Cosmology Seminar: Perturbative Non-Equilibrium Thermal Field Theory.

Peter Millington, Physics Department, University of Sheffield

Wednesday 15 May 2013, 1400-1500
Cavendish Colloquium Room

We present a new perturbative formulation of non-equilibrium thermal quantum field theory. The resulting time-dependent diagrammatic perturbation series are built from non-homogeneous free propagators and time-dependent vertices, which are able to encode the explicit space-time dependence of the statistical background. These perturbative expansions remain free of so-called pinch singularities, without the need for effective resummation of finite widths. Thus, our approach does not suffer from the mathematical pathologies previously thought to spoil truly perturbative treatments of non-equilibrium field theory. Arriving at a physically meaningful definition of particle number densities that does not rely upon quasi-particle approximations, we derive master time evolution equations for statistical distribution functions, which are valid to all orders in perturbation theory and to all orders in a gradient expansion. For a scalar model, we truncate these evolution equations in a perturbative loopwise sense, whilst still capturing the dynamics on all time-scales. We show that the early-time transient behaviour is dominated by non-Markovian energy-violating processes, which result from the systematic incorporation of finite-time effects.