Quantum Gravity

The problem of time is often discussed as an obstacle in the canonical quantisation of gravitational systems: general covariance means there is no preferred time parameter with respect to which evolution could be defined. We can instead characterise dynamics in relational terms by defining one degree of freedom to play the role of an internal clock for the other variables; this leads to a multiple choice problem of which variable should play the role of clock. I will review recent results obtained in a quantum cosmological model with three dynamical degrees of freedom: a volume or scale factor variable for the geometry, a massless scalar matter field, and a perfect fluid. Each of these variables can be used as a clock for the other two. We obtain three different theories which, if we require them to have unitary time evolution with respect to the given clock, make very different statements about the fate of the Universe. Only one resolves the classical singularity, and only one leads to a quantum recollapse of the Universe at large volume. Nonclassical behaviour arises whenever a classical solution terminates in finite time so that reflecting boundary conditions are needed to make the theory unitary. We discuss general implications for a canonical quantisation of gravitational systems.

Zoom Link: https://pitp.zoom.us/j/95556457739?pwd=S0dZUVYwTTBOaFNpNGcrc2ladHZ5QT09

The information loss paradox is usually stated as an incompatibility between general relativity and quantum mechanics. However, the assumptions leading to the problem are often overlooked and, in fact, a careful inspection of the main hypothesises suggests a radical reformulation of the problem. Indeed, I will present a thought experiment that shows the existence of an incompatibility between (i) the validity of the laws of general relativity to describe infalling matter far from the Planckian regime, and (ii) the so-called central dogma which states that as seen from an outside observer a black hole behaves like a quantum system whose number of degrees of freedom is proportional to the horizon area. We critically revise the standard arguments in support of the central dogma, and argue that they cannot hold true unless some new physics is invoked even before reaching Planck scales. This leads to the counterintuitive result that the information loss problem, in its current formulation, is not necessarily related to any loss of information or lack of unitarity. Semiclassical general relativity and quantum mechanics can be perfectly compatible before reaching the final stage of the black hole evaporation where, instead, a consistent theory of quantum gravity is needed to make any prediction.

Zoom Link: https://pitp.zoom.us/j/94637978136?pwd=VmlWa1BJU3I1UGVRNjliMEpUTlo2dz09

In the asymptotic safety approach to quantum gravity it is conjectured that a nonperturbative ultraviolet completion exists for Einstein's metric gravity and its perturbative effective description in four dimensions. Such completion would circumvent the notorious problems of renormalizability and predictivity at high energies of the quantum theory. I want do discuss the conjecture armed with pragmatism and avoiding preconceived ideologies. I also want to present some perturbative results in two dimensions, that can be used to argue the validity of the conjecture above two dimensions, and to discuss the implications for the current status of the approach.

The incorporation of classical general relativity into the framework of quantum field theory yielded a rather surprising result -- thermodynamic particle production. In short, for fundamental deformations in the structure of spacetime, quantum mechanics necessitates the creation of thermalized particles from the vacuum. One such phenomenon, known as the Unruh effect, causes empty space to effervesce a thermal bath of particles when viewed by an observer undergoing uniformly accelerated motion. These highly accelerated systems will also have an associated Rindler horizon which produces this Unruh radiation at the celebrated Fulling-Davies-Unruh temperature. For accelerated charges, the emission and absorption of this Unruh radiation will not only affect the associated Rindler horizon in accordance with the Bekenstein-Hawking area-entropy law, but will also imprint the FDU temperature on any photons emitted and subsequently detected in the laboratory. A recent series of high energy channeling experiments carried out by the NA63 collaboration at CERN have finally brought about the first observations and insights into the nature of the Unruh effect. In this presentation, I will discuss the various aspects of acceleration-induced thermality measured by these experiments at NA63.

Zoom Link: https://pitp.zoom.us/j/97257949405?pwd=Ung4TXVwbHJDdm9LbEVRSExQTzI4Zz09

The advent of black hole imaging has opened a new window into probing the horizon scale of black holes. An important question is whether string theory results for black hole physics can predict interesting and observable features that current and future experiments can probe.

I will discuss the physical properties of four-dimensional, string-theoretical, horizonless “fuzzball” geometries by means of imaging their shadows. Their microstructure traps light rays straying near the would-be horizon on long-lived, highly redshifted chaotic orbits. In fuzzballs sufficiently near the scaling

limit this creates a shadow much like that of a black hole, while avoiding the paradoxes associated with an event horizon.

Finally, I will consider comparing such fuzzball images to their black hole counterparts. In particular, detailed measurements of higher order photon rings have the potential to discriminate between fuzzballs and black holes in future observations.

This talk is devoted to the geometric approach to supergravity and applications in the framework of loop quantum gravity. Among other things, this approach leads to a reformulation of the theory in which (part of) supersymmetry manifests itself in terms of a gauge symmetry. Using the interpretation of supergravity in terms of a super Cartan geometry, we will derive the Holst variant of the MacDowell-Mansouri action for N=1 and N=2 AdS supergravity in D=4 for arbitrary Barbero-Immirzi parameters. We will show that these actions provide unique boundary terms that ensure local supersymmetry invariance at boundaries. The chiral case is special. The action is invariant under an enlarged gauge symmetry, and the boundary theory is a super Chern-Simons theory. The action also implies boundary conditions that link the super electric flux through, and the super curvature on, the boundary. Applications we have in mind are supersymmetric black holes and loop quantum cosmology. To this end, we will study a class of symmetry reduced models of chiral supergravity. The enlarged gauge symmetry of the chiral theory is essential as it allows for nontrivial fermionic degrees of freedom even if one imposes spatial isotropy. The quantization of the theory yields a natural state space and allows a consistent implementation of the constraint algebra.

Finally, we will give an outlook on applications towards a quantum description of supersymmetric black holes in the context of LQG and possible relations to superstring theory.

The spacetime in the interior of a black hole can be described by an homogeneous line element, for which the Einstein–Hilbert action reduces to a one-dimensional mechanical model. We have shown that this model exhibits a symmetry under the (2+1)-dimensional Poincaré group. The existence of this symmetry provides a powerful criterion to discriminate between different regularization and quantization schemes. It also unravels new aspects of symmetry for black holes, and opens the way towards a rigorous group quantization of the interior. Remarkably, the physical ISO(2,1) symmetry can be seen as a broken infinite-dimensional symmetry. This is done by reinterpreting the action for the model as a geometric action for the BMS3 group, where the configuration space variables are elements of the algebra bms3 and the equations of motion transform as coadjoint vectors.

Zoom Link: https://pitp.zoom.us/j/99646733321?pwd=U05kYU85V0Q4VCtrZ1BNV2JZbE1DUT09

The inclusion of the cosmological constant is one of the main questions faced by quantum gravity. In three dimensions, non-perturbative approaches to quantum gravity including loop quantum gravity (LQG),  combinatorial quantization and  spinfoam path integrals encode the cosmological constant as a deformation parameter in a quantum group structure. In this talk, I will focus on the LQG approach: I will explain the Poisson-Lie structure of the classical phase space and how its quantization naturally leads to the emergence of quantum groups. I will use the holonomy-flux algebra and its spinorial presentation introduced in the series of work by Bonzom, Dupuis, Girelli, Livine and myself. This allows to construct the Hamiltonian constraint, understand its matrix elements as Turaev-Viro amplitudes. This connects LQG to the other approaches in a unified mathematical setting.

Strong gravity tests indicate that general relativity is a very accurate description of the classical dynamics of spacetime even at extreme regimes. Yet, the same dynamics can be described by "alternative" versions of general relativity such as unimodular gravity. In the quest for a quantum theory of the gravitational field, it is unclear if the quantization of such classically equivalent theories leads to the same physical predictions. In this talk, I will report on some recent results regarding this issue in the framework of continuum and perturbative quantum field theory. With a view towards ultraviolet completion, I will discuss some evidence for asymptotic safety in unimodular quantum gravity. Moreover, I will comment on the role of matter fields which couple very differently to gravity in those settings.   

Asymptotically flat spacetimes are invariant under an infinite-dimensional symmetry group comprised of superrotations and supertranslations. These symmetries are spontaneously broken, leading to an infinite degeneracy of gravitational vacua in asymptotically flat spacetimes. Starting from an analysis of four-dimensional asymptotically flat gravity in first order formulation, I will describe how superrotation parametrization modes labelling distinct superrotation vacua are governed by an Alekseev–Shatashvili action on the celestial sphere. I will also comment on a recent construction of a two-dimensional effective action for the Goldstone modes of broken supertranslation invariance.

We introduce the basic elements of tensorial group field theories (TGFTs) for quantum gravity, emphasizing how they encode quantum geometry and their relation with canonical loop quantum gravity and spin foam models. Next, we discuss briefly the issue of continuum limit and how it could be understood in this framework.

In the bulk of the talk, we overview the work on the extraction of an effective cosmological dynamics from TGFTs, inspired by the idea of the universe as a quantum gravity condensate. In this context, we emphasize: the need for appropriately coarse-grained states capturing collective dynamics and the role of relational observables and their construction. We discuss what the theory says (so far) about the fate of the big bang singularity at the beginning of our universe and how it suggests a quantum gravity origin for (phantom) dark energy.

An extension of general relativity (GR) obtained by adding local quadratic terms to the action will be considered. Such theory can be a viable UV completion of GR. The additional terms soften gravity above a certain energy scale and render gravity renormalizable. The presence of 4 derivatives implies via the Ostrogradsky theorem that the classical Hamiltonian is unbounded from below. Nevertheless, I will argue that the relevant solutions are not unstable, but metastable: when the energies are much below a threshold (that is high enough to describe the whole cosmology) runaways are avoided. Remarkably, the chaotic inflation theory of initial conditions ensures that such bound is satisfied and testable implications for the early universe will be discussed. I will also argue that the basic unitarity condition is satisfied when the theory is correctly formulated at the quantum level. Moreover, thanks to the UV softening of gravity, sufficiently light objects must be horizonless and I will discuss explicit analytic examples of horizonless ultracompact objects, which have interesting physical implications.