# Talks

If you can’t explain it to a six year old, you don’t understand it yourself.

A list of scientific talks I have given over the years on my various projects.

Contrary to the above quote, these are not for six year olds; they only have the slides and nothing of what I said during the talks.

## Infrared finite scattering in QFT & quantum gravity

The "infrared problem" is the generic emission of an infinite number of low-frequency quanta in any scattering process with massless fields. The "out" state contains an infinite number of such quanta which implies that it does not lie in the standard Fock representation. Consequently, the standard S-matrix is undefined as a map between "in" and "out" states in the standard Fock space. This fact is due to the existence of a low-frequency tail of the radiation field i.e. the memory effect. In massive QED, the Faddeev-Kulish representations have been argued to yield an I.R. finite S-matrix. We clarify the "preferred " status of such representations as eigenstates of the conserved "large gauge charge'' at spatial infinity. We prove a "No-Go" theorem for the existence of a suitable Hilbert space analogously constructed in massless QED, QCD, linearized quantum gravity with massive/massless sources, and in full quantum gravity. We then suggest an "infrared-finite" formulation of scattering theory in terms of correlation functions without any a priori choice of "in/out" Hilbert spaces.

## Asymptotic quantum fields at spatial infinity

I will consider the asymptotic behaviour of massless scalar quantum fields at spatial infinity in Minkowski spacetime. The bulk scalar fields can be written in terms of massive scalar de Sitter fields on the hyperboloid of directions at spatial infinity. This gives a formulation of the QFT purely in terms of fields defined at spatial infinity which can be generalised to any asymptotically-flat spacetime. I will argue that quantum states with different values of the asymptotic “scalar charge” live in different unitarily-inequivalent Hilbert spaces. This gives a precise formulation of charge superselection. I will comment on generalization to asymptotic quantization in the gravitational case and the relation to null infinity.

## Angular momentum at null infinity in Einstein-Maxwell theory

On Minkowski spacetime, the angular momentum flux through null infinity of Maxwell fields, computed using the stress-energy tensor, depends not only on the radiative degrees of freedom, but also on the Coulombic parts. This flux cannot be written as the change of an angular momentum charge computed purely on cross-sections of null infinity. We investigate the angular momentum charge and flux in full Einstein-Maxwell theory. Using the prescription of Wald and Zoupas, we compute the charges associated with any BMS symmetry on cross-sections of null infinity. The change of these charges along null infinity then provides a flux. For Lorentz symmetries, the Maxwell fields contribute an additional term to the charge on a cross-section, compared to the charge in vacuum general relativity. With this additional term, the flux associated with Lorentz symmetries, e.g. the angular momentum flux, is purely determined by the radiative degrees of freedom of the gravitational and Maxwell fields. In fact, the contribution to this flux by the Maxwell fields is given by the purely radiative Noether current flux and not by the stress-energy flux.

## Asymptotic symmetries & charges at spatial infinity

We analyze the asymptotic symmetries and their associated charges at spatial infinity in 4-dimensional asymptotically-flat spacetimes. The asymptotic fields and symmetries are treated in the covariant Ashtekar-Hansen formalism and live on the 3-manifold of spatial directions at spatial infinity, represented by a timelike unit-hyperboloid (or de Sitter space). Using the covariant phase space formalism adapted to the above description of spatial infinity, we derive the charges corresponding to asymptotic supertranslations and Lorentz symmetries at spatial infinity. We expect that our charge expressions will be suitable to prove the matching of the Lorentz charges at spatial infinity to those defined on null infinity, as has been recently shown for the supertranslation charges.

## Conservation of supermomentum from \(\mathscr I^-\) to \(\mathscr I^+\)

We show that asymptotic supertranslations and supermomenta on past null infinity can be matched to those on future null infinity using the Ashtekar-Hansen structure of spatial infinity. This provides an infinite number of conservation laws for nonlinear gravitational scattering in a general class of spacetimes as has been recently conjectured by Strominger.

## Conservation of asymptotic charges from \(\mathscr I^-\) to \(\mathscr I^+\)

We show that asymptotic symmetries and charges on past null infinity can be matched to those on future null infinity using the Ashtekar-Hansen structure of spatial infinity. This provides a conservation law for asymptotic charges from past to future null infinity in a general class of spacetimes. We focus on electromagnetic fields and comment on the generalization to supertranslations in General Relativity. We also comment on the extension to the full BMS group of symmetries.

## Supertranslation charge & flux on null surfaces

## Canonical energy & linear stability of Schwarzschild

Consider linearised perturbations of the Schwarzschild black hole in 4 dimensions. Using the linearised Newman-Penrose curvature component, which satisfies the Teukolsky equation, as a Hertz potential we generate a ‘new’ metric perturbation satisfying the linearised Einstein equation. We show that the canonical energy, given by Hollands and Wald, of the ‘new’ metric perturbation is the conserved Regge-Wheeler-like energy used by Dafermos, Holzegel and Rodnianski to prove linear stability and decay of perturbations of Schwarzschild. We comment on a generalisation of this strategy to prove the linear stability of the Kerr black hole.

## The first law of black hole mechanics for fields with internal gauge freedom

We derive the first law of black hole mechanics for physical theories based on a local, covariant and gauge-invariant Lagrangian where the dynamical fields transform non-trivially under the action of some internal gauge transformations. The theories of interest include General Relativity formulated in terms of tetrads, Einstein-Yang-Mills theory and Einstein-Dirac theory. Since the dynamical fields of these theories have some internal gauge freedom, we argue that there is no natural group action of diffeomorphisms of spacetime on such dynamical fields. In general, such fields cannot even be represented as smooth, globally well-defined tensor fields on spacetime. Consequently the derivation of the first law by Iyer and Wald cannot be used directly. Nevertheless, we show how such theories can be formulated on a principal bundle and that there is a natural action of automorphisms of the bundle on the fields. These bundle automorphisms encode both spacetime diffeomorphisms and internal gauge transformations. Using this reformulation we define the Noether charge associated to an infinitesimal automorphism and the corresponding notion of stationarity and axisymmetry of the dynamical fields. We first show that we can define certain potentials and charges at the horizon of a black hole so that the potentials are constant on the bifurcate Killing horizon, giving a generalised zeroth law for bifurcate Killing horizons. We further identify the gravitational potential and perturbed charge as the temperature and perturbed entropy of the black hole which gives an explicit formula for the perturbed entropy analogous to the Wald entropy formula. We then obtain a general first law of black hole mechanics for such theories. The first law relates the perturbed Hamiltonians at spatial infinity and the horizon, and the horizon contributions take the form of a “potential times perturbed charge” term. We also comment on the ambiguities in defining a prescription for the total entropy for black holes.

## First Law for fields with Internal Gauge

We extend the analysis of Iyer and Wald to derive the First Law of blackhole mechanics in the presence of fields charged under an ‘internal gauge group’. We treat diffeomorphisms and gauge transformations in a unified way by formulating the theory on a principal bundle. The first law then relates the energy and angular momentum at infinity to a potential times charge term at the horizon. The gravitational potential and charge give a notion of temperature and entropy respectively.

## First Law for fields with Internal Gauge

## Growth rate of Black Hole instabilities

## Growth rate of Black Hole instabilities

Hollands and Wald showed that dynamic stability of stationary axisymmetric black holes is equivalent to positivity of canonical energy on a space of linearised axisymmetric perturbations satisfying certain boundary and gauge conditions. Using a reflection isometry of the background, we split the energy into kinetic and potential parts. We show that the kinetic energy is positive. In the case that potential energy is negative, we show existence of exponentially growing perturbations and further obtain a variational formula for the growth rate.

## Growth rate of Black Hole instabilities

## Positive energy & Black Hole stability

Hollands and Wald showed that dynamic stability of stationary axisymmetric black holes is equivalent to positivity of canonical energy on a space of linearised axisymmetric perturbations satisfying certain boundary and gauge conditions. We show that the “kinetic energy” — the energy of the perturbations that are odd under reflection in \( t \) and \(\phi \) — is positive. We discuss implications of having a positive kinetic energy for proving exponential growth in the case where the “potential energy” can be made negative.

## Gauge, energy & Black Hole stability

## Gauge conditions & Black Hole stability

Hollands and Wald showed that dynamic stability of a black hole is equivalent to the positivity of canonical energy on a space of linearised perturbations satisfying certain boundary conditions and gauge conditions. The boundary/gauge conditions are naturally formulated on the space of initial data for the perturbations in terms of orthogonality to gauge transformations. These perturbations can be uniquely specified in terms of transverse-traceless tensors. Using these transverse-traceless data, positivity of kinetic energy for perturbations can be proven.

## Gauge conditions & Black Hole stability

Hollands and Wald showed that dynamic stability of a black hole is equivalent to the positivity of canonical energy on a space of linearised perturbations satisfying certain boundary conditions and gauge conditions. The boundary/gauge conditions are naturally formulated on the space of initial data for the perturbations in terms of orthogonality to gauge transformations. These perturbations can be uniquely specified in terms of transverse-traceless tensors. Using these transverse-traceless data, positivity of kinetic energy for perturbations can be proven.