Lecture notes prepared for the 2021 SAGEX PhD School in Amplitudes hosted by the
University of Copenhagen August 10th through 13th. Topics covered include: the …

We review the role that infinite-dimensional symmetries arising at the boundary of
asymptotically flat spacetimes play in the context of the celestial holography program. Once …

A bstract All 4D gauge and gravitational theories in asymptotically flat spacetimes contain an
infinite number of non-trivial symmetries. They can be succinctly characterized by …

A short, graduate-level synthesis of recent developments in theoretical physics, from a
pioneer in the field Lectures on the Infrared Structure of Gravity and Gauge Theory presents …

A bstract Celestial amplitudes represent 4D scattering of particles in boost, rather than the
usual energy-momentum, eigenstates and hence are sensitive to both UV and IR physics …

We study solutions of the Klein-Gordon, Maxwell, and linearized Einstein equations in R 1,
d+ 1 that transform as d-dimensional conformal primaries under the Lorentz group SO (1, d+ …

A bstract The operator product expansion of massless celestial primary operators of arbitrary
spin is investigated. Poincaré symmetry is found to imply a set of recursion relations on the …

The soft limits of scattering amplitudes have been extensively studied due to their essential
role in the computation of physical observables in collider physics. The universal …

Celestial holography proposes a duality between the gravitational $\mathcal {S} $-matrix
and correlators in a conformal field theory living on the celestial sphere. In this white paper …

A bstract The four-dimensional S-matrix is reconsidered as a correlator on the celestial
sphere at null infinity. Asymptotic particle states can be characterized by the point at which …