When studying gauge theories in the presence of boundaries, local symmetry transformations are typically classified as gauge or physical depending on whether the associated charges vanish or not. Here, we propose that physical charges should further be refined into “dynamical” or “kinematical” depending on whether they are associated with evolution equations or not. To support this proposal, we analyze (A)dS gravity with boundary Weyl rescalings and compare the solution spaces in Bondi-Sachs and Fefferman- Graham coordinates. In line with previous accounts, we show that the Weyl charge vanishes in the Bondi-Sachs gauge but not in the Fefferman-Graham gauge. Conversely, the charges arising from the metric Chern-Simons Lagrangian behave in the opposite way. This indicates that the gauge-dependent Weyl charge differs fundamentally from charges like mass and angular momentum. This interpretation is reinforced by two key observations: the Weyl conformal factor does not satisfy any evolution equation, and the associated charge arises from a corner term in the symplectic structure. These properties justify assigning the Weyl charge a kinematical status. These results can also be derived using the field-dependent diffeomorphism that maps between the two gauges. Importantly, this diffeomorphism does not act tensorially on the variational bi-complex due to its field dependency, and is able to “toggle” charges on or off. This provides an example of a large diffeomorphism $\textit{between}$ gauges, as opposed to a residual diffeomorphism $\textit{within}$ a gauge.

Canada

Luca Ciambelli

France

Marc Geiller

Field-dependent diffeomorphisms and the transformation of surface charges between gauges

The symmetries of the gravitational scattering are intimately tied to the symmetries which preserve asymptotic flatness at null infinity. In Penrose’s definition of asymptotic flatness, a central role is played by the notion of asymptotic simplicity and the ensuing peeling behavior which dictates the decay rate of the Weyl tensor. However, there is now accumulating evidence that in a generic gravitational scattering the peeling property is broken, so that the spacetime is not asymptotically-flat in the usual sense. These obstructions to peeling can be traced back to the existence of $\textit{universal}$ radiative low frequency observables called “tails to the displacement memory”. As shown by Saha, Sahoo and Sen, these observables are uniquely fixed by the initial and final momenta of the scattering objects, and are independent of the details of the scattering. The universality of these tail modes is the statement of the classical logarithmic soft graviton theorem. Four-dimensional gravitation scattering therefore exhibits a rich infrared interplay between tail to the memory, loss of peeling, and universal logarithmic soft theorems.

India

Alok Laddha

Céline Zwikel

Symmetries of the gravitational scattering in the absence of peeling

We present an action for minimal massive gravity (MMG) in three dimensions in terms of a dreibein and an independent spin connection. Furthermore, the construction provides an action principle for an infinite family of so-called third-way consistent generalizations of the three-dimensional Einstein field equations, including exotic massive gravity and new higher-order generalizations. It allows us to systematically construct the matter couplings for these models, including the couplings to fermions, depending on the spin connection. In particular, we construct different supersymmetric extensions of MMG, and derive their second order fermionic field equations. This establishes a new class of three-dimensional supergravity theories and we discuss their limit to topological massive supergravity. Finally, we identify the landscape of (anti–)de Sitter vacua of the supersymmetric models. We analyze the spectrum and the unitarity properties of these vacua. We recover the known AdS vacua of MMG which are bulk and boundary unitary.

Türkiye

Austria

Nihat Sadik Deger

Croatia

Jan Rosseel

Henning Samtleben

Minimal massive supergravity and new theories of massive gravity

We present a Sugawara-type construction for boundary charges in 4d BF theory and in a general family of related TQFTs. Starting from the underlying current Lie algebra of boundary symmetries, this gives rise to well-defined quadratic charges forming an algebra of vector fields. In the case of 3d BF theory (i.e. 3d gravity), it was shown in [1] that this construction leads to a two-dimensional family of diffeomorphism charges which satisfy a certain modular duality. Here we show that adapting this construction to 4d BF theory first requires to split the underlying gauge algebra. Surprisingly, the space of well-defined quadratic generators can then be shown to be once again two-dimensional. In the case of tangential vector fields, this canonically endows 4d BF theory with a diff(S 2) × diff(S 2) or diff(S 2) ⋉ vect(S 2)ab algebra of boundary symmetries depending on the gauge algebra. The prospect is to then understand how this can be reduced to a gravitational symmetry algebra by imposing Plebański simplicity constraints.

Florian Girelli

Germany

Christophe Goeller

Panagiotis Tsimiklis

Diffeomorphisms as quadratic charges in 4d BF theory and related TQFTs

Minimal massive gravity in three dimensions propagates a single massive spin-2 mode around an anti–de Sitter vacuum. It is distinguished by allowing for vacua with positive central charges of the asymptotic conformal algebra and a bulk graviton of positive energy. We present a new action for the model (and its higher order extensions) in terms of a dreibein and an independent spin connection. From this, we construct its supersymmetric extension. Surprisingly, all vacua complying with bulk and boundary unitarity appear to break supersymmetry spontaneously. In contrast, all supersymmetric vacua have a negative central charge whenever the bulk graviton has positive energy.

Minimal Massive Supergravity

We study the finite distance boundary symmetry current algebra of the most general first order theory of 3D gravity. We show that the space of quadratic generators contains diffeomorphisms but also a notion of dual diffeomorphisms, which together form either a double Witt or centerless <math><mrow><msub><mrow><mi>BMS</mi></mrow><mrow><mn>3</mn></mrow></msub></mrow></math> algebra. The relationship with the usual asymptotic symmetry algebra relies on a duality between the null and angular directions, which is possible thanks to the existence of the dual diffeomorphisms.

Dual diffeomorphisms and finite distance asymptotic symmetries in 3D gravity

We introduce a new gauge and solution space for three-dimensional gravity. As its name Bondi-Weyl suggests, it leads to non-trivial Weyl charges, and uses Bondi-like coordinates to allow for an arbitrary cosmological constant and therefore spacetimes which are asymptotically locally (A)dS or flat. We explain how integrability requires a choice of integrable slicing and also the introduction of a corner term. After discussing the holographic renormalization of the action and of the symplectic potential, we show that the charges are finite, symplectic and integrable, yet not conserved. We find four towers of charges forming an algebroid given by   <math> <mi>vir</mi> <mo>⊕</mo> <mi>vir</mi> <mo>⊕</mo> </math>  $$ \mathfrak{vir}\oplus \mathfrak{vir}\oplus $$  Heisenberg with three central extensions, where the base space is parametrized by the retarded time. These four charges generate diffeomorphisms of the boundary cylinder, Weyl rescalings of the boundary metric, and radial translations. We perform this study both in metric and triad variables, and use the triad to explain the covariant origin of the corner terms needed for renormalization and integrability.

3d gravity in Bondi-Weyl gauge: charges, corners, and integrability

We show that the phase space of three-dimensional gravity contains two layers of dualities: between diffeomorphisms and a notion of “dual diffeomorphisms” on the one hand, and between first order curvature and torsion on the other hand. This is most elegantly revealed and understood when studying the most general Lorentz-invariant first order theory in connection and triad variables, described by the so-called Mielke-Baekler Lagrangian. By analyzing the quasi-local symmetries of this theory in the covariant phase space formalism, we show that in each sector of the torsion/curvature duality there exists a well-defined notion of dual diffeomorphism, which furthermore follows uniquely from the Sugawara construction. Together with the usual diffeomorphisms, these duals form at finite distance, without any boundary conditions, and for any sign of the cosmological constant, a centreless double Virasoro algebra which in the flat case reduces to the BMS3 algebra. These algebras can then be centrally-extended via the twisted Sugawara construction. This shows that the celebrated results about asymptotic symmetry algebras are actually generic features of three-dimensional gravity at any finite distance. They are however only revealed when working in first order connection and triad variables, and a priori inaccessible from Chern-Simons theory. As a bonus, we study the second order equations of motion of the Mielke-Baekler model, as well as the on-shell Lagrangian. This reveals the duality between Riemannian metric and teleparallel gravity, and a new candidate theory for three-dimensional massive gravity which we call teleparallel topologically massive gravity.

Chile

Nelson Merino

Most general theory of 3d gravity: covariant phase space, dual diffeomorphisms, and more

In the tetrad formulation of gravity, the so-called simplicity constraints play a central role. They appear in the Hamiltonian analysis of the theory, and in the Lagrangian path integral when constructing the gravity partition function from topological BF theory. We develop here a systematic analysis of the corner symplectic structure encoding the symmetry algebra of gravity, and perform a thorough analysis of the simplicity constraints. Starting from a precursor phase space with Poincaré and Heisenberg symmetry, we obtain the corner phase space of BF theory by imposing kinematical constraints. This amounts to fixing the Heisenberg frame with a choice of position and spin operators. The simplicity constraints then further reduce the Poincaré symmetry of the BF phase space to a Lorentz subalgebra. This picture provides a particle-like description of (quantum) geometry: the internal normal plays the role of the four-momentum, the Barbero-Immirzi parameter that of the mass, the flux that of a relativistic position, and the frame that of a spin harmonic oscillator. Moreover, we show that the corner area element corresponds to the Poincaré spin Casimir. We achieve this central result by properly splitting, in the continuum, the corner simplicity constraints into first and second class parts. We construct the complete set of Dirac observables, which includes the generators of the local   <math> <mi>sl</mi> <mfenced> <mn>2</mn> <mi>ℂ</mi> </mfenced> </math>  $$ \mathfrak{sl}\left(2,\mathrm{\mathbb{C}}\right) $$  subalgebra of Poincaré, and the components of the tangential corner metric satisfying an   <math> <mi>sl</mi> <mfenced> <mn>2</mn> <mi>ℝ</mi> </mfenced> </math>  $$ \mathfrak{sl}\left(2,\mathrm{\mathbb{R}}\right) $$  algebra. We then present a preliminary analysis of the covariant and continuous irreducible representations of the infinite-dimensional corner algebra. Moreover, as an alternative path to quantization, we also introduce a regularization of the corner algebra and interpret this discrete setting in terms of an extended notion of twisted geometries.

Laurent Freidel

Italy

Daniele Pranzetti

Edge modes of gravity. Part III. Corner simplicity constraints

In this second paper of the series we continue to spell out a new program for quantum gravity, grounded in the notion of corner symmetry algebra and its representations. Here we focus on tetrad gravity and its corner symplectic potential. We start by performing a detailed decomposition of the various geometrical quantities appearing in BF theory and tetrad gravity. This provides a new decomposition of the symplectic potential of BF theory and the simplicity constraints. We then show that the dynamical variables of the tetrad gravity corner phase space are the internal normal to the spacetime foliation, which is conjugated to the boost generator, and the corner coframe field. This allows us to derive several key results. First, we construct the corner Lorentz charges. In addition to sphere diffeomorphisms, common to all formulations of gravity, these charges add a local   <math> <mi>sl</mi> </math>  $$ \mathfrak{sl} $$ (2, ℂ) component to the corner symmetry algebra of tetrad gravity. Second, we also reveal that the corner metric satisfies a local   <math> <mi>sl</mi> </math>  $$ \mathfrak{sl} $$ (2, ℝ) algebra, whose Casimir corresponds to the corner area element. Due to the space-like nature of the corner metric, this Casimir belongs to the unitary discrete series, and its spectrum is therefore quantized. This result, which reconciles discreteness of the area spectrum with Lorentz invariance, is proven in the continuum and without resorting to a bulk connection. Third, we show that the corner phase space explains why the simplicity constraints become non-commutative on the corner. This fact requires a reconciliation between the bulk and corner symplectic structures, already in the classical continuum theory. Understanding this leads inevitably to the introduction of edge modes.