Quantum Gravity

In their seminal 1977 paper, Gibbons and Hawking (GH) audaciously applied concepts of quantum statistical mechanics to ensembles containing black holes, finding that a semiclassical saddle point approximation to the partition function recovers the laws of black hole thermodynamics. In the same paper they insouciantly applied the formalism to the case of boundary-less de Sitter space (dS), obtaining  the expected temperature and entropy of the static patch. To what  ensemble does the dS partition function apply? And why does the entropy of the dS static patch decrease upon addition of Killing energy? I’ll answer these questions, and then generalize the GH method to find the approximate partition function of a ball of space at any fixed proper volume. The result is the exponential of the Bekenstein-Hawking entropy of its boundary.

Zoom link:  https://pitp.zoom.us/j/91961890091?pwd=R3lZWHNIQUUzSldzS3kyclJKR3JXdz09

Understanding gravity in the framework of quantum mechanics is one of the great challenges in modern physics. Along this line, a prime question is to find whether gravity is a quantum entity subject to the rules of quantum mechanics. It is fair to say that there are no feasible ideas yet to test the quantum coherent behaviour of gravity directly in a laboratory experiment. I will introduce an idea for such a test based on the principle that two objects cannot be entangled without a quantum mediator.  I will show that despite the weakness of gravity, the phase evolution induced by the gravitational interaction of two micron size test masses in adjacent matter-wave interferometers can detectably entangle them even when they are placed far apart enough to keep Casimir-Polder forces at bay. I will provide a prescription for witnessing this entanglement, which certifies gravity as a quantum coherent mediator, through simple correlation measurements between two spins: one embedded in each test mass. Fundamentally, the above entanglement is shown to certify the presence of non-zero off-diagonal terms in the coherent state basis of the gravitational field modes.

Zoom link:  https://pitp.zoom.us/j/99584743899?pwd=aHl1cVlpK29ZVDkrdFZyM01GemJJdz09

In the holographic approach to quantum gravity, quantum information theory plays a fundamental role in understanding how semiclassical gravity emerges from the microscopic description.  The map (sometimes called the dictionary) between these two descriptions has the structure of a quantum error correcting code.  In the context of an evaporating black hole, this code can be arbitrarily far from an isometry.  Such codes are novel from a quantum information standpoint, and their properties are not yet well understood.  I will describe a simple toy model of an evaporating black hole which allows for an explicit construction of the dictionary using the Euclidean gravity path integral.  I will also describe the sense in which this dictionary is a non-isometric code, explain its basic properties, and comment on implications for semiclassical physics in the black hole interior.

Zoom link:  https://pitp.zoom.us/j/94869738394?pwd=dGNBWXpmTTZaRSs3c0NQUDA1UkZCZz09

Cosmological models and black holes belong to classes of space-time metrics defined in terms of a finite number of degrees of freedom, for which the Einstein–Hilbert action reduces to a one-dimensional mechanical model. We investigate their classical symmetries and the algebra of the corresponding Noether charges. These dynamical symmetries have a geometric interpretation, not in terms of spacetime geometry, but in terms of motion on the field space. Moreover, they interplay with the fiducial scales, introduced to regulate the homogenous model, suggesting a relationship with the boundary symmetries of the full theory.

Finally, the existence of these symmetries unravels new aspects of the physics of black holes and cosmology. It opens the way towards a rigorous group quantization of the reduced model and to the study of their holographic properties. It might have significant consequences on the propagation of test fields and the corresponding perturbation theory.

Zoom link:  https://pitp.zoom.us/j/92846533238?pwd=cERGUjd6OXB5S0ZaSzVIdVJyMHZxUT09

A natural way to extend Einstein's General Relativity to the high-energy regime is to introduce quadratic curvature operators, which give rise to a renormalizable gravitational Lagrangian in four dimensions. However, this theory turns out to be pathological due to an additional massive spin-2 ghost that causes classical instabilities and breaks perturbative unitarity at the quantum level. In this talk, we investigate which principles of QFT are usually affected when higher-order derivative operators are present in a Lagrangian, and show that one possibility to avoid ghosts is giving up locality. In fact, the emergence of non-local physics at short distances and high energies is a common prediction for various quantum gravity approaches. We propose a model-independent approach to constrain the non-local Lagrangian in quantum gravity, thus providing a novel scenario in which different quantum gravity programs can be fruitfully contrasted in terms of foundational concepts and computational methods.

Zoom link:  https://pitp.zoom.us/j/96167652193?pwd=OWFDV1JXVGxLdE1qMHo4N3ZKcDByQT09

I will discuss information theoretic properties of planar black hole microstates in 2 + 1 dimensional asymptotically anti-de Sitter spacetime, modeled by black holes with an end-of-the-world brane behind the horizon. The von Neumann entropy of sufficiently large subregions in the dual CFT exhibits a time-dependent phase, which from a doubly-holographic perspective corresponds to the appearance of quantum extremal islands in the brane description. Considering the case where dilaton gravity is added to the brane, we show that tuning the associated couplings affects the propagation of information in the dual CFT state. By requiring that information theoretic bounds on the growth of entanglement entropy are satisfied in the dual CFT, we can place bounds on the allowed values of the couplings on the brane.

Zoom link:  https://pitp.zoom.us/j/96173295706?pwd=REdGajBXbTlPYVFhL0s1c3lINXY5Zz09

Inspired by the emergence of bulk diffeomorphism invariance in holography, I will give an explicit description how entanglement between quantum subsystems can lead to emergent gauge symmetry in a classical limit. Along the way, I will provide a precise characterisation of when it is consistent to treat a quantum subsystem classically in such a limit, and show that this gives strong constraints on the entanglement structure of classical states. I will explain how this generically leads to emergent fundamentally non-local classical degrees of freedom, which may nevertheless be accounted for in a kinematically local way if one employs an appropriately redundant description. The mechanism I describe is general and elementary, but for concreteness I will exhibit a toy example involving three entangled spins at high angular momentum, and I will also describe a significant generalisation of this toy example based on coadjoint orbits. If there is time, I will discuss evidence for the role this phenomenon plays in gravity. This talk is based on arXiv:2209.03979.

Zoom link:  https://pitp.zoom.us/j/92066956880?pwd=OTRySTlOVGgvM3RCRmkzWHFVSUF3Zz09

Extensions of asymptotic phase spaces and their corresponding asymptotic symmetry groups have become a topic of increasing interest in recent years, due to results that connect them with a wide spectrum of areas, such as symmetries of the S-matrix, soft theorems, corner symmetries, double copy maps and celestial holography. The study of these extensions aims to characterize the degrees of freedom of the physical theories at the classical level, gathering information on how their symmetries can be upgraded to the quantum theories. In this talk, I will describe extensions of phase spaces at null infinity for gravity and gauge theories, such that the charges are consistent with tree-level soft (graviton, gluon or photon) theorems and act canonically. First, as motivation, I will show how the Geroch group for cylindrically symmetric general relativity can be upgraded as a quantum symmetry, exploiting the integrability of the system (arXiv:1906.04856 [gr-qc]). Second, I will review the construction of an extended phase space where the generalized BMS symmetry group acts canonically (arXiv:2002.06691 [gr-qc]). I will show that this construction is consistent with the extended corner symmetry approach (via the embedding maps). Finally, I will show that a similar approach can be done in Yang Mills, by extending the phase space with "Goldstone modes" that transform inhomogeneously under linearized O(r) symmetries (arXiv:2111.00973 [hep-th]). Some preliminary results regarding the extension to higher order O(r^n) will be discussed (arXiv:2211.12991 [hep-th]).

Zoom link:  https://pitp.zoom.us/j/95866794894?pwd=UEtQYlBpV0NCM3cyeWpDcTI2WVp6UT09

I will describe a holographic model for the three dimensional de Sitter static patch where the boundary theory is the so called $T\bar{T}+\Lambda_2$ deformation of the conformal field theory dual to AdS_3 quantum gravity. This identification allows us to obtain the cosmic horizon entropy from a microstate count, and the microstates themselves are a dressed version of those that account for the entropy of certain black holes in AdS space. I will also show how the effect of this dressing at the cosmic horizon is to replace the spacetime dependence of the fields of the undeformed holographic CFT with dependence on the indices of large matrices.

Zoom link:  https://pitp.zoom.us/j/95396921570?pwd=NGFoOGlGY1ZDU2pnNFRwWit3b2w0Zz09