Recordings of our Seminars and Other Events

2024

Abstract: 

After an introduction to black hole evaporation, I will review that Hawking's calculation of particle production is based on the semi-classical limit of a fixed metric. This approximation can break down after a finite time as the black hole evolves due to back-reaction. Therefore, I shall argue that two far-reaching questions remain to be answered:
(1) How long is the semi-classical description valid?
(2) What happens after a potential breakdown?
Attempting to answer them, I will subsequently present simple analogue systems, which share important quantum properties of a black hole, such as its entropy. Numerical non-perturbative time evolution reveals indications that (1) the semi-classical description can break down long before half of the mass is lost and that (2) evaporation slows down drastically at this point. As a particular consequence, small primordial black holes can both become viable dark matter candidates and provide a unique window on quantum gravity.

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In semiclassical gravity, lower bounds of the smeared stress-energy tensor, so-called quantum energy inequalities, allow to conclude generalized singularity theorems a la Penrose and Hawking and exclude exotic spacetime configurations like wormholes and warp drives. In this regard, they provide a powerful stability criterion selecting physically reasonable geometries sourced by quantum matter. However, to treat self-interacting theories has been largely unsuccessful so far. In this talk, I will present numerical and analytical results in this direction, focussing on integrable QFT models in 1+1 dimension, in particular the O(N)-nonlinear-sigma and sinh-Gordon model.

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In this seminar I will bring into contact the Renormalization Group and the Asymptotic Safety scenario for quantum gravity and the problem of observables in gravity.

Geometrical information of the quantum spacetime which we try to observe is crucially carried by physical fields. Observables cannot be properties of the “quantum spacetime” alone, but must also depend on the experimental setup that is used in order to observe or probe them.

After giving an introduction about Asymptotic Safety, I will introduce an approach to compute the RG flow of relational observables which evolve from their microscopic expressions towards the full quantum expectation value.

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In this talk, I will start by motivating the introduction of new- (quantum) physics effects in black hole spacetimes through a principled-parameterized approach. I will then discuss how these new-physics effects can affect the formation of non-singular black holes by applying the principled-parameterized approach to gravitational collapse. This will motivate to study non-singular black hole parameterizations beyond circularity. Finally, I will present how quantum gravity effects linked to these guiding principles can be traced back to observational imprints in synthetic (ng)EHT images, in particular in photon rings.

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Description: 

A panel discussion with Alejandro Perez and Leopoldo A. Pando Zayas representing two different views (LQG vs. AdS/CFT and string theory) on black hole entropy. 

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Celestial Holography is a proposal for applying the holographic principle to asymptotically flat spacetimes which conjectures that a quantum theory of gravity in asymptotically flat backgrounds should be dual to a codimension two Celestial Conformal Field Theory living in the celestial sphere at null infinity. This proposal emerged from the observation that the bulk S-matrix written in a basis of boost eigenstates behaves in many ways as a two-dimensional conformal correlator. In particular, soft theorems allow for the identification of certain soft modes in the bulk theory as two-dimensional currents and a candidate stress tensor in the boundary theory. In this talk I will review the motivation for Celestial Holography that emerged from soft symmetries, and present some of its features. Finally I will review some recent developments suggesting that Celestial Holography naturally emerges from AdS/CFT in the flat space limit of the latter. This perspective may provide new tools for the understanding of Celestial Holography by the application of the wide range of results from holography in asymptotically AdS spacetimes.

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Several approaches to quantum gravity have area variables as fundamental degrees of freedom. Motivated by this, ‘effective spin foam models’ are defined as geometrical path integrals for quantum gravity based on discrete area variables. These models have simple amplitudes while maintaining the universal features of quantum geometry and thus allow for fast computations, they are the fastest to date.  

The study of quantum gravitational dynamics reveals a very rich semi classical regime as an interplay between the parameters of the model. The study also provides a resolution of `flatness problem’ appearing in asymptotic analysis of spin foam models. These results are very promising in the hope of emergence of general relativity from spin foam models. I will discuss these developments in the talk.

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Twistor theory was introduced in the mid-1960s as an approach to combining quantum theory with space-time structure. A driving force behind the introduction of Twistor Theory was to combine the quantum-field theoretic requirement of positive frequency with the structure of space-time. In order to achieve this, the notion of twistor space was introduced to codify the structure of space-time in a way which related it to the splitting of the twistor space into two halves, one representing positive frequency, and the other representing negative frequency. Standard twistor theory involves a complex projective 3-space PT which naturally divides into two halves PT+ and PT– , joined by their common 5-real-dimensional boundary PN. The points of the space PN represent light rays in Minkowski space-time. However, this splitting has two quite different basic physical interpretations, namely positive/negative helicity and positive/negative frequency, which ought not to be confused in the formalism, and the notion of “bi-twistors” is introduced to resolve this issue. It is found that quantized bi-twistors have a previously unnoticed G2* structure, which enables the split octonion algebra to be directly formulated in terms of quantized bi-twistors, once the appropriate complex structure is incorporated. 

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Covariant Loop Quantum Gravity is a tentative, but complete, quantum theory of gravity. I review its conceptual structure and the equations that define it. I discuss the numerous open issues in the theory.  I illustrate its current tentative applications in cosmology and black hole physics. 

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Researchers in quantum gravity look for a theoretical and quantitative understanding of the dynamics of spacetime near the ultrashort Planck scale, which is believed to be governed by large quantum fluctuations and therefore cannot be described by perturbation theory about solutions to the classical Einstein equations. A powerful nonperturbative methodology is that of "random geometry", the study of continuum limits of statistical ensembles of piecewise flat simplicial manifolds consisting of equilateral triangular building blocks. While systems of two-dimensional random geometry in Riemannian signature provide analytically soluble toy models and illustrate the viability of the method, physical quantum gravity requires an implementation in four dimensions and Lorentzian signature.

The formulation of Causal Dynamical Triangulations (CDT), which builds on the principles of random geometry, is a quantum-gravitational analogue of what lattice QCD is to nonabelian gauge theory. It has allowed us to move away from formal considerations in quantum gravity to extracting quantitative results on the spectra of invariant quantum observables, describing physics near the Planck scale. A breakthrough result of CDT quantum gravity in four dimensions is the emergence, from first principles, of a nontrivial, nonperturbative vacuum state with properties of a de Sitter universe. I will summarize these findings, highlighting the unusual character of observables in quantum gravity, and describe some of the interesting physical questions that are being explored using the new notion of quantum Ricci curvature.

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I introduce the group field theory formalism for quantum gravity, in its basic features as well as in its links with other quantum gravity approaches. Then, I survey a few research directions and recent results, focusing in particular on continuum limit and renormalization, and on the extraction of effective cosmological dynamics from the fundamental quantum dynamics.

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String Field Theory is Batalin-Vilkovisky (BV) by construction. I will motivate and describe this construction, starting with the relativistic point particle, which leads to BV-quantum field theory, and then describe the challenges of extending this to string theory. 

No prior knowledge of String Theory is required. 

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Starting from a simple and modern introduction to the main features of string theory, we will then focus on the many ways in which it can be used to tackle the long-standing problem of formulating a theory of quantum gravity. Specific attention will be dedicated to the most recent developments of the so-called Swampland Program, which aims at highlighting the features of quantum field theories that can be consistently coupled to quantum gravity in the ultraviolet regime.

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In this talk, I will present the status of the asymptotic safety program for quantum gravity. I will review part of the results that give significant evidence for an ultraviolet fixed point in quantum gravity, both for pure gravity and gravity-matter systems. I will also discuss some open challenges in the field.

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Asymptotically safe quantum gravity might provide a unified description of the fundamental dynamics of quantum gravity and matter. The realization of asymptotic safety, i.e., of scale symmetry at high energies, constraints the possible interactions and dynamics of a system. In this talk, I will confront asymptotically safe quantum gravity with several phenomenological consistency tests. These consistency tests are inspired, for example by the observation of light fermions at low energies, or the presence of a non-trivial Abelian gauge sector. I will present indications that asymptotically safe quantum gravity might pass these consistency tests and that the interplay of quantum gravity and matter might impose a lower bound on the number of fermions in our universe, or even constrain fundamental parameters of our universe. Finally, I will highlight how non-dynamical scalar fields can be used to investigate whether lattice methods, in particular Euclidean dynamical triangulations, are a suitable tool to investigate asymptotic safety.

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One of the open questions in theoretical physics is whether or not there is a quantum theory of gravity. Standard perturbative renormalization techniques require an infinite number of counterterms, demanding that an infinite number of experiments fix that. On the other hand, an asymptotically-safe theory of quantum gravity could render a non-perturbative renormalization of general relativity, restoring the predictive power at higher energies. The asymptotic-safety community has been finding indications for that. In this talk, I will introduce the concept of asymptotic safety in quantum gravity by exploring its machinery and landscape, focusing on its predictive power and the interplay between gravity and matter.

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Physical effects like Hawking radiation, as well as more conceptual questions like the strong cosmic censorship conjecture, are motivations to study quantum field theory on curved spacetimes. The appropriate framework to do so is algebraic quantum field theory. I will present some basic ideas of this formalism, and explain why in this framework choosing a state for a concrete problem can be very difficult. Finally, I will give an outlook on how this difficulty can be overcome in the example of a Kerr-de Sitter spacetime. 

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Fixed, non-dynamical backgrounds, on which QFTs may be defined, come with their own causal structure; and while it is an axiom that spacelike separated observables of any QFT commute, it is still a nontrivial question in what sense causality is incorporated in quantum field theory. Indeed, as an old protocol by Sorkin demonstrates, a priori perfectly reasonable local quantum operations (e.g, non-selective measurements) might still allow for superluminally signal. In this talk, I will present a class of physically reasonable local and fully causal quantum operations for QFTs on fixed backgrounds. If time permits, I will indicate how they can be used to perform causal measurements of local observables.

This talk is based on joint work with H. Bostelmann and C. J. Fewster.

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General relativity admits singular solutions, where the predictability of evolution of physical degrees of freedom is expected to break down. This notion is challenged by presenting classical solutions that satisfy the existence and uniqueness theorems of differential equations at the big bang singularity, thereby surviving through it. This is presented for the class of homogeneous anisotropic cosmological models, namely the Bianchi IX universe, coupled to stiff matter sources such as scalar fields, using the ADM formalism of general relativity. The talk focuses on developing the prerequisites for this work, in particular the 3+1 decomposition of spacetime (& consequently the ADM formalism) as well as an introduction to the homogeneous universes. The talk will conclude by showing the results for the case of a scalar field minimally coupled to gravity whose classical solutions survive through the big bang singularity. Possible future directions will be mentioned.

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Holography - also referred to as AdS/CFT correspondence or Gauge/Gravity duality - is an attractive new concept providing surprising cross-relations between various active research areas. In the talk, I will mainly focus on AdS/CFT dualities linking (conformal) quantum field theories to higher-dimensional quantum gravity theories in asymptotically Anti-deSitter spacetimes. On one hand, those dualities shed new light on quantum gravity aspects, while on the other, they provide tools for studying strongly coupled systems in a variety of areas in physics.

After a short introduction into the basic building blocks of such AdS/CFT dualities, I discuss quantum gravity aspects of such dualities. Finally, I will provide an outlook on the important next steps in this research direction.