Week 05.03.2023 – 11.03.2023

Quantum coherence plays essential role in diverse phenomena of relevance in the early universe. Examples include activation of sterile neutrinos, electroweak baryogenesis, resonant leptogenesis and particle production in phase transitions and during the (p)reheating stage after inflation. After a general introduction I will concentrate on the particle production problem. I will show how one can derive coupled, renormalized and tractable quantum kinetic equations for the scalar field 1- and 2-point functions starting from the CTP-formalism, working in the Hartree approximation of the 2PI-action. I will then apply these equations to study particle production and the back-reaction from the non-equilibrium modes on the dynamics of the one-point function. We will see both spinodal and parametric resonances taking place and sometimes overlapping in a novel way, and we follow the process of decoherence and thermalization. Overall, I argue that advanced quantum transport equations are necessary for an accuarate description of many systems of acute interest in cosmology.

I will discuss the classical and quantum symmetries of TTbar-deformed CFTs and their manifestations in holography. These symmetries are infinite in number and, in a certain basis, organise into a Virasoro x Virasoro algebra with the same central charge as that of the undeformed CFT. I will present a quantum, abstract proof of the existence of these symmetries and three different explicit classical perspectives: Hamiltonian, Lagrangian and holographic. I will then discuss the relationship between the single-trace TTbar deformation and the asymptotically linear dilaton background in string theory, and show that the asymptotic symmetries of this background take an identical form to those of TTbar-deformed CFTs, further strengthening this proposed connection.

I will describe progress in formulating and solving QFT non-perturbatively in the infinite volume limit using Hamiltonian Truncation. I will present new results for time-dependent observables in a certain class of both Lagrangian and non-Lagrangian theories in 1+1d. I will also present our study of chaos and thermalization for a scalar theory, where we test the Eigenstate Thermalization Hypothesis (ETH). While we do find results which are broadly consistent with ETH, at weak coupling we also find a set of “scar” states, which do not satisfy Random Matrix Statistics, and which can be distinguished from the rest of the thermal states by the expectation value of local operators.