Conference talks – Probing the spacetime fabric: from concepts to phenomenology

You can now download the presentations from this shared folder. The folder will be updated with the remaining presentations as soon as they are provieded by the speakers.

INVITED SPEAKERS

Niayesh Afshordi, University of Waterloo/Perimeter Institute

Title: (Astro-)Physical Quantum Gravity

Abstract: I plan to discuss how modern astrophysical observations can transform theories of quantum gravity from fantasy into physics. I will provide two examples: First, why cosmic acceleration may be linked to the fuzzball structure of black horizons, and how it can be probed by gravitational wave and galaxy surveys. Second, how pulsar timing observations severely constrain the UV structure of our fundamental theories.
Emanuele Alesci, SISSA

Title: The Emergent Bouncing Universe

Abstract: Quantum Reduced Loop Gravity (a gauge fixed version of LQG) allows to describe the cosmological models using more structure of the full theory respect LQC i.e. the standard cosmological implementation of LQG. In particular the kinematical Hilbert space can be characterized by density matrices controlling the quantum superposition over the graphs that identify homogeneous Universes. Ordinary LQC results can be derived using particular choices of density matrices. However general density matrices motivated by statistical arguments lead to new striking features for Friedmann: the Big Bounce scenario of LQC is replaced by the Emergent Bouncing Universe. Our classical Universe emerges with consecutive bounces followed by a last Big Bounce from a metastable quantum phase in which the Universe is confined in a scale of order 100 Planck volumes. In this region gravitational attractive forces are dynamically competing with pure quantum gravity repulsive forces.
Giovanni Amelino-Camelia, Sapienza, Università di Roma

Title: Probing spacetime structure with GRB photons and neutrinos

Abstract: Probing spacetime structure with GRB photons and neutrinos.
Markus Aspelmeyer, University of Vienna

Title: Quantum Optomechanics – new opportunities for gravity (quantum) experiments

Abstract: tba
Carlo Baccigalupi, SISSA

Title: Cosmic Gravitational Waves and Microwave Background Polarization

Abstract: We review the cosmological sources of B-modes in CMB anisotropies, focusing on the imprint of cosmological GWs. We outline the present status of measurements, challenges in the analysis of data, ongoing and planned observations till the end of the next decade.
Carlos Barceló, Andalusian Institute of Astrophysics, Granada

Title: Probing gravity in stellar collapse

Abstract: The standard way to understand quantum corrected black holes leads to the information loss paradox and the lifetime dilemma. A radical way out of this situation is to give up a hypothesis which is tacitly assumed in the vast majority of works on the subject: that the classical singularity is substituted by something effectively acting as a sink for a long period of time, as seen by asymptotic observers. Eliminating this characteristic changes drastically much of the physics now associated to black holes. A nice feature of the new hypothesis it that it offers a clear possibility of experimental falsifiability with upcoming gravitational waves observations. In this talk I will discuss these possibilities.
Aurélien Barrau, Laboratoire de Physique Subatomique et de Cosmologie, Grenoble

Title: Loop Quantum Cosmology

Abstract: I will review some aspects of the background dynamics and perturbations in loop quantum cosmology. I will also briefly discuss some issues related to back holes.
Alessio Belenchia, University of Vienna

Title: Probing Spacetime Non-Locality with Quantum Systems

Abstract: In this talk I will discuss several instances of non-locality phenomenology by way of quantum (non)relativistic systems. In particular, I will briefly introduce the concept of non-locality — which stands for kinetic terms containing infinitely many space and time derivatives — through the example of causal set theory. In doing so, the class of non-local field theories will be divided in two broad sub-classes, based on the analytical properties of the kinetic terms. In the main part of the talk I will illustrate how, exploiting opto-mechanical oscillators and Unruh-DeWitt particle detectors, it is possible (at least in principle) to cast stringent constraints on the non-locality scale, i.e., the free parameter of the models under consideration.
Dionigi Benincasa, SISSA

Title: An Overview of Causal Set Theory

Abstract: Causal sets form the basis for a sum-over-histories approach to quantum gravity. They were discovered independently by ’t Hooft, Myrheim, Sorkin and Finklestein in the 70s and 80s, and have been championed by R. Sorkin ever since. In the past decade there have been important developments both in the understanding of quantum field theory on (fixed, background) causal sets as well as defining the dynamics of causal sets themselves via an Einstein-Hilbert like action. After an introduction to the subject, I will review these advances and, when relevant, discuss some of their phenomenological implications.
N. Emil Bjerrum-Bohr, University of Copenhagen (NBIA)

Title: Scattering in Quantum Gravity

Abstract: We discuss how general relativity can be quantised as an effective field theory. We consider scattering of light-like matter in this theory.
Robert Brandenberger, McGill University, Montréal

Title: String Gas Cosmology: An Update

Abstract: I will review the current status of String Gas Cosmology.
Caslav Brukner, University of Vienna

Title: Quantum theory with no global causal order

Asbtract: One of the most deeply rooted concepts in science is causality: the idea that events in the present are caused by events in the past and, in turn, act as causes for what happens in the future. If an event A is a cause of an effect B, then B cannot be a cause of A. The possible interplay between quantum theory and general relativity may, however, require superseding such a paradigm. I will review the framework of “process matrices”, which allows describing “superpositions of causal order”, where one cannot say that A is before or after B. The framework reduces to the standard quantum formalism whenever the causal order is fixed. I will show that indefinite causal structures offer advantage in communication and computation, and discuss their realization in the gravitational field of a massive object in a spatial superposition.
David Edward Bruschi, York Centre for Quantum Technologies

Title: Bose-Einstein Condensates as spacetime probes

Abstract: Quantum experiments are reaching relativistic regimes. Quantum communication protocols have been demonstrated at long lengths scales and experiments are underway to distribute entanglement between Earth and Satellite-based links. At these regimes the Global Positioning System requires relativistic corrections. Therefore, it is necessary to understand how motion and gravity will affect long-range quantum experiments. Interestingly, relativistic effects can also be observed at small lengths scales. Some effects have been demonstrated in superconducting circuits involving boundary conditions moving at relativistic speeds and quantum clocks have been used to measure time dilation in table-top experiments. In this talk we will present a formalism for the study of gravitational effects on quantum technologies. This formalism is also applicable to the development of new quantum technologies that can be used to deepen our understanding of physics in the overlap of quantum theory and relativity. Examples include accelerometers, gravitational wave detectors and spacetime probes underpinned by quantum field theory in curved spacetime.
Sean Carroll, California Institute of Technology

Title: Emergence of the Universe from the Wave Function

Abstract: Quantum mechanics, at least in its Everettian incarnation, is a theory of vectors in Hilbert space evolving smoothly through time. Often we construct quantum theories by starting from a set of classical variables, including space itself, and quantizing them, but presumably Nature doesn’t work that way. I will discuss some tentative progress toward the goal of seeing how space itself can emerge from the quantum wave function, and how that can be connected to quantum gravity.
Richard Dawid, University of Stockholm

Title: Non-Empirical Confirmation in Fundamental Physics

Abstract: In recent decades, non-empirical arguments have assumed an increasingly important though not entirely uncontroversial role in assessing a theory’s status. This is true in the case of string theory, which has acquired a high degree of trust in the absence of empirical confirmation. It is also true in the context of cosmology in cases where data in support of a theory is sometimes complemented by strong non-empirical arguments. The talk will analyze the structure of these arguments. It will be argued that non-empirical confirmation is closely related to lines of reasoning that are applied when assessing the significance of empirical confirmation.
Sebastian De Haro, University of Cambridge

Title: The Emergence of Space, Illustrated by Random Matrix Models

Abstract: There are two sorts of issues about emergence of spacetime. First, most notions of emergence are informal. Second, there is a lack of good examples illustrating emergence of spacetime explicitly. In this talk, I will report new work towards resolving these two issues. My main example will be random matrix models, where the emergence of a Riemann surface from an algebraic structure can be explicitly studied. I will comment on how this example bears on attempts to give more precise accounts of the notion of emergence.
Bianca Dittrich, Perimeter Institute

Title: Bootstrapping quantum gravity

Abstract: I will present the consistent boundary framework and show how it leads to a bootstrap approach for the construction of consistent quantum gravity amplitudes in a truncation scheme. Here the truncation is informed by the dynamics of the system. This feature also ensures that the scheme corresponds to an expansion around a physically interesting state that can be identified with the physical vacuum state.

I will explain how this helps to identify the most useful variables and boundary states for the formulation of a quantum gravity dynamics.
Astrid Eichhorn, University of Heidelberg

Title: Matter and gravity in the asymptotic safety paradigm

Asbtract: I will discuss the asymptotic safety paradigm and will highlight recent advances towards understanding whether an ultraviolet complete model of gravity and Standard-Model matter can be constructed within this paradigm. I will also explore how the inclusion of matter in the study of quantum spacetime could allow us to forge a link between physics at the Planck scale and physics at the electroweak scale, thus imposing observational constraints on microscopic quantum gravity models.
Daniele Faccio, Heriot-Watt University, Edinburgh

Title: Rotating Black Holes in Fluids of Light

Abstract: We will overview recent experimental work aimed at creating artificial spacetimes in 2D superfluids made of light. By controlling the flow of the superfluid we can create a vortex structure with an inward radial flow. Measurements allow to distinguish between the ergosphere and horizon. We are currently studying the scattering of waves with angular momentum, searching for features of Penrose superradiance from a rotating spacetime with quantised angular momentum.
Uwe R. Fischer, Seoul National University

Title: Experimental Quantum Cosmology: Probing the Inflationary Scale Invariance in Ultracold Dipolar Gases

Abstract: We argue that an example of the kinematical phenomena of quantum fields in curved spacetime, which analogue gravity can explore, is furnished by a hallmark signature of inflationary cosmology, the scale invariance of the power spectrum of inflaton field correlations. It is demonstrated that an analogue de Sitter cosmos in an expanding quasi-two-dimensional Bose-Einstein condensate, with dominant dipole-dipole interactions between the atoms or molecules in the ultracold gas, allows to investigate whether excitation spectra deviating from Lorentz invariance at trans-Planckian momenta violate scale invariance. It is demonstrated that scale invariance experiences strong modifications when at the initial stage of expansion the spectrum displays a roton minimum. Dipolar quantum gases thus furnish a viable laboratory tool to experimentally access, with well-defined and controllable initial conditions, the question whether primordial oscillation spectra deviating from Lorentz invariance at trans-Planckian momenta violate a standard prediction of inflationary cosmology.
Steven Giddings, UC Santa Barbara

Title: Nonviolent unitarization: postulates to soft quantum structure of black holes

Abstract: tba
Sabine Hossenfelder, Frankfurt Institute for Advanced Studies

Title: Yet another model for modified gravity

Abstract: I discuss Erik Verlinde’s recently proposed emergent gravity and its prospects for explaining dark matter and dark energy. In particular, I will construct a covariant Lagrangian that reproduces Verlinde’s ansatz and show what additional insights this brings.
Nick Huggett, University of Illinois at Chicago

Title: The Edges of the Universe

Abstract: One typically conceptualizes the breakdown of an effective description as occurring at a time, place, or scale — in spatiotemporal terms. But news ways of thinking are required when classical spacetime itself is the effective entity, for when it breaks down there may be no times, places, or scales. I will discuss the issue in the context of string theory models of black holes and big bang.
Vincent Lam, The University of Queensland

Title: Functionalist perspective on quantum gravity

Abstract: Many research programs in quantum gravity seem to suggest a radical picture according to which crucial spatio-temporal features (and possibly even space and time themselves) are not part of the fundamental physical ontology. This may raise two related families of conceptual worries. The first concerns the very possibility of empirical evidence in a context without spacetime and the precise sense in which spacetime quantities can emerge from a fundamental non-spatio-temporal ontology. The second type of conceptual worries has to do with the very characterization of this possibly non-spatio-temporal physical ontology. In particular, it should be clarified what makes the non-spatio-temporal entities described by quantum gravity concrete physical entities, rather than merely abstract mathematical ones. The two standard criteria for concreteness, which rely on spacetime and causation respectively, do not seem appropriate in this context. However, they together suggest a mixed strategy that combine certain aspects of the two criteria: to focus on the spacetime functions—that is, on the spatio-temporal or ‘spacetime-like’ roles, in some broad functional rather than causal sense—the quantum gravity entities may instantiate in certain circumstances. This talk aims to evaluate to what extent such a functionalist perspective can help to alleviate the conceptual worries mentioned above.
Renate Loll, Radboud University Nijmegen

Title: Quantum Ricci Curvature

Abstract: Curvature provides a very important way of understanding the properties of classical spacetime. However, in the context of nonperturbative quantum gravity, the realization of meaningful quantum curvature observables in a Planckian regime has not received much attention. I will report about new work to define and operationally construct “quantum Ricci curvature”, inspired by a classical notion of sphere distance. The proposal has been tested on a variety of piecewise flat geometries and in two-dimensional, dynamically triangulated quantum gravity, so far with promising results.
Francesco Marino, CNR – Istituto Nazionale di Ottica

Title: Optomechanical tests of quantum-gravity-induced nonlocality

Abstract: Several quantum gravity scenarios lead to physics below the Planck scale characterized by non-local, Lorentz invariant equations of motion. In the non-relativistic limit such effective field theories lead to a non-local generalization of the Schrödinger equation. In this framework, we discuss a series of opto-mechanical experiments based on macroscopic quantum oscillators, aimed at constraining non-locality scales and possibly detect signatures of non-local effects.
Mercedes Martín-Benito, Instituto de Astrofísica e Ciências do Espaço, Universidade de Lisboa

Title: The quantum echo of the early Universe

Abstract: In this talk I will discuss the transmission of information and correlations through quantum fields in cosmological backgrounds. With this aim, I will make use of quantum information tools to quantify the classical and quantum correlations induced by a quantum massless scalar field in particle detectors. I will first identify a signature of quantum gravitational effects that survives from the early universe to the current era: Fluctuations of quantum fields as seen by comoving observers are significantly influenced by the history of the early universe. In particular I will show how the existence (or not) of a quantum bounce leaves a trace in the background quantum noise that is not damped and would be non-negligible even nowadays. Secondly, I will focus on the violation of the strong Huygens principle in cosmology, to show that much more information reaches us through timelike channels than it is carried by the massless quanta.
Eduardo Martin-Martinez, University of Waterloo

Title: Engineering negative stress-energy densities with quantum energy teleportation

Abstract: We show how to use quantum energy teleportation in the light-matter interaction as an operational means to create quantum field states that violate energy conditions and have negative local stress-energy densities. We show that the protocol is optimal in the sense that it scales in a way that saturates the quantum interest conjecture and violates energy conditions maximally. We will briefly discuss the backreaction of this protocol on spacetime curvature, and the gravitational properties of the negative energy density created with this process.
David Mattingly, University of New Hampshire

Title: Is there a (realistic) future for quantum gravity phenomenology?

Abstract: tba
Daniele Oriti, Max Planck Institute for Gravitational Physics (Albert Einstein Institute)

Title: The microstructure of an emergent spacetime in the group field theory formalism

Abstract: I introduce the group field theory formalism for quantum gravity and I summarize recent results on the emergence of spacetime in this framework.
Renaud Parentani, Laboratoire de Physique Théorique d’Orsay

Title: Black hole thermodynamics in Lorentz-violating theories

Abstract: Black hole thermodynamical are still poorly understood in Lorentz-violating theories. One early problem (identified by Jacobson in 2001) comes from a lack of predictability. This problem (essentially) disappears in some theories of modified gravity, such as Horava and Einstein-Aether theories, because spherically-symmetric black holes possess a second inner horizon which separates field configurations into two disconnected classes. This then raises the question of which of the two horizons governs the temperature of the Hawking radiation. To answer this question, we compute the late time radiation emitted by a dispersive field propagating in such background. We fix the initial conditions by considering a regular collapsing geometry, and imposing that the state inside the infalling shell is vacuum. We find that the mode pasting across the shell is adiabatic at late time (large inside frequencies). This implies that large black holes emit a thermal flux with a temperature fixed by the surface gravity of the Killing horizon. In turn, this suggests that the new inner horizon should play no role in the thermodynamical properties of these black holes.
Tsvi Piran, The Hebrew University of Jerusalem

Title: Obervationa limits on Lorentz Invariance Violation

Abstract: I will review limits from GRBs and AGN as well as from neutrino observations on Lorentz Invariance Violations at high energies.
Daniele Pranzetti, SISSA

Title: Generalized GFT condensates and horizon entropy

Abstract: I present the construction, in the group field theory formalism, of a generalized class of quantum gravity condensate states, that allows the description of continuum homogeneous quantum geometries within the full theory. They are defined by an infinite superposition of graph-based states encoding in a precise way the topology of the spatial manifold. The construction is based on the definition of refinement operators on spin network states, written in a second quantized language. I apply the construction to the case of a continuum spherically symmetric quantum geometry of an horizon and I show how the associated reduced density matrix manifestly exhibits an holographic behavior, reproducing the Bekenstein-Hawking formula.
Carlo Rovelli, Centre de Physique Theorique de Luminy, Marseille

Title: A possibly observable quantum gravity phenomenon: Planck Stars

Asbtract: Black holes are stable in classical GR, evaporate very slowly in QFT with backreaction on the classical geometry, but can also explode suddenly in quantum gravity, leading to a potentially observable phenomenon, characterized by a very peculiar observational signature. I review the state of theory and observations on this phenomenon.
Mairi Sakellariadou, King’s College London

Title: Quantum Gravity and Cosmology

Asbtract: tba
Francesco Salamida, Università degli Studi dell’Aquila

Title: Highlights from the Pierre Auger Observatory

Abstract: The Pierre Auger Observatory, based in Mendoza province, Argentina is the largest ultrahigh-energy cosmic ray Observatory ever built. After more than ten years of operation the exposure reached almost 60000 km2 sr yr and the unprecedented quality data set spans three orders of magnitude.
Chris Smeenk, Western University Canada

Title: Underdetermination in the Early Universe

Asbtract: In the history of physics, available data has often been sufficient to justify a theory as correct, at least within some domain. Yet in many areas of current fundamental physics, our lack of access to the most revealing regimes implies that experiments and observations may not provide useful guidance. Here I will briefly defend a response to this situation that contrasts with Dawid’s recent defence of non-empirical confirmation. I concur with Dawid that limits to underdetermination are an essential part of justifying theories. But I have a different assessment of how physicists have successfully responded to underdetermination in the past, and what this implies for current physics. I will discuss early universe cosmology to illustrate these contrasting approaches.
P. Kyle Stanford, University of California, Irvine

Title: But What if we Can’t Confirm Theories of Quantum Gravity? A Plea for Modest Instrumentalism

Abstract: Regarding many scientific theories across many natural domains, it seems perfectly reasonable to adopt the so-called ‘scientific realist’ view that their impressive empirical and practical achievements show that at least the most central and fundamental claims of those theories represent accurate descriptions of how things really stand in otherwise inaccessible domains of nature. But for theories of quantum gravity it seems we are in a position to assess only their comparative practical or pragmatic theoretical virtues (e.g. simplicity) and not their epistemic virtues (e.g. empirical adequacy). It therefore does not seem plausible or attractive to adopt such a realist attitude towards theories of quantum gravity, rather than the competing ‘instrumentalist’ view that these theories are instead simply powerful cognitive tools or instruments for achieving our goals. But here I argue that the leading form of such instrumentalism familiar from the philosophical literature, Bas van Fraassen’s ‘Constructive Empiricism’, is simply not responsive to the sorts of reasons we have for resisting realism in the case of theories of quantum gravity and it therefore seems no more plausible or attractive an epistemic attitude to take towards such theories than is realism itself. I will go on to sketch a competing form of such instrumentalism and argue that it represents an attractive and plausible epistemic attitude concerning theories of quantum gravity.
Carlo A. Trugenberger, SwissScientific

Title: Emergence of topologically ordered geometric space and matter from random qubits

Abstract: I will describe a quantum gravity model in which geometric space emerges from random bits in a quantum phase transition driven by the combinatorial Ollivier-Ricci curvature and corresponding to the condensation of short cycles in random graphs. This quantum critical point defines quantum gravity non-perturbatively. In the ordered geometric phase at large distances the action reduces to the standard Einstein-Hilbert term. I will then show that this emergent geometric space can be viewed as as a topologically ordered quantum entanglement pattern with quantum dimension 32 in four Euclidean space dimensions. “Quasi-particle” excitations are thus organized in a 32-dimensional representation. A 16 “Weyl” spinor of SO(10) is the unique representation in which exactly all matter content of one standard model generation fits. While parity violation is not accounted for in this simple model, this suggests the idea that all matter particles are made by local entanglement patterns.
Matteo Viel, SISSA

Title: Our Universe Large Scale Structure as a Fundamental Physical Probe

Abstract: I will review the state-of-the-art of the use of cosmic structures to address fundamental physical questions as: variation of fundamental constants, the nature of dark matter, modifications of gravity, dark energy evolution, possible extensions of the standard cosmological model, measurements of neutrino mass. The presentation will focus on current and future observational efforts that could provide breakthrough discoveries in this field.
Matt Visser, Victoria University of Wellington

Title: Emergent gravity

Abstract: The word “emergence” means many different things to different people. I will present a general overview of “emergent gravity” indicating what is possible, what is plausible, and what avenues seem worth exploring.
Silke Weinfurtner, The University of Nottingham

Title: Observation of superradiance in a vortex flow

Abstract: Wave scattering phenomena are ubiquitous to almost all Sciences, from Biology to Physics. When an incident wave scatters off of an obstacle, it is partially reflected and partially transmitted. Since the scatterer absorbs part of the incident energy, the reflected wave carries less energy than the incident one. However, if the obstacle is rotating, this process can be reversed and waves can be amplified, extracting energy from the scatterer. Even though this phenomenon, known as superradiance, has been thoroughly analysed in several theoretical scenarios (from eletromagnetic radiation scattering on a rotating cylinder to gravitational waves incident upon a rotating black hole), it has never been observed. Here we describe in detail the first laboratory detection of superradiance. We observed that plane waves propagating on the surface of water are amplified after being scattered by a draining vortex. The maximum amplification measured in the experiment was 20%, obtained for 3.70 Hz waves, in a 6.25 cm deep fluid. Our results are consistent with superradiant scattering caused by rapid rotation. In particular, a draining fluid can transfer part of its rotational energy to incident low-frequency waves. Our experimental findings will shed new light on Black Hole Physics, since shallow water waves scattering on a draining fluid constitute an analogue of a black hole. We believe, especially in view of the recent observations of gravitational waves, that our results will motivate further research (both theoretical and experimental) on the observation of superradiance of gravitational waves.

BRIEF TALKS

Raúl Carballo-Rubio, Department of Mathematics & Applied Mathematics, University of Cape Town

Title: Horizonless alternatives to black holes

Abstract: The detection of gravitational waves has revived interest in theoretical scenarios in which black holes are replaced by horizonless ultra-compact configurations. While the theoretical status of these hypothetical objects is yet unclear, at the same time they remain virtually unconstrained by observations. In this talk I will provide additional details regarding these assertions, and discuss possible ways to improve our current physical understanding of these alternatives.
Esteban Castro, University of Vienna

Title: Entanglement of quantum clocks through gravity

Abstract: In general relativity, the picture of space–time assigns an ideal clock to each world line. Being ideal, gravitational effects due to these clocks are ignored and the flow of time according to one clock is not affected by the presence of clocks along nearby world lines. However, if time is defined operationally, as a pointer position of a physical clock that obeys the principles of general relativity and quantum mechanics, such a picture is, at most, a convenient fiction. Specifically, we show that the general relativistic mass–energy equivalence implies gravitational interaction between the clocks, whereas the quantum mechanical superposition of energy eigenstates leads to a nonfixed metric background. Based only on the assumption that both principles hold in this situation, we show that the clocks necessarily get entangled through time dilation effect, which eventually leads to a loss of coherence of a single clock. Hence, the time as measured by a single clock is not well defined. However, the general relativistic notion of time is recovered in the classical limit of clocks.
Goffredo Chirco, Max Planck Institute for Gravitational Physics (Albert Einstein Institute)

Title: Group field theory and tensor networks: holographic entanglement entropy in full quantum gravity

Abstract: We investigate the use of tensor networks techniques and quantum information theory for the study of the entanglement properties of quantum gravity states. In the language of the group field theory, we describe spin network states as quantum many-body systems. The very connectivity of such states, encoded in the links of the underlying graphs, is associated with the entanglement between the fundamental quanta constituting them. We establish a dictionary between group field theory states and (generalised) random tensor networks. With such a dictionary at hand, we target the calculation of the Ryu-Takayanagi formula for the entanglement entropy in the full quantum gravity formalism of group field theory.
Marco de Cesare, King’s College London

Title: Cosmological implications of the group field theory approach to quantum gravity

Abstract: I will discuss the impact of quantum gravity effects for cosmology, considering the emergent spacetime scenario based on the group field theory framework. Considering some simple models, I will show that there are significant departures from the standard picture for the history of our Universe, both at early and late times. In particular, I will show how it is possible to achieve a bounce and an early epoch of accelerated expansion in this approach.
Flaminia Giacomini, University of Vienna

Title: Quantum mechanics in quantum reference frames

Abstract: In all our known theories, our description of the physical world always relies on the existence of a classical, ideal reference frame. If we abandon this idealised view and consider a reference frame as a physical system, we need to take into account the dynamical degrees of freedom of the reference frame. Here, we address the question of describing physics within quantum reference frames. By quantum reference frames we mean a physical system showing quantum properties such as superposition and entanglement. We describe how it is possible to change perspective from the classical external reference frame to a quantum reference frame, we show how the quantum state transforms, and we address the measurement in quantum reference frames. Moreover, we consider the dynamical laws and derive the Schrödinger equation in quantum reference frames. In particular, we analyse the situation in which the new reference frame is in a superposition of velocities from the point of view of the old one, and show that the transformation corresponding to a “superposition of Galilean boosts” preserves the covariance of the Schrödinger equation.
Lisa Glaser, Radboud University Nijmegen

Title: The spectrum of geometries

Abstract: A large part of the information about non-commutative geometries is contained in the spectrum of the Dirac operator. The question is then, how can we recover useful information from this spectrum? I will show some promising results on this for the ensemble of random non-commutative geometries, using the spectral dimension and the zeta function.
Daniel Grimmer, University of Waterloo

Title: A model of open dynamics for relativistic quantum fields

Abstract: The dynamics of open quantum systems, i.e., of systems interacting with an environment, forms the basis of numerous active areas of research. Open dynamics is central to foundational questions such as the quantum measurement problem and gravitational decoherence. This is particularly relevant in the interaction of quantum fields with atoms. We will analyze the entropy flows in the emergent open dynamics of a particle detector undergoing rapid repeated interactions with a quantum electromagnetic field. We show that there are strong constraints on the ability of the interaction to allow for entropy flows from the detector to the quantum field. We will also show how a detector repeatedly interacting with a fully relativistic quantum field can display a range of thermodynamic behaviors such as thermalization, purification, and dephasing. We will discuss the impact of our results in some of the most common setups in Relativistic Quantum Information. We will pay special attention to the usage of the Gaussian formalism in the context of information flows between particle detectors and fields.
Aaron Held, Institute for Theoretical Physics, Heidelberg University

Title: Top mass from asymptotic safety

Abstract: Asymptotically safe quantum gravity could predict the top-quark-mass. For a broad range of microscopic gravitational couplings, quantum gravity could provide an ultraviolet completion for the Standard Model by triggering asymptotic freedom in the gauge couplings and bottom-Yukawa and asymptotic safety in the top-Yukawa and Higgs-quartic coupling. We find that in a part of this range, a difference of the top-mass and bottom-mass of approximately 170 GeV is generated and the Higgs mass is determined in terms of the top mass. Assuming no new physics below the Planck scale, we construct explicit Renormalization Group trajectories for Standard Model and gravitational couplings which link the transplanckian regime to the electroweak scale and yield a top mass of M_t ≈ 178 GeV.
Philipp Hoehn, Institute for Quantum Optics and Quantum Information, Vienna

Title: Lorentz transformations from quantum communication

Abstract: In most approaches to fundamental physics, spacetime symmetries are postulated a priori and then explicitly implemented in the theory. This includes Lorentz covariance in quantum field theory and diffeomorphism invariance in quantum gravity, which are seen as fundamental principles to which the final theory has to be adjusted. In this talk, we suggest, within a much simpler setting, that this kind of reasoning can actually be reversed, by taking an operational approach inspired by quantum information theory. We consider observers in distinct laboratories, with local physics described by the laws of abstract quantum theory, and without presupposing a particular spacetime structure. We ask what information-theoretic effort the observers have to spend to synchronize their descriptions of local physics. If there are “enough” observables that can be measured universally on several different quantum systems, we show that the observers’ descriptions are related by an element of the orthochronous Lorentz group O^+(3,1), together with a global scaling factor. We explain how this result embeds into an informational perspective on spacetime and may contribute to emergent spacetime approaches.
Alexander Kegeles, Max Planck Institute for Gravitational Physics (Albert Einstein Institute)

Title: Inequivalent representations of Group Field Theory

Abstract: Group field theory is a field theoretical formulation of spin networks and simplicial geometry, in which the states are associated with excitations of basic geometrical degrees of freedom over a vacuum of the theory. The simplest type of vacuum is the state of “No geometry”. However, that is not the only possible vacuum and there are many more. In my talk I will present an algebraic formulation of Group Field Theory in which the study of different vacua of the theory can be rigorously addressed and studied. I then show the existence and explicit examples of different inequivalent vacua, based on coherent states of group field theory.
Maria Papageorgiou, Institute for Quantum Computing, University of Waterloo

Title: On the implementation of a covariant ultraviolet cutoff

Abstact: General relativity describes spacetime as a smooth manifold, while quantum field theory is built with ultraviolet divergences that suggest an ultraviolet cutoff, presumably at the Planck scale. A cutoff in the form of a minimum length cannot be consistent with Lorentz symmetry. We will discuss a model for a covariant cutoff that respects the Lorentz symmetry of QFT’s, and can be naturally generalized for curved spacetimes.
Dennis Rätzel, University of Vienna

Title: Gravitational Properties of Light

Abstract: As Einstein’s equations tell us that all energy is a source of gravity, light must gravitate. However, because changes of the gravitational field propagate with the speed of light, the gravitational effect of light differs significantly from that of massive objects. In particular, the gravitational force induced by a laser pulse is due only to its creation and annihilation and decays with the inverse of the distance to the pulse. We can expect the gravitational field of light to be extremely weak. However, the properties of light are premises in the foundations of modern physics: they were used to derive special and general relativity and are the basis of the concept of time and causality in many alternative models. Studying the back-reaction of light on the gravitational field could give new fundamental insights to our understanding of space and time as well as classical and quantum gravity. A brief overview is given of the gravitational field of laser pulses in the framework of linearized Einstein gravity. A glimpse is caught of the gravitational interaction of two single photons, which turns out to depend on the degree of their polarization entanglement.
Allison Sachs, Institute for Quantum Computing, University of Waterloo

Title: Divergenceless methods to quantify vacuum correlations in quadratically coupled fields

Abstract: The vacuum state of a quantum field possesses correlations, both classical and quantum, between spacelike separated regions . By reading out these correlations, we can gather information about the structure of spacetime . Additionally, vacuum correlations can, in principle, be used as a resource for quantum communication and other quantum information tasks. Past works have studied this phenomenon, called entanglement harvesting , in the case of detectors coupling linearly to a bosonic field; e.g. two atoms coupled to the electromagnetic field. We present new divergence-free methods to study correlations harvested from quadratically coupled fields to a particle detectors (such as Unruh-DeWitt). These methods become relevant in the study of vacuum entanglement of fermionic fields and interacting bosonic theories. For example, the entanglement structure of the fermionic vacuum has not yet been studied in detail. The chief reason is that we lacked an adequate divergence-free equivalent to the Unruh-DeWitt particle detector model for fermionic fields. We expect that these studies will shed light on the nature of fermionic field vacuum entanglement, which displays distinctive features not present in the bosonic case as observed in the study of the Unruh effect.
Sehar Sahebdiavn, University of Vienna

Title: Non-classical Features of Gravity in a Double Slit Experiment

Abstract: In classical gravity, as described by general relativity or its Newtonian limit, the gravitational field of a single body cannot be in a superposition. As long as we deal with macroscopic masses, there is no place for a conflict with quantum mechanics. However, in the case of quantum rigid objects, like a mega-molecule, in which their centre of mass can be in a spatial superposition state, a paradox arises. The paradox originates from the question whether gravitational fields corresponding to the centre of the mass can also be in superposition. If gravity has an underlying quantum nature, it should possess other quantum characteristics like superposition and entanglement. Despite the weakness of gravity there is a chance, in principle, to observe these features without direct observation of the graviton. An intermediate theoretical model and experimental protocol are needed, in the low energy regime to address this question. We propose a theoretical model based on modified Schrödinger-Newton equation and an experimental protocol in the form of gravitational double slit setup to verify the collective behaviour of the system of two gravitationally interacting quantum rigid bodies.
Mehdi Saravani, University of Nottingham

Title: Nonlocal field theory: revisited

Abstract: I will introduce a class of nonlocal field theories that are mostly overlooked in the literature and describe why they might be the key to solve one of the outstanding problems in theoretical physics: dark matter.
Petar Simidzija, University of Waterloo

Title: Information carrying capacity of cosmological constant

Abstract: A question of fundamental importance in Relativistic Quantum Information asks what is the information carrying capacity of a quantum field. Previous studies have discussed that in curved spacetimes and in particular in cosmology, information flows decouple from energy flows. Remarkably, these studies show that information can travel in massless fields between timelike separated observers. In other words, the propagation of light signals leaves a timelike echo in the entire future lightcone of the emitter. In this setting we can imagine ourselves to be Bob, a late observer in an expanding de Sitter Universe attempting to retrieve information that was imprinted in a massless quantum field by an early time emitter Alice in our timelike past. We will show that, contrary to intuition, the faster the exponential expansion of the Universe the more information Alice can send Bob through a timelike communication channel via the quantum field. Even more remarkably, we will see that the channel capacity does not decay with Alice and Bob’s spatial or temporal separations, in contrast with both the capacity of conventional lightlike channels (suggesting that it is possible to gather information about the early Universe from timelike signals with much greater efficiency than by pointing our telescopes to distant light signals) and with the timelike channel capacity in slower, polynomially-expanding cosmologies. In this talk, we will try to convince you that if we wait a billion years before reading Alice’s message, we could (in principle) recover the same amount of information as if we read the message today.
Manuele Tettamanti, Università degli Studi dell’Insubria

Title: Hawking radiation in BECs: an exactly solvable model

Abstract: Bose-Einstein condensates (BECs) are one of the most promising experimental setups used in the field of analogue gravity in order to detect the Hawking effect. Exploiting a peculiar state of hard core bosons in 1D (i.e. the Tonks-Girardeau gas) we are able to obtain the exact solution of a BEC flowing against an obstacle and we examine it in the framework of sonic black holes; in this limit we recover Hawking result without making use of the gravitational analogy and we find that a precise correspondence between the emission of phonons in the upstream region and the Hawking-like mechanism requires additional conditions to be met. Finally we study the correlations between the Hawking quanta and the in-falling partner, recovering the expected pattern.

POSTERS

Philippe Allard Guérin, University of Vienna

Title: Frames of reference in causally indefinite quantum processes

Abstract: Quantum mechanics allows for situation in which the causal order between events is not well-defined. For example, in the so-called quantum switch, a qubit controls the orders “Alice before Bob”, and “Bob before Alice”. We introduce a notion of change of causal reference frames, that allows to describe a non-causal quantum process from the point of view of one of the parties. We also apply this notion of reference frame to more exotic processes, which violate causal inequalities. This allows us to extract qualitative differences between such processes and the quantum switch.
Rory Conboye, American University, Washington DC

Title: Approximating smooth continuum-limit geometry in Regge calculus

Abstract: The piecewise flat geometry of Regge calculus has played an important role in a number of approaches to quantizing space-time. However, deficit angles and the Regge equations are not directly comparable with smooth concepts of curvature. Here, new piecewise flat curvature constructions are presented, obtained by using a similar approach to smooth curvature. Evidence has shown these constructions to converge to the correct smooth curvatures for sequences of piecewise flat manifolds limiting to smooth, and give good approximations even for course triangulations of smooth manifolds. These constructions should provide a convenient tool for investigating any emergent smooth behaviour of piecewise flat geometries obtained as classical limits of quantum space-times.
Adrià Delhom I Latorre, Universitat de Valencia – IFIC

Title: Is spacetime Riemannian? Accelerator constraints on non-metricity

Abstract: Non-metricity is a geometric property that plays an important role in metric-affine theories of gravity but which has received little attention in applications involving fermionic fields. In this talk, I will introduce the Dirac Lagrangian that must be used in a non-Riemannian background and will then show that contact 4-fermion interactions arise setting a lower energy scale in which metric-affine theories of gravity are compatible with accelerator experiments.
Michele Doronzo, University of Insubria

Title: The Hopfield-Kerr model and analogue black hole radiation in dielectrics

Abstract: We propose a model for the study of the Hawking effect in the presence of high-frequency dispersion. In particular, we are interested in describing a black hole analog induced by a refractive index perturbation propagating in a dielectric medium. The model we adopt is a non-linear version of the covariant Hopfield model. We show that, in this framework, the presence of an Hawking-like effect can be inferred, where the emission temperature is found to be proportional to the surface gravity of the analog black hole.
Florian Gerhardt, Max Planck Institute for Gravitational Physics (Albert Einstein Institute)

Title: Long wavelength cosmological perturbations in GFT within the separate universe approach

Abstract: One way to describe the emergence of the universe is to picture spacetime as a condensate of fundamental building blocks. The many-body states of those fundamental building blocks can be given by a QFT on a group manifold. This approach is called Group Field Theory (GFT). The Friedmann equations were calculated for a simple condensate Ansatz for an isotropic and homogeneous universe. In this poster, we show how to make use of those Friedmann equations to gain insights into, at least, long wavelength cosmological perturbations in a straightforward manner using the separate universe approach.
Colin MacLaurin, University of Queensland

Title: Time slicings of black holes

Abstract: Before studying quantum fields on curved spacetime, much less full quantum gravity, we need to be clear on the 3+1-splitting. Many descriptions of Schwarzschild-Droste spacetime rely on only the *static* foliation, but this restriction to a single choice runs the risk of
“Newtonian” misconceptions. We compare the spacetime slicing due to a family of timelike observers, all free-falling radially with the same “energy per mass” *e* (that is, the invariant for geodesic motion derived
from the usual timelike Killing vector field). This corresponds to a slice of constant “time” in generalised versions of Gullstrand-Painleve and Lemaitre coordinates.

Specifically, the usual (static) radial proper distance (1-2M/r)^{-1/2}dr becomes 1/|e| dr under the new slicing, and the 3-volume follows trivially. The curvature of 3-space is depicted by a funnel (Flamm’s paraboloid) in the usual slicing, but this becomes a cone. Test particles take infinite time to cross the horizon according to Schwarzschild *t*, however an
observer at infinity determines only finite time under the alternate simultaneity convention. The “swapping of space and time” interpretation is
also coordinate-dependent. My presentation is conceptual and pedagogical. In future I hope to study the dependence of quantum vacuum states and Hawking radiation on these 3+1-splittings.
Gustavo Pazzini de Brito, Centro Brasileiro de Pesquisas Físicas

Title: Renormalizability, Unitarity and the Interparticle Potencial Energy in Higher-Derivative Models of Quantum Gravity

Abstract: In this work we consider an extensive classification regarding renormalizability and unitarity of higher-derivative models of quantum gravity. It is verified that perturbative renormalizability and tree-level unitarity cannot be conciliated. In addition, we discuss an interesting connection of the aforementioned features with the behavior of the interparticle potential energy for small distances.

Guillaume Thiam, Max Placnk Institute for Gravitational Physics (Albert Einstein Institute)

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Jessica Santiago Silva, Victoria University of Wellington

Title: Vaidya-like model for Hawking radiation

Abstract: We construct a Vaidya-like model for Hawking radiation by patching together, at a certain radius R(t), an exterior radiating Vaidya spacetime with an interior infalling Vaidya spacetime. In this model we impose a positive energy flux going to future null infinity. However, instead of backtracking these photons all the way back to the horizon, we consider the possibility that they were actually created at a certain finite radius outside the black hole. To complete the physical picture, the interior Vaidya region represents a negative energy flux falling into the black hole.
Sebastian Schuster, Victoria University of Wellington

Title: Fully covariant formulation of electrodynamics of continuous media on a general space-time

Abstract: Based on a version of the Bel decomposition for the electrodynamic constitutive tensor in non-vacuum media, we present a fully covariant formulation of the electrodynamics of continuous media on a general space-time. While partially covariant formulations have been known for a long time, these presentations usually involved explicit regression to 3+1 dimensions, explicit lists of components, or the introduction of new indices for the field excitations. We demonstrate that this is not necessary, and that a fully covariant formalism is achievable. We also show how to look for possible effective metrics within such a general continuous medium, and show what is gained from this “reductionist” formalism.