The hydrohedron: rigorous bounds on transport from causality

• Benjamin Withers

Room: Room D As an effective theory constructed in a gradient expansion, relativistic hydrodynamics is fixed by symmetries up to a set of transport coefficients. I will utilise bootstrap techniques to rule out theories that are inconsistent with microscopic causality. What remains is a universal convex geometry in the space of transport coefficients, "the hydrohedron”. I will also discuss the necessary role played by non-hydrodynamic degrees of freedom in achieving causality. Based on 2212.07434 and 2305.07703.

Analyticity and causality in Lorentz-breaking backgrounds: scattering vs correlators

• Alessandro Podo
• Shengjia Zhou

Room: Room D We will describe the analytic structure and positivity properties of the perturbative S-matrix of excitations in a relativistic phi^4 superfluid. Motivated by this, we will then discuss how microcausality implies non-perturbative analyticity properties of correlators in a mixed (t,k) representation. This result applies to generic relativistic QFTs in homogeneous and isotropic backgrounds, including finite temperature or density, and cosmological spacetimes.

Dressing and Screening in de Sitter

• Lorenzo Di Pietro

Room: Room D For a gauge theory in dS, I will discuss how to define and compute properly dressed late-time correlation functions of charged fields, and an analogue of Debye screening that affects the late-time decay of the electric field. I will briefly review the use of conformal symmetry to reveal analyticity and positivity properties of late-time correlation functions, and discuss these properties in the gauge theory example.

An Open System Approach to Gravity and Cosmology

• Enrico Pajer

Room: Room D Cosmological models and predictions rely extensively on the well-established field theory framework of particle physics. However, a qualitatively new challenge arises: cosmological systems inherently contain substances with poorly constrained macroscopic properties and entirely unknown microphysics, such as the inflaton sector, dark matter, and dark energy. This results in a rich array of novel phenomena, including dissipation, stochastic fluctuations, out-of-equilibrium dynamics, and non-unitary macroscopic evolution. Moreover, since gravitational observables are of primary interest, and gravity universally couples to all forms of matter, a precise description of all cosmic constituents is required—something feasible only in the simplest toy models. To address these challenges, I propose an open system approach to cosmology. I begin with a pedagogical introduction to open quantum system techniques, formulated within the Schwinger-Keldysh path integral framework. Then, I present the open effective field theory of inflation as a general class of theories of single-field inflation in the presence of an unknown medium. This local dissipative single-field effective theory yields a new class of predictions for cosmological correlators, generalizing existing models. I then tackle the challenge of formulating general relativity in the presence of an unspecified medium. As a warmup, I present a Schwinger-Keldysh formulation of electromagnetism in a medium, incorporating dissipation and fluctuations while ensuring a consistent treatment of gauge symmetries within an open system framework. Building on these results, I introduce the general and systematic construction of dissipative extensions of general relativity and explore their implications for modeling open dark energy and the late-time evolution of the universe. Finally, I study the implications for the dissipative propagation of gravitational waves through the dark sector medium.

Bispectrum Islands

• Sadra Jazayeri

Room: Room D Motivated by the success story of the modern S-matrix bootstrap in mapping out the space of scattering amplitudes consistent with fundamental principles, I explore how a similar framework can be applied to cosmological correlators generated during inflation. In particular, I focus on the bispectrum of curvature perturbations sourced by interactions with a generic hidden sector during inflation, and I show that the result must obey a set of positivity constraints stemming from the interplay between unitarity and de Sitter isometries. While I assume certain broad features of the hidden sector, I remain agnostic about its microscopic details. As a result, the derived positivity bounds are broadly applicable to a wide range of inflationary UV completions—whether the bispectrum arises from the exchange of weakly coupled fields, such as Kaluza-Klein modes or moduli in extra-dimensional models, or from operators with continuous spectra, as in conformal field theories.

Graviton loops and negativity

• Julio Parra-Martinez

Room: Room D I will explain some challenges introduced by loop corrections to gravitational EFT bounds on Wilson coefficients and how they can be overcome. Time permitting, I will show that bounds without IR divergences from massless graviton exchange can be obtained under certain reasonable assumptions.

IFPU Colloquium: Spontaneously broken spacetime symmetries and a quantum bound on transparency

• Alberto Nicolis

Room: Room D I will show how the low-energy dynamics of different phases of matter---different "materials"---can be conveniently described within the framework of relativistic quantum field theory, in particular in terms of spontaneous symmetry breaking, Goldstone excitations, and effective field theory. As an application of the formalism, I will show how to derive an absolute bound on how transparent a material can be.