Théorie

Long-range interactions, mediated for instance by photons and/or gravitons, force exclusive S-matrix elements to vanish in D=4 flat space-time, due to infrared divergences. This poses a challenge to programs, such as positivity and S-matrix bootstrap, that directly rely on the properties of 2-to-2 amplitudes. In this talk, I will introduce stripped amplitudes as IR-finite, analytic, crossing-symmetric and Regge-behaved avatars of standard amplitudes, associated with a physical detector scale \Lambda. In the regime in which the latter is taken exponentially small than all other scales, they also satisfy a form of unitarity, allowing to derive IR-finite positivity bounds on EFTs, in presence of long-range interactions.

Dissipation and noise arise across physics, from condensed matter to cosmology, whenever a system evolves in the presence of an unknown medium. The Schwinger–Keldysh formalism provides a systematic framework for describing such non-equilibrium and open-system dynamics. In this talk, I present an approach to cosmology that combines General Relativity with the Schwinger-Keldysh formalism. I show how symmetry, locality, and unitarity constrain the possible forms of dissipation and noise, and use these principles to derive the most general conservative and dissipative dynamics of scalar and tensor perturbations in a Friedmann–Lemaître–Robertson–Walker background. I then discuss how dissipative effects leave characteristic imprints on primordial non-Gaussianities and gravitational waves, and conclude with future prospects for dissipative dark sectors in the late universe.

Theory Seminar