez Benito

Parallel sessions: S5

40 On the canonical equivalence of Jordan and Einstein frames

A longstanding issue is the equivalence between the Jordan and the Einstein frames. It is believed, but not completely proved, that the cosmological physical observables give the same results in the two frames. Our aim is to tackle this problem from the perspective of the Hamiltonian formalism. For this reason, we will perform the Hamiltonian analysis of the Brans-Dicke theory with Gibbons-Hawking-York boundary term both in the Jordan and the Einstein frames. Contrary to many claims made in the literature in the past, it will be shown that the transformations from the Jordan to the Einstein frames are not Hamiltonian canonical transformations. We will show that if we will perform a gauge fixing of the lapse and shifts functions and implement them as secondary Dirac’s constraints in the ADM formalism, the primary first-class constraints will become second class. In this way, we can eliminate these degrees of freedom replacing the Poisson brackets with the Dirac’s brackets and solve the second-class constraints. On this reduced phase space, the Hamiltonian transformations from the Jordan to the Einstein frames are Hamiltonian canonical transformations. In our opinion, this does not mean that Jordan and Einstein frames, from Hamiltonian point of view, are physically equivalent. In fact, we have only shown that solutions of the equations of motion in the Einstein frame can generate solutions of the equations of motion in the Jordan frame. Furthermore, we will see that the Jordan Frame is Hamiltonian canonical equivalent, under some transformations called anti-Newtonian transformations, to a frame whose solutions, in the limit, behave as Carroll Gravity. This abstract is partially based on the following papers: arXiv:2003.04304 Phys. Rev. D 103 024022 (2021), arXiv:2110.12222 Phys. Rev. D 105 084008 (2022), arXiv:2112.02098 Universe 8 (2022) and others to come…

Speaker: Gabriele Gionti, S.J. (Specola Vaticana (Vatican Observatory) and LNF-INFN)

GiontiQGNijmegen2023 Gabriele Gionti, S.J.pdf

41 Impact of matter fields on quantum geometry

The model of Causal Dynamical Triangulations (CDT) is a background-independent and diffeomorphism-invariant approach to quantum gravity, which provides a lattice regularization of the formal gravitational path integral. The framework does not require any coordinate system but uses only geometric invariants. We introduce coordinates via scalar fields with periodic boundary conditions with a jump that match a universe with a toroidal spatial topology. I will present the impact of dynamical matter fields on the geometry of a typical quantum universe in the four-dimensional CDT model. In particular, I will discuss a phase transition induced by the change in the amplitude of the scalar field jump.

Speaker: Andrzej Görlich (Jagiellonian U. )

GoerlichNijmegen2023b Andrzej Görlich.pdf

42 Holographic mappings from quantum gravity states

Holographic transport and reconstruction properties as in the AdS/CFT correspondence are conjectured to be a key ingredient in characterising physical states of quantum gravity. With the goal of characterising states in the Hilbert space of spin networks with holographic properties, we extend the usual quantum informational notion of Jamiolkowski-Pillis-Choi channels to the setting of algebras with nontrivial center. With this tool, we define bulk and boundary subsystems of spin networks and study mappings between them induced by states of quantum geometry. Using developments from Random tensor network theory, we can then analyse criteria for typical states to induce holographic maps, and study some of their geometric properties. OR The building blocks of a Group Field Theory are, by their conceptual definition, finite chunks of space with boundary. Therefore, the study of finite size boundary effects in gravity is crucial to understand the composition of these chunks into larger spaces. In this talk, I review the basic kinds of effects that become relevant when boundaries are present, highlight which of these are already naturally incorporated within the GFT framework, and point out some possible extensions to it.

Speaker: Simon Langenscheidt

2_SimonLangenscheidtSpinsPres_2 Simon Langenscheidt.pdf

43 Quantum theory and the maximum compactness of stars.

Very compact stars seem to be forbidden in General Relativity. While Buchdahl's theorem sets an upper bound on compactness, further no-go results rely on the existence of two light rings, the inner of which is associated to gravitational instabilities. However, little is known about the role of QFT in these strong gravity regimes. We show that the renormalized stress tensor for CFTs diverges faster than the classical source as the star's surface approaches the Buchdahl radius rather than the Schwarzschild radius. The backreaction of quantum fields in this regime therefore cannot be ignored.

Speaker: Ignacio Reyes (University of Amsterdam)

Radboud_anomaly Ignacio Reyes.pdf

44 Everpresent Λ Cosmology

I will discuss some aspects of the Everpresent Λ cosmological model arising from fundamental principles in causal set theory and unimodular gravity. In this framework the value of the cosmological constant (Λ) fluctuates, in magnitude and in sign, over cosmic history. At each epoch, Λ stays statistically close to the inverse square root of the spacetime volume. Since the latter is of the order of H^2 today, this provides a way out of the cosmological constant puzzle without fine tuning. I will review the theoretical background of this idea. I will also describe a phenomenological implementation of this model, and discuss recent results on the statistics of its simulations and observational tests of it.

Speaker: Yasaman Yazdi (Imperial College London)

Yasaman_QGJuly Yasaman Yazdi.pdf

45 How to constrain the Universe anisotropies across a Big Bounce

We study the implementation of Polymer Quantum Mechanics (PQM) to a system decomposed into a quasi-classical background and a small quantum subsystem, according to the original Vilenkin proposal. We develop the whole formalism in the momentum representation that is the only viable in the continuum limit of the polymer paradigm and we generalize the fundamental equations of the original Vilenkin analysis in the considered context. Then, we provide a Minisuperspace application of the theory, first considering a Bianchi I cosmology and then extending the analysis to a Bianchi IX model in the limit of small anisotropies. In both these cases, the quasi-classical background is identified with an isotropic bouncing Universe whereas the small quantum subsystem contains the anisotropic degrees of freedom. When the Big Bounce scenario is considered, we obtain that in the Bianchi I model the anisotropies standard deviation is regular at $t=0$ but still increases indefinitely, whereas in the presence of the harmonic Bianchi IX potential such same quantity is bounded and oscillate around a constant value. As a consequence, we demonstrate that the picture of a semiclassical isotropic Bounce can be extended to more general cosmological settings if the spatial curvature becomes relevant when the anisotropic degrees of freedom are still small quantum variables.

Speaker: Eleonora Giovannetti (CPT Marseille)

QGNijmegen_Giovannetti eleonora giovannetti.pdf

46 The problem of time in quantum cosmology from different approaches

When canonically quantising general relativity, one is faced with the problem of frozen dynamics, also known as the problem of time, which arises due to the presence of constraints in the theory. In this talk, I will provide an overview of the problem of frozen dynamics in general relativity and explain why it is a significant challenge in the field. To study this problem, we have limited ourselves to highly symmetric spacetimes, such as cosmological toy models, which allow us to explore the these issues in a more tractable setting. The problem of time is usually solved by gauge fixing the constraint classically (reduced quantisation) or solving the constraint after quantising the theory, for example, by projecting from the too big kinematical Hilbert space to the physical Hilbert space using the constraint operator (Dirac programme). In these toy models, it has already been proven that ambiguites in the reduced quantisation procedure will lead to different quantum theories. In our work we have focused in solving the constraint in the quantum level and we have seen how ambiguities in the definition of the kinematical Hilbert space and the constraint operator may also lead to different physical Hilbert spaces. These ambiguities arise from the different boundary conditions resulting from considering unitary evolution of our universes. In this talk, I will recap the principal results of our work, explain their significance for the broader field of quantum gravity. Additionally, I will briefly introduce other approaches to the problem of time. In particular, I will focus in the main alternative to canonical quantisation, namely, path integral quantisation. I will introduce how the problem of time and unitarity conditions may be manifested in this approach to quantum gravity and discuss the advantages and limitations of this approach compared to other quantisation methods. Overall, my talk will provide a comprehensive overview of the problem of frozen dynamics in general relativity and the various approaches that have been developed to address it.

Speaker: Lucía Menéndez-Pidal (Complutense university)

LuciaMenendez-Pidalqg2023 Lucía Menéndez-Pidal.pdf

Parallel sessions: S6 - Young Researchers session

47 Alleviation of anomalies from the non-oscillatory vacuum in Loop Quantum Cosmology

It has been shown that Loop Quantum Cosmology (LQC) has the potential to alleviate anomalies related to large scale power suppression, the lensing amplitude and te parity asymmetry present in observations of the CMB. As a consequence of the pre-inflationary dynamics, some modes reach the onset of inflation in an excited state with respect to the Bunch-Davies vacuum, resulting in a scale dependence of the primordial power spectrum for large scales. The choice of vacuum state in the pre-inflationary regime and free parameters of the theory impact the concrete predictions. In this work we investigate the consequences of choosing the non-oscillatory vacuum of cosmological perturbations. We perform a Bayesian analysis of the hybrid LQC model, contrasting with observations from the CMB, so that we can obtain constraints on free parameters and find whether whether it is capable of alleviating these anomalies.

Speaker: Rita Neves (Universidad Complutense de Madrid)

QG23-RitaNeves Rita Neves.pdf

48 Axion wormhole leading to expanding baby universes

The existence of Euclidean wormholes contributing to the Euclidean path integral, and especially their (in)stability, is a long standing puzzle. It has implications both phenomenologically (by breaking the axion global shift symmetry, thus making it a possible dark matter candidate or large-field inflaton, and solving the strong CP-problem of QCD) and from a theoretical perspective, e.g. through the weak gravity conjecture. In this talk, I will describe a new type of Euclidean wormhole solutions in axion-gravity theories which lead to the nucleation of an expanding baby universe in Lorentzian time. This contrasts with the well-known Giddings-Strominger (GS) solutions that lead to a contracting baby universe in Lorentzian time. I will describe the key properties of these new solutions such as their action-to-charge ratio, and I will compare them to more standard GS-like wormhole solutions.

Speaker: Caroline Jonas (Albert Einstein Institute, Potsdam)

CarolineJonas Caroline Jonas.pdf

49 Asymptotic quantum correlations of field modes in time-dependent backgrounds

Studying quantum fields in strong gravity poses several fundamental questions, like the quantum-classical transition of primordial density perturbations in the early Universe and the information paradox. To understand these questions, in this talk, we consider a massive quantum scalar field in a time-dependent background and study various quantum correlation measures — entanglement entropy, quantum fidelity, Loschmidt echo, etc. The leading order behavior of these measures is found to be related via simple expressions at late times, and also serves as a diagnostic tool for instabilities in the system. We quantify such instabilities in terms of scrambling time and Lyapunov exponents and show that the system mimics classicality under certain conditions. We also show that the entropy scaling oscillates between the area-law and volume-law for a scalar field that undergoes a global quench. We then discuss its implications for the quantum-classical transition of scalar perturbations in the early Universe. [Refs: https://doi.org/10.1103/PhysRevD.107.025003]

Speaker: Sree Mahesh Chandran (Indian Institute of Technology Bombay)

SreeMaheshChandranQG2023 S Mahesh Chandran.pdf

50 Macroscopic Microscopic Effects from Quantum Gravity

Quantum Gravity induced corrections at the classical level are usually too small to be relevant for phenomena that can be probed by current observations. In this talk, I will focus on spherically symmetric solutions of Quadratic Gravity. I will discuss a mechanism that ensures that even if corrections to the Schwarzschild solution from General Relativity are small, they become dominant just outside the would-be event horizon - even if the corrections are Planck-sized to begin with. Confronting the Quadratic Gravity solutions, that are all sensitive for this mechanism, with observations from the Event Horizon Telescope allows to rule out a considerable portion of the phase space.

Speaker: Jesse Daas (Radboud University)

QGconference2023Talk_Jesse Jesse Daas.pdf

51 Gaussian states in group field theory and semiclassical properties

Approaches to quantum gravity describing spacetime as emergent, i.e., as the collective behaviour of some discrete fundamental structures, must allow for quantum states which manifest semiclassical features. In this talk I present a realisation of this idea in the context of group field theory (GFT), where we depart from the familiar coherent states by studying properties of the more general Gaussian states in simple cosmological models. We show that while these states do not minimise uncertainty inequalities, they have small quantum fluctuations. I will also discuss thermal states and the interpretation of thermodynamic equilibrium, and different notions of semiclassicality in two quantisation schemes in GFT.

Speaker: Andrea Calcinari (University of Sheffield)

GaussianStatesGFT_AndreaC Andrea Calcinari.pdf

52 A gauge for graviton fluctuations in the WKB approach

We will show how to construct the proper low-energy limit of QFT in curved spacetime from a canonically quantized gravity-matter system, discussing four emerging points of previous literature. To address the related shortcomings, we will consider the gravitational sector as composed of a classical background (i.e. a vacuum Bianchi I universe) and quantum fluctuations on top (gravitons) regarded as “slow” quantum components in a Born-Oppenheimer approximation, while the matter scalar field will have a “fast” quantum character. Via a WKB expansion of the full system, the quantum matter dynamics will recover a functional Schrödinger form after averaging over quantum gravitational effects, thanks to a gauge condition imposed on the gravitons’ wave functional, which is equivalent to the gravitational Wheeler-DeWitt constraint implemented a priori in previous approaches.

Speaker: Giulia Maniccia (Sapienza University of Rome, INFN Roma1)

Maniccia_TalkNijmegen Giulia Maniccia.pdf

53 Discussion block

15:30 Coffee/tea break

Parallel sessions: S7

54 Quantum Gravity in de Sitter Space

Quantum information techniques are playing a crucial role in our understanding of quantum aspects of spacetime. In particular for de Sitter space, I will illustrate this through two different examples. Firstly, I will show why momentum-space entanglement is an essential concept in early universe cosmology, as a quantifier for non-unitary evolution, and derive its conceptual ramifications for quantum corrections to cosmological observables. On the other hand, I will discuss why one needs to go beyond entanglement entropy and explore complexity for the Euclidean vacuum in de Sitter. Time permitting, I will briefly highlight why this quantum informatic measure is pivotal for establishing cosmic ER=EPR.

Speaker: Suddhasattwa Brahma (Higgs Centre, University of Edinburgh)

Brahma_QG2023_Nijmegen Suddhasattwa Brahma.pdf

55 Quantum Gravity in de Sitter Space

I discuss two striking new insights into the fundamentals of the Hartle-Hawking no-boundary wave function. First, I show that the Kontsevich–Segal criterion, applied to the complex saddles that specify the semiclassical no-boundary wave function, acts as a selection mechanism on inflationary scalar field potentials. This saddle criterion effectively bounds the tensor-to-scalar ratio of cosmic microwave background fluctuations to be less than 0.08, in line with current observations. Second, moving beyond the semiclassical approximation, I explain how quantum corrections to the entropy of four-dimensional de Sitter space can be computed using the AdS/CFT correspondence.

Speaker: Thomas Hertog (KU Leuven)

QGNijmegen_Hertog Thomas Hertog.pdf

56 Discussion Block

Parallel sessions: S8 - Young Researchers session

57 Young ISQG - Career Panel

During the career panel, the panelists

Maximilian Becker (Radboud University)
Flaminia Giacomini (ETH Zurich)
Luca Buoninfante (NORDITA)
Kasia Rejzner (University of York)

will share their experiences and insights on applying for jobs in and out of academia, specifically focused on young researchers doing QG. This session will include a Q&A where attendees can ask career related questions.

Speakers: Flaminia Giacomini (ETH Zurich), Kasia Rejzner, Luca Buoninfante (Nordita), Maximilian Becker (Radboud University, Nijmegen)

58 Young ISQG - Networking Session

This session offers a networking space opportunity specially designed for young researchers, allowing them to connect with like-minded peers, establish valuable contacts, and foster collaborations that may shape their future trajectory. The meeting will be conducted in the informal atmosphere of the cultuur cafe.

Thursday, 13 July

Registration: Registration and Information desk

59 Loop Quantum Gravity and Spin Foams | Review

This talk will provide an introduction to Loop Quantum Gravity and Spin Foam models. Loop Quantum Gravity is founded on a gauge-theoretic approach to the quantization of gravity. This approach leverages the huge diffeomorphism invariance of General Relativity to make the Wilson loops of gauge theory into practical probes of spacetime curvature and centers focus on the metrical areas of surfaces, rather than the usual lengths of curves. The simplest example of a loop quantization of geometry is the quantum tetrahedron. I will report on advances connecting classical and quantum tetrahedra to elliptic curves. In addition to the compelling connections with the quantization of geometry and gauge theories, recent years have seen substantial developments in the discrete geometry path integrals known as Spin Foam models. I will overview recent advances in the numerical treatment of these path integrals. This work is demonstrating how these path integrals can be used to address dynamical questions. I will emphasize the multifaceted approach of Loop Gravity, its connections to other approaches to quantum gravity, and some open questions.

Speaker: Hal Haggard

HaggardQG2023LoopQuantumGravityAndSpinFoams Hal Haggard.pdf

60 Loop Quantum Gravity and Spin Foams | Focus: Quantum Gravitational Dynamics: An Electric Shift in Perspective

Classical gravitational evolution in time admits an elegant re-expression in terms of of certain generalizations of Lie derivatives with respect to a triple of spatial vector fields called the Electric Shift. This property of classical evolution naturally
dovetails into a new construction of a quantum dynamics for Euclidean gravity within the canonical quantization approach known as Loop Quantum Gravity (LQG). In my talk I will discuss these new results, their import and avenues for further exploration.

Speaker: Madhavan Varadarajan

lqgrad1 Madhavan Varadarajan.pdf

10:30 Coffee/tea break

61 Holographic Descriptions of Spacetime | Review

Speaker: Jan de Boer

nijmegen0723 Jan de Boer.pdf

62 Holographic Descriptions of Spacetime | Focus: Exactly solvable lower-dimensional gravity models

I will discuss three interrelated gravitational models in 1+1 dimensions: Jackiw-Teitelboim (JT) gravity, double-scaled SYK, and Liouville gravity. In spite of their very distinct formulation, I will illustrate that their exact quantum amplitudes show a strong resemblence in terms of the underlying representation theory. This observation can be appreciated upon formulating each of them in terms of a dilaton gravity model, suggesting a modified version of holography for the different models. Based largely on arXiv:2212.07696 and arXiv:2306.00941.

Speaker: Thomas Mertens

PresQG2023ThomasMertens Thomas Mertens.pdf

63 Poster prize ceremony

12:40 Lunch

Parallel sessions: S10

64 Quantum Gravity meets Cosmology

The dawn of gravitational wave astronomy provided multiple channels for probing the physics at work in the very early universe as well as for testing gravity itself. They can be categorised into resolved and unresolved gravitational waves sources, the latter of which embodies a stochastically distribution of signals (similar to the CMB) called the stochastic gravitational wave background (SGWB). In this talk, I review particular ways in which Quantum Gravity (QG) will manifest itself in gravitational waveforms and the SGWB. I present analytic tools and features of the signals that can be utilised to enhance faint signatures of QG originating from the early universe and black hole mergers, and briefly talk about their applications in preliminary works.

Speaker: David Maibach (Heidelberg U.)

QG2023_Nijmegen_Maibach David Maibach.pdf

65 Remembrance of Things Past

I show how the coincidence limits of the massless, minimally coupled scalar propagator and its first two derivatives facilitate the summation of large logarithms from inflationary gravitons using a variant of Starobinsky's stochastic formalism. I derive analytic approximations for these correlators that are valid for any cosmology which has undergone an epoch of primordial inflation. A fascinating feature of these correlators is that they transmit the high scales of primordial inflation to arbitrarily late times. I demonstrate the method by explicitly evolving a nonlinear sigma model (which has the same derivative couplings as gravity) through primordial inflation to late times. This talk is based on arXiv:2305.17641, 2302.11528, 2302.04808 and 2110.08715.

Speaker: Richard Woodard (Department of Physics/University of Florida)

Woodard Richard Woodard.pdf

66 The Noise of Gravitons

When the gravitational field is treated quantum-mechanically, the classical trajectories of falling objects are subject to random fluctuations ("noise"). This fundamental noise might be observable at gravitational wave detectors and, if detected, would provide experimental evidence for the quantization of gravity. The noise also modifies the Raychaudhuri equation, thereby enhancing the formation of caustics.

Speaker: Maulik Parikh (Arizona State University)

The Noise of Gravitons - Maulik Parikh Maulik Parikh.pdf

67 Running relational observables in asymptotically safe gravity

We introduce an approach to compute the renormalization group flow of relational observables in quantum gravity which evolve from their microscopic expressions towards the full quantum expectation value. This can be achieved by using the composite operator formalism of the functional renormalization group. These methods can be applied to a large class of relational observables for different physical coordinate systems. As a first application we consider four scalar fields coupled to gravity to represent the physical coordinate frame from which relational observables can be constructed, such as the relational scalar curvature. We evaluate the scaling dimensions at the fixed point, both in the standard renormalization group scheme and in the essential scheme. This represents the first steps to describe running observables within Asymptotic Safety and allows access to universal critical exponents of the observables. As such the computation of these exponents can serve as a way to compare with different approaches to quantum gravity.

Speaker: Renata Ferrero (Johannes Gutenberg University Mainz )

FerreroNijmegen2023 Renata Ferrero.pdf

68 Discussion Block

Parallel sessions: S9

69 Quantum Black Holes: Covariance in effective models of quantum black holes

There are many proposals in the context of loop quantum gravity to effectively describe the main effects predicted by the theory by performing certain specific modifications in the Hamiltonian of general relativity. However, in general, such modifications produce anomalies in the constraint algebra, which imply a loss of covariance of the effective theory. In this work we focus on spherical symmetry and we present a family of effective Hamiltonians, which indeed form a closed constraint algebra. In this way, the corresponding metric tensor can be constructed in a covariant way, so that different gauge choices in phase space simply correspond to different spacetime coordinates. This model is then used to study in detai spherical black holes (with charge and cosmological constant) and, in particular, to analyze the possible resolution of the classical singularity.

Speaker: David Brizuela (University of the Basque Country)

brizuela David Brizuela.pdf

70 Quantum Black Holes: Unitarity and quantum resolution of gravitational singularities

I will summarise some recent work illustrating a fundamental clash between requiring unitarity in quantum theories of gravity and the notion of general covariance: unitarity means well-defined evolution for arbitrarily long time, but as different notions of time may remain finite or infinite, e.g., on approach to a cosmological singularity, this may or may not require significant departures from classical evolution in the quantum theory. I will mostly focus on cosmological models but also discuss other scenarios, such as the black hole interior. I will also comment on how different approaches to quantization might offer different views on this question.

Speaker: Steffen Gielen (University of Sheffield)

QG2023 Steffen Gielen.pdf

71 Quantum Black Holes: Quantum gravity lessons from black hole thermodynamics

Black hole thermodynamics lies at the crossroads of quantum gravity, information theory, and relativistic field theory, and provides fundamental clues about the microscopic degrees of freedom of gravity. In this talk, I will discuss the insights we have gained from our attempts to understand black holes as thermodynamic systems. I describe the phase structure of black hole spacetimes, and the role of the Euclidean path integral in their study. I further discuss the dual CFT description, and show how important non-perturbative effects can be understood by studying various modifications to the bulk phase structure in the context of gauge-gravity duality. I close with some comments regarding the so-called black hole information paradox, and suggest avenues for future exploration.

Speaker: Fil Simovic (Macquarie University)

Simovic QG 2023 Fil Simovic.pdf

72 Quantum Black Holes: Regular black holes and the first law of black hole mechanics

A complete theory of quantum gravity is generally expected to cure the singularities inherent to the mathematical black holes predicted by general relativity. In the absence of such a theory, singularity-free models of so-called regular black holes have become a popular alternative to avoid the nontrivial causal structures typically associated with mathematical black holes. In this talk, we derive a generic condition that spherically symmetric dynamical regular black holes must satisfy to be compatible with the first law of black hole mechanics and investigate the thermodynamic consequences of the singularity resolution. We examine the dynamical generalizations of models typically considered in the literature, and demonstrate that none of them satisfies the necessary condition required to be consistent with the first law. [Based on https://arxiv.org/abs/2304.05421]

Speaker: Sebastian Murk (Okinawa Institute of Science and Technology (OIST))

QG23-96-230713-1440-Murk Sebastian Murk.pdf

73 Discussion Block

15:30 Coffee/tea break

Parallel sessions: S11

74 Quantum geometry and entanglement from corner symmetry

In this talk, I will show that In gravity, the subregion entanglement is controlled by a symmetry group called the corner symmetry group, which follows from invariance under diffeomorphism of the total space. This universal symmetry group gives us semi-classical phase space tools to understand quantum geometry in the continuum. It also constrains the entanglement structure of subregions. I will describe how this approach, called local holography, provides a bottom-up approach to quantum gravity where finding the corner symmetry group representations amount to quantizing geometry. If time permits, I will show that dynamics along null surfaces and asymptotic dynamics can be understood as charge conservation for a Carrollian fluid.

Speaker: Laurent Freidel (Perimeter Institute)

Niejemen-Short Laurent Freidel.pdf

75 Partition function for a volume of space

We consider the quantum gravity partition function that counts the dimension of the Hilbert space of a spatial region with topology of a ball and fixed proper volume, and evaluate it in the leading order saddle point approximation. The result is the exponential of the Bekenstein-Hawking entropy associated with the area of the saddle ball boundary, and is reliable within effective field theory provided the mild curvature singularity at the ball boundary is regulated by higher curvature terms. This generalizes the classic Gibbons-Hawking computation of the de Sitter entropy for the case of positive cosmological constant and unconstrained volume, and hence exhibits the holographic nature of nonperturbative quantum gravity in generic finite volumes of space.

Speaker: Manus Visser (University of Cambridge)

Manus Visser - Quantum Gravity 2023 Manus Visser.pdf

76 Multipartite Entanglement of Random Spin Networks in Quantum Gravity

Quantum spin networks are a phenomenal quantitative tool to investigate the operational content of the gravitational field at the quantum scale. In this light, spin network entanglement has recently become a central resource to characterize dynamical signatures and holographic behaviour of quantum spacetime geometry, providing a new tool to study the emergence of classical spacetime geometry from its quantum description. We review a series of recent results focusing on the notion of random spin network intended as a proxy of a complex quantum many-body description of quantum geometry concerning the emergence of holographic behaviour in a regime of quantum typicality. For such states, we discuss the notion of quantum negativity as a good witness of multipartite entanglement in an open-quantum state description of bounded 3D-space regions.

Speaker: Goffredo Chirco (University of Naples Federico II)

chirco-qg23nijmegen Chirco.pdf

77 Discussion Block

Parallel sessions: S12

78 Quantum reference frames: a relational perspective on nonclassical spacetime

Understanding the fundamental nature of gravity at the interface with quantum theory is a major open question in theoretical physics. Recently, the study of gravitating quantum systems, namely quantum systems sourcing a gravitational field and interacting gravitationally, has attracted a lot of attention, thanks to the possibility of realising these scenarios in the laboratory in the near future. When a quantum system sources a gravitational field, spacetime is fundamentally nonclassical. In this situation, also the notion of a reference frame needs to be generalised to account for quantum features of spacetime, thus realising a quantum reference frame (QRF).

In this talk, I will introduce a formalism to associate a QRF to the quantum state of a particle, and to transform between different QRFs that are in a quantum relationship with each other. I will argue that QRFs can contribute to formulate physics on nonclassical spacetime, and highlight some promising future directions.

Speaker: Flaminia Giacomini (ETH Zurich)

QG2023_Giacomini Flaminia Giacomini.pdf

79 Observables in Quantum Gravity: UV Effects Mixed into the IR: Observational Signatures of Quantum Gravity

Traditionally observational signatures of quantum gravity (QG) have been considered far out of reach of observation. We take another look at this question, and discuss how UV/IR mixing, in theoretically controlled contexts (such as fluid gravity near horizons and shockwave backgrounds), could give rise to the appearance of the IR scale in QG effects in experiments.

Speaker: Kathryn Zurek (Caltech)

23_QG_Zurek Kathryn Zurek (1).pdf

80 Understanding Ryu-Takayangi as entropy without invoking holography

In appropriate semiclassical limits, the so-called island formula computes the entropy of non-gravitational quantum systems entangled with a gravitational theory. This is a special case in which the quantum-corrected Ryu-Takayanagi formula has been shown to compute a von Neumann entropy using only properties of the gravitational path integral and, in particular, without relying on the existence of a holographic dual field theory. It is thus natural to claim that a similar conclusion should hold more broadly, and that any asymptotically-AdS gravitational theory will define an algebra for any boundary region such that, in appropriate limits, the entropy of any state on that algebra is computed by the quantum-corrected Ryu-Takayanagi formula. Recent works by Chandrasekaran, Pennington and Witten have used the theory of von Neumann algebras to derive results of this form in various special contexts. We argue here that the above claim holds more generally, whenever the Euclidean path integral of the gravitational theory satisfies a set of standard axioms. We thus allow finite values of all coupling constants and do not require taking any special limits. Since our axioms do not restrict ultra-violet bulk structures, they may be expected to hold equally well for successful formulations of string field theory, spin-foam models, or any other approach to constructing a UV-complete theory.

Speaker: Eugenia Colafranceschi (University of California, Santa Barbara)

talk Eugenia Colafranceschi Eugenia Colafranceschi.pdf

81 Discussion Block

19:00 Social Dinner

Friday, 14 July

Registration: Registration and Information desk

82 Causal Structure in Quantum Gravity | Review: Causal Structure in Quantum Gravity

Classical spacetimes have Lorentzian metrics, and the resulting
light cone structure is crucial to a broad range of results,
from singularity theorems to the propagation of gravitational
waves. In quantum gravity, though, we expect quantum
fluctuations to smear light cones, blurring the causal
structure. I will briefly review some of the problems -- and
some of the potential benefits -- presented by this blurring,
and discuss a few efforts, such as the discrete causal set
approach, to address causal structure in quantum gravity.

Speaker: Steve Carlip

QG23_Carlip Steve Carlip.pdf

83 Causal Structure in Quantum Gravity | Focus: The search for a UV fixed point in four-dimensional CDT

The phase diagram of four-dimensional CDT hints the existence of a UV fixed point which could in principle connect this lattice theory to the asymptotic safety scenario seen by using the functional renormalization group. The present status and future perspectives will be discussed.

Speaker: Jan Ambjørn

ambjorn-QG2023 Jan Ambjørn.pdf

10:30 Coffee/tea break

84 Field Theory on and of Spacetime | Review

Speaker: Kasia Rejzner

QG2023-v2 Kasia Rejzner.pdf

85 Field Theory on and of Spacetime | Focus: Modern (discrete) 3rd quantization of gravity and emergent cosmology

I recall the old idea of 3rd quantization of gravity, generalizing the gravitational path integral to a sum over topologies, and proceed to illustrate how one could try to make mathematical sense of it, going discrete. I emphasize how this forces to adopt a different perspective, in line with emergent spacetime scenarios. I then introduce tensorial group field theories (TGFTs) for quantum gravity. They are a higher-dimensional generalization of random matrix models (and closely related to lattice quantum gravity and canonical loop quantum gravity), which gave the first concrete realization of the idea of 3rd quantization. Finally, I report on recent results showing the emergence of bouncing cosmologies with late-time cosmic acceleration, from the mean field approximation of quantum geometric TGFT models.

Speaker: Daniele Oriti

talk-QG2023 Daniele Oriti (1).pdf

12:30 Lunch

Parallel sessions: S13

86 The problem of time in quantum cosmology with a quantum clock

We revisit the problem of time in the context of quantum cosmology. By considering more than one scalar matter field propagating on an FLRW cosmology, we identify several Dirac observables from which two evolving constants of motion (ECM) are constructed. One of these ECMs will play the role of the clock and against which the other ECM is measured. We quantize both of these ECMs which amounts to quantizing both the observable and the clock. We then present the consequences of such a quantum model, and in particular, the implications of quantization of the clock.

Speaker: Saeed Rastgoo (University of Alberta)

Saeed Rastgoo - QG 2023 Saeed Rastgoo.pdf

87 Towards the phase structure of first order Lorentzian Palatini gravity via Group field theory

The Barrett-Crane (BC) spin foam and GFT model is a state-sum model which provides a tentative quantization of first order Lorentzian Palatini gravity written as a constrained BF-theory. It is conjectured that this model gives rise to continuum spacetime with General Relativity as an effective description for the dynamics at criticality via phase transition. In this talk, we discuss how phase transitions in this model can be studied using Landau-Ginzburg mean-field theory. In a first step, we demonstrate this by restricting the building blocks of the model such that the Feynman diagrams are dual to spacelike triangulations. This setting lays the groundwork to study the critical behavior when arbitrary Lorentzian building blocks are incorporated and also paves the way for the analysis of the phase structure of the complete model via functional renormalization group techniques in future research. This work is based on arXiv:2112.00091, arXiv:2206.15442, arXiv:2209.04297 and arXiv:2211.12768.

Speaker: Andreas Pithis (LMU Munich)

Presentation Pithis Andreas Pithis.pdf

88 Scalar cosmological perturbations from full quantum gravity

Extracting the physics of cosmological inhomogeneities and anisotropies from full quantum gravity is a crucial step to make contact with observations. I address this problem within the group field theory (GFT) formalism for quantum gravity by studying the perturbative mean-field effective dynamics of small relational inhomogeneities of GFT condensates. I show how these perturbations give rise to local volume and matter relational inhomogeneities and compare their dynamics with that of scalar cosmological perturbations from general relativity, highlighting similarities and differences that may lead to observational consequences.

Speaker: Luca Marchetti (University of New Brunswick)

23_07_14_marchetti Luca Marchetti.pdf

89 General Relativistic Decoherence with Applications to Dark Matter

Quantum mechanics allows for states in macroscopic superpositions, but they ordinarily undergo rapid decoherence due to interactions with their environment. A system that only interacts gravitationally, such as an arrangement of dark matter (DM), may exhibit slow decoherence. In this work, we compute the decoherence rate of a quantum object within general relativity; a robust and rigorous quantum general relativistic result. We present several applications, include galactic dark matter, superpositions of phase oscillations, Boson stars, etc. This is based on Phys. Rev. Lett. 127, 031301 (2021), Phys. Rev. D 103, 104053 (2021), and JCAP 07 (2020) 056.

Speaker: Mark Hertzberg (Tufts Institute of Cosmology)

TalkQG2023c Mark Hertzberg.pdf

Parallel sessions: S14

90 Tensor eigenvalue distributions through field theoretical methods

Tensor models provide an interesting direction in the pursuit of quantum gravity. Understanding their dynamics would be an important part of the pursuit, which could be partly captured by distributions of tensor eigenvalues in tensor models, similarly to Wigner's semi-circle law in matrix models. We show how the distributions can be computed by field theoretical tools, and discuss some possible extensions of the procedure.

Speaker: Naoki Sasakura (Yukawa Institute for Theoretical Physics, Kyoto University)

NaokiSasakura NaokiSasakura.pdf

91 Spinor fields, CPTM symmetry and smallness of cosmological constant in framework of extended manifold

A model of an extended manifold for the Dirac spinor field is considered. Two Lagrangians related by CPTM (charge-parity-time-mass) symmetry are constructed for a pair of the Dirac spinor fields with each spinor field defined in a separate manifold. An interaction between the matter fields in the manifolds is introduced through gravity. A fermionic effective action of the general system is constructed and a tadpole one-loop spinor diagram and part of the one-loop vacuum diagrams with two external gravitational off-shell fields which contribute to the effective action are calculated. It is demonstrated that among different versions of the second spinor Lagrangian there is a special one for which a cancellation of the mentioned diagrams in the total effective action takes place. As a result, the diagrams do not contribute to the cosmological constant, as well there is a zero contribution of the zero point energies of the spinor fields to the action. The non-zero leading order value of the cosmological constant for each manifold in the framework is proportional to the trace of an momentum-energy tensor of each separated manifold or difference of manifolds tensors, depending on the chosen model of interaction of gravitational fields with fermions. An appearance of the dark matter in the model is shortly discussed as well as further applications of the approach.

Speaker: Sergey Bondarenko (Ariel University, Israel)

HollandCCSmallness.pdf

92 Invariant observables in quantum gravity and corrections to 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 Newtonian gravitational potential of a point particle using these observables. The one-loop results 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, and references therein.

Speaker: Markus B. Fröb (ITP, Universität Leipzig, Germany)

talk-froeb Markus Fröb (1).pdf

93 Second law from the Noether current on null hypersurfaces

I study the balance law equation of surface charges in the presence of background fields. The construction allows a unified description of Noether's theorem for both global and local symmetries. From the balance law associated with some of these symmetries, I will discuss generalizations of Wald's Noether entropy formula and general entropy balance laws on null hypersurfaces based on the null energy conditions, interpreted as an entropy creation term. The entropy is generally the so-called improved Noether charge, a quantity that has recently been investigated by many authors, associated to null future-pointing diffeomorphisms. These local and dynamical definitions of entropy on the black hole horizon differ from the Bekenstein-Hawking entropy through terms proportional to the first derivative of the area along the null geodesics. Two different definitions of the dynamical entropy are identified, deduced from gravity symplectic potentials providing a suitable notion of gravitational flux which vanish on non-expanding horizons. The first one is proposed as a definition of the entropy for dynamical black holes by Wald and Zhang, and it satisfies the physical process first law locally. The second one vanishes on any cross section of Minkowski's light cone. I study general properties of its balance law. In particular, I look at first order perturbations around a non expanding horizon. Furthermore, I show that the dynamical entropy increases on the event horizon formed by a spherical symmetric collapse between the two stationary states of vanishing flux, i.e the initial flat light cone and the final stationary black hole. I compare this process to a phase transition, in which the symmetry group of the stationary black hole phase is enlarged by the supertranslations.

Speaker: Antoine Rignon-Bret (Aix-Marseille Université)

Presentation_Nijmegen_Rignon_Bret Antoine RIGNON-BRET.pdf

Parallel sessions: S15

94 Vacuum energy, the Casimir effect, and Newton's non-constant

We explore two hypotheses. First, the possibility that the quantum vacuum energy density of the Casimir effect contributes to a (local) gravitational vacuum energy density. Second, the possibility that a change in the gravitational coupling implies a change in the cosmological constant. We parametrize these two possibilities in a covariant framework and show that the next generation of Casimir experiments have a surprisingly good chance of exploring this parameter space.

Speaker: Benjamin Koch (TU-Wien ITP und Atominstitut)

CasimirAndVacuum-Nijmegen-2023 Benjamin Koch.pdf

95 Loop quantization as a continuum limit: challenges for quantum gravity

In 2005 an article with a similar title by Manrique et al presented an implementation of Wilson’s renormalization group and continuum limit tailored to loop quantization together with a nontrivial example leading to an interacting relativistic quantum field theory. I will review that proposal, compare with other proposals, give new examples and critically examine the application of this framework to quantum gravity.

Speaker: Jose A. Zapata (Universidad Nacional Autónoma de México)

LQasAcontLimQG2023 Jose Zapata.pdf

96 Testing The Cosmological Principle In A Quantum Universe

There are not many tools to quantitatively monitor the emergence of classical geometric features from a quantum spacetime, whose microscopic structure may be a highly quantum-fluctuating “spacetime foam”. To improve this situation,we introduce new quantum observables that allow us to measure the absolute and relative homogeneity and isotropy of geometric properties of a nonperturbative quantum universe, as function of a chosen averaging scale. This opens a new way to compare results obtained in full quantum gravity to descriptions of the early universe that assume homogeneity and isotropy at the outset. Our construction is purely geometric and does not depend on a background metric. We illustrate the viability of the new observables by a nontrivial application to two-dimensional Lorentzian quantum gravity formulated in terms of a path integral over Causal Dynamical Triangulations, and find some evidence of quantum inhomogeneity and anisotropy. This paves the way for future test of these observables in the emergent universe of 4 dimensional CDT, where current results indicate that a quasi-De Sitter spacetime emerges from full quantum gravity with no a priori symmetry assumptions.

Speaker: Agustín Silva (Radboud University)

QG2023___A__Silva - Final Version Agustín Silva.pdf

97 Quantum computations in Loop Quantum Gravity

One of the possible applications of quantum computers in the near future are quantum simulations of physical systems. In Loop Quantum Gravity the quantum geometry of space is represented by superposition of the so-called spin networks. A construction of quantum circuits that generate states of the Ising-type spin networks is described. The results of the implementation of the approach on the IBM superconducting quantum computers are presented. Moreover, some theoretical quantum information properties of spin networks are described.

Speaker: Grzegorz Czelusta (Jagiellonian University)

Czelusta Grzegorz Czelusta.pdf

98 Closing Discussion: Round Table

15:40 Coffee/tea