From Scattering to Expansion

Scientific Programme

Title and Description of talks.

Hayden Lee: Inflationary Correlators from Symmetries and Singularities

In flat space, scattering amplitudes in perturbation theory are highly constrained by from Lorentz symmetry, locality, and unitarity. In this talk, I will show how the analogous story works in the case of inflationary correlators. Specifically, I will describe the symmetries and singularities of tree-level three- and four-point functions in slow-roll inflation, and explain how these principles can be used to fully determine the final answer without reference to bulk time evolution.

Mark Trodden: Extending the Double Copy

I will discuss several recent papers on the double copy. I will first recap work extending the classical double copy correspondence to maximally symmetric curved spacetimes, clarifying the interpretation of these copies using several examples, and pointing out some peculiar features. I will then discuss how to generalize and extend the procedure relating gauge and gravity theories through color-kinematics replacements to relationships between pions and special Galileons, and between bi-adjoint scalar radiation and non-linear sigma model radiation. Finally, I will introduce work studying the double copy beyond leading order, leading into the following talk.

Mariana Carrillo: Shift symmetries, soft theorems, and the double copy

The non-linear sigma model and the special galileon are theories with non-linearly realized symmetries, which satisfy enhanced soft limits, and are related through the double copy. In this talk, I will explain how the equivalence of these properties breaks when treating the theories as effective field theories. I will also give some insights for understanding the differences and the required constraints to reconcile the equivalence.

Sam Wong: Cosmological soft theorem and soft bootstrap.

We consider the implication of non-linearly realized symmetries in cosmology for correlation functions, which is a set of soft theorems analogous to that in scattering amplitude. We introduce the physical mode condition in order for the soft theorems to apply on equal time correlation functions. A set of identities is derived for the quasi-single field scenario and they can be employed to constraint correlation functions.

Charlotte Sleight: a Mellin space approach to boundary correlators in (A)dS

Massimo Taronna: Bootstrapping dS exchanges in Mellin space

In this talk I will discuss how to bootstrap dS exchange amplitudes generalizing corresponding ideas from the AdS case. I will then discuss, as an application, their soft-limit and the corresponding Inflationary 3pt correlators.

Anson Hook: Minimal signatures of the Standard Model at the cosmological collider

We show that the leading coupling between a shift symmetric inflaton and the Standard Model fermions leads to an induced electroweak symmetry breaking due to particle production during inflation, and as a result, a unique oscillating feature in non-Gaussianities. The result is a one parameter model that determines both the mass and coupling signatures at the cosmological collider.

Jaroslav Trnka: Towards the Gravituhedron

I will show the evidence for the existence of the geometric picture behind graviton scattering amplitudes in four dimensions. In particular, I will present a new compact formula for the 6pt NMHV tree-level graviton amplitude, which appears to be very similar in structure to the geometric triangulation of the Amplituhedron representing the analogous gluon amplitude. This gives an indirect evidence for the existence of the yet-to-be discovered Gravituhedron geometry, where the new formula for the graviton amplitude corresponds to a geometric triangulation of such space.

Grant Remmen: Extremal Black Holes and EFTs

Higher-dimension operators in the action modify the extremality condition for black holes. In this talk, I will explore implications for these extremality corrections as a consequence of bounds on Wilson coefficients coming from scattering amplitudes. I will discuss connections to the Weak Gravity Conjecture and generalizations to dyonic, spinning, and BTZ black holes, as well as bounds on Wilson coefficients coming from consistency of black hole entropy.

Julio Parra Martinez: Universality in the classical limit of gravitational scattering

In this talk I will describe recent progress in calculating the gravitational deflection angle in theories with varying amounts of supersymmetry. I will explain how this quantity can be extracted from the large angular momentum limit of four point scattering amplitudes, as well as, how some old and new S-matrix theory techniques can make this task simpler. I will show through explicit calculation that the classical corrections to the massless angle are universal through two loops. Similarly, I will provide evidence that the same is true in the massive case at high energies. Finally, I will discuss the consequences of these results for the Post-Minkowskian (PM) Effective One Body (EOB) resummation techniques.

Michael Zlotnikov: Poincaré Constraints on Celestial Amplitudes

Finding new ways to express scattering amplitudes has the potential to reveal hidden features that were not accessible from other viewpoints. In flat space holography the plane wave basis in Minkowski space is transformed to a conformal primary wave function basis on the celestial sphere, allowing to map four dimensional Minkowski space amplitudes to two dimensional correlator like objects -- celestial amplitudes. Considering that asymptotic states that serve as input to scattering amplitudes originate at or near the celestial sphere, it is natural to seek an amplitude description entirely on this boundary, never referring to the bulk of Minkowski space. Poincaré symmetry generators act non-trivially on the celestial sphere. In this talk we will consider the constraints Poincaré symmetry imposes on lower point celestial amplitudes of massless particles of various spin, as well as massive scalars.

Enrico Herrmann: UV properties of gravitational scattering amplitudes

I will give a gentle introduction to the study of ultraviolet properties of (super-)gravity scattering amplitudes. Upon briefly reviewing standard expectations and the status of UV divergences in supergravity, I am going to present some integrand-level evidence of unexpected "enhanced cancellations" in the ultraviolet (special to four spacetime dimensions). These studies are based certain scaling properties of unitarity cuts that probe the infinite loop momentum region of amplitudes. With a broad audience in mind, I aim to explain the main tools that enter this UV analysis.

Shruti Paranjape: Soft Bootstrap and Galileons

The soft bootstrap utilizes behaviour of scattering amplitudes in the limit of vanishing momentum of one of the external particles to study effective field theories. We will focus on three applications of the soft bootstrap to Galileon theories: classification of theories, supersymmetrizations and higher derivative corrections.

Callum Jones: Born-Infeld at One-Loop

In this talk I will explain how aspects of the Born-Infeld model of nonlinear electrodynamics, obscured in the traditional formulation of Lagrangian field theory, are clarified by directly studying the on-shell S-matrix. In particular in 3+1-dimensions, classical Born-Infeld has an electromagnetic duality symmetry which manifests in tree-level scattering amplitudes as the conservation of a chiral charge. Whether this conservation law can be preserved under perturbative quantization is presently unknown. I will describe how generalized unitarity together with supersymmetric decomposition can be used to explicitly calculate infinite classes of one-loop amplitudes, and explain how the results are consistent with the existence of a quantum electromagnetic duality.

Mikhail Solon: Binary Black Holes and Scattering Amplitudes

Gravitational wave physics presents an opportunity for particle physicists to make an impact through novel application of tools from quantum field theory such as effective field theory and scattering amplitudes. I will give an overview of recent developments and open questions.

Marilena LoVerde: Separate Universe Techniques for Nonlinear Gravitational Evolution

I will introduce the "separate universe" approach to determining some aspects of nonlinear gravitational evolution: specifically the bias of tracers such as halos and voids and squeezed limit bispectra. I will motivate and define the separate universe, and then show how it can be generalized for cosmologies with multiple fluctuating components in the energy density -- either different fluids or adiabatic and isocurvature modes. Finally, I'll describe results using these techniques that explicitly demonstrate how small-scale observables depend on the growth-history of long-wavelength density fluctuations and new small-scale observables that can arise.

Chia-Hsien Shen: From Scattering to Inspiral

Many hidden simplicities of field theories are found in the program of scattering amplitudes. In this talk, I will describe how combining these advances can lead to direct applications in gravitational wave experiment. I will review the first calculation of two-body Hamiltonian at third post-Minkowskian order, and compared with the state-of-the-art calculation in the literature.

Andrés Luna-Godoy: A bestiary of classical double copies.

In this talk, I will describe various manifestations of the classical double copy,
from the Kerr-Schild approach to static black holes to very recent applications to the binary inspiral problem.

Victor Gorbenko: On light scalar fields in dS

I will show how the longstanding issue of infrared divergences present in theories of light selfinteracting scalar fields on dS background is resolved.

Suna Zekioğlu: Color-dual building blocks for higher derivative corrections

Higher derivative operators in gauge theory and gravity, important for renormalizability and capturing potential new UV physics, can prove to be challenging to study due to complexity in their tensor structures. I will introduce constructive building blocks for building these directly on-shell. Only three bi-colored scalar building blocks are required to saturate all higher-derivative corrections to the tree-level supersymmetric open string at four-points, and only a few more gauge building blocks reproduce the bosonic open string to all orders in alpha-prime.

Zhewei Yin: Novel constructions of non-linear sigma model amplitudes

Recent years have seen a surge of ideas on alternative ways to construct on-shell amplitudes of scalar effective field theories, including the non-linear sigma model (NLSM), and we present the latest developments on this subject. The soft recursion relation, also called the soft bootstrap, is known to reproduce leading-order single-trace amplitudes of NLSM up to all multiplicity. We extend it to the multi-trace case and discover a new series of amplitudes that satisfy Adler’s zero condition, distinct from the single-trace ones. We also generalize the method to the sub-leading interactions and find out that the soft bootstrap can recover the most general 4-derivative operators of a NLSM, including the Wess-Zumino-Witten term that encodes the chiral anomaly. The degrees of freedom allowed by the on-shell method exactly matches the results of the traditional Lagrangian approach. Moreover, our latest results show that there exists an amplitude relation between the single-trace and multi-trace amplitudes. A novel representation for 4-pt 4-derivative amplitudes is also discovered, which makes the flavor-kinematic duality manifest.

Daniel Baumann: Amplitudes Meet Cosmology

Cosmology is famously an observational rather than an experimental science. No experimentalists
were present in the early universe, and the experiment of the birth and subsequent evolution of
the universe cannot be repeated. Instead, we can only measure the spatial correlations between
cosmological structures at late times. A central challenge of modern cosmology is to construct
a consistent history of the universe that explains these correlations. In this colloquium, I will describe a new approach to determine cosmological correlations from consistency conditions alone, following a perspective familiar from the study of scattering amplitudes. I will also discuss more generally how recent breakthroughs in the field of scattering amplitudes could lead to new insights into the structure of cosmological correlations.