Scalar charges and black hole hair
Black holes can have no hair. Scalar charges, which do not seem to obey any conservation law and have no consistent definition, are considered hair. Nevertheless, many black holes seem to have this kind of hair because there are very special values of the scalar charge which seem to be allowed by the no-hair conjecture because they are considered “secondary hair.” It has been argued that these scalar charges play a role in the first law of black-hole mechanics, in spite of their lacking a consistent definition.
In this talk I will propose a consistent definition of scalar charge in stationary black-hole spacetimes which I will use to determine its possible allowed value and formulate no-hair theorems. In order to describe this definition, I will review the definition of conserved charges associated to local and global symmetries in a pedagogical way.constraints on the coupling of the TT̄bar deformation in two dimensions.
26 September 2023, 14:30
Bootstrapping bulk RG flows from the boundary
For QFTs in AdS the boundary correlation functions remain conformal even if the bulk theory has a scale. This property allows one to constrain RG flows with numerical conformal bootstrap methods. We analyze two-dimensional flows between different CFTs with a focus on deformations of the tricritical and ordinary Ising models. We provide non-perturbative constraints for the boundary correlation functions of these flows and compare them with conformal perturbation theory in the vicinity of the fixed points. We also discuss bounds on deformations around arbitrary CFTs, in particular reproducing constraints on the coupling of the TT̄bar deformation in two dimensions.
15 June 2023, 10:00
From scattering amplitudes to gravitational waves
Scattering amplitudes of elementary particles exhibit a fascinating simplicity, which is entirely obscured in textbook Feynman-diagram computations. While amplitudes find their primary application to collider physics, describing the dynamics of the tiniest particles in the universe, they also characterise the interactions among some of its heaviest objects, such as black holes. Violent collisions among black holes occur where tremendous amounts of energy are emitted in the form of gravitational waves, and it is now crucial to develop novel efficient methods for highly needed high-precision predictions. Thanks to their inherent simplicity, amplitudes are ideally suited to this task, but one faces the challenge of extracting classical contributions efficiently.
I will describe how working in a Heavy-mass Effective Field Theory (HEFT) allows to achieve this goal, dropping quantum-suppressed terms and hyper-classical corrections from the get go, i.e. before any integrations are performed. I will then discuss two important applications of the HEFT amplitudes: the computation of the two-loop (or third post-Minkowskian) scattering angle of two spinless black holes; and the determination of the subleading (one-loop) gravitational waveform in the frequency and time domains. These computations are performed using HEFT amplitudes, in conjunction with the Bern-Carrasco-Johansson double copy and generalised unitarity. I will conclude with prospects for future work.
8 June 2023, 14:30
The Witten Diagram Bootstrap for Holographic Defects
We compute correlation functions of two local operators and one defect in holographic conformal field theories. In the supergravity description, the local operators are dual to certain Kaluza-Klein modes, while the defects are dual to probe branes preserving part of the supersymmetry. For concreteness, this talk focuses on correlators of two chiral-primary operator and one Maldacena-Wilson line in N=4 SYM. To obtain the leading strong-coupling correction to this correlator, we make an ansatz in terms of Witten diagrams and fix it with the help of superconformal Ward identities and localization. The result admits a closed-form expression, which takes a surprisingly simple form in Mellin space.
31 May 2023, 14:30
Hexagons, Wilson Loops and Q-functions
The hexagonalization procedure arose in the context of AdS/CFT correspondence in order to compute correlation functions in planar N = 4 SYM, a particular corner where the theory is known to become integrable. This approach, formalized by Basso, Vieira and Komatsu in 2015 is based upon a tessellation of the dual closed string worldsheet that permits to obtain two building blocks that can be bootstrapped using the power of integrability. After an introduction about the integrable structures arising in the AdS/CFT correspondence, I will review the hexagonalization technique and how we can suitably modify this procedure in order to compute correlation functions on non-trivial backgrounds such as a Wilson loop and the possible links with other well-known integrability techniques, namely the Q-functions approach and the functional separations of variable (FSoV).
30 May 2023, 11:00
Anomaly constraints on heterotic strings and supergravity
The known string landscape covers a small subset of all consistent-looking effective field theories of (quantum) gravity. A great deal of indications suggest that anything not covered by the landscape is actually inconsistent, usually due to subtle violations of unitarity when quantum gravity effects are at play. The ambitious program of understanding this principle of "string universality" has been quite successful, starting from theories with 32 supercharges and gradually moving down in symmetries and dimensions, but minimal supersymmetry in six dimensions is a prominent roadblock. In this talk I will present some preliminary results in this setting: new anomalies of the Dai-Freed type seem to significantly constrain six dimensional supergravities that would otherwise look consistent, while a large class of heterotic string constructions remain remarkably safe.
23 May 2023, 14:30
Poisson gauge theory
Motivated by noncommutative gauge theory, I will discuss a novel approach to gauge theory with underlying non-trivial Poisson structure over space-time. The goal is to formulate a gauge theory which reproduces the standard one in the 'commutative' limit (e.g. with trivial Poisson bracket) and such that the algebra of gauge parameters closes under Poisson brackets. The case of U(1) gauge theory shall be considered in some detail.
16 May 2023, 14:30
Bounds on photon scattering
We study 2-to-2 scattering amplitudes of massless spin one
particles in four spacetime dimensions, like real world photons. We use
full nonlinear unitarity to construct numerical bounds on the Wilson
coefficients, which describe these amplitudes at low energies. Some of the
bounds can be recovered analytically by using the optical theorem, others
cannot. Finally, some Wilson coefficients cannot be bounded, and we discuss
why.
09 May 2023, 14:30
A bundle geometric perspective on covariant phase space methods for diffeomorphisms symmetry: Applications to gravitational dressings & gravitational edge modes
We emphasise the elementary bundle geometry of the space of fields, F, supporting the action of diffeomorphisms. We first stress that the Lagrangian of a theory with Diff(M) symmetry is a section of a line bundle, associated to F, of a peculiar type: it is a “twisted bundle” generalising the standard notion of associated vector bundle. Then, we sketch how the so-called "dressing field method" allows to build basic forms on F, which then descend as forms on the orbit space F/Diff(M). Finally, we show that it is a unifying framework for gravitational dressings à la Giddings, relational variables à la Rovelli, and gravitational edge modes à la Donnelly-Freidel-Speranza (& others). In the latter case, it allows to systematically build the “extended” covariant phase space of a Diff(M)-theory. We stress the limits of the approach.
04 May 2023, 14:30
Recent advances in precision holography
I will present ongoing efforts to probe the AdS4/CFT3 correspondence beyond the strict large N limit. On the gauge theory side, I will summarize some recent conjectures for the partition functions of 3d SCFTs preserving at least N=2 supersymmetry on various compact Euclidean 3-manifolds. The proposed expressions are perturbatively exact in 1/N, and they are supported by consistency with available analytic results as well as detailed numerical studies. On the gravity side, I will explain how higher-derivative corrections and loop effects account for subleading terms in the large N expansion of boundary observables. Such precision tests of AdS/CFT have important consequences for our understanding of M-theory and quantum aspects of AdS black holes.
02 May 2023, 14:30