• stefan-boltzmann law
• quantum field theory
• particle density
• out of equilibrium

Out of equilibrium physics is a branch of modern QFT currently under development.
Some powerful formalisms have been developed - the Keldysh one, for example, is extremely useful and widely applied - yet the lack of a good framework and the complexity of the calculations limit the potentiality of these approaches.
This thesis has been written with the hope that some important quantities can be computed even far from equilibrium in a simple manner. The driving idea was to adopt the formalism of quantum field theory to look for corrections to known laws when a small interaction drives a system out of equilibrium. In particular we consider two-component systems. Each of the components is in contact with a heat reservoir and the reservoirs are in general at different temperatures and chemical potentials. Switching on an interaction drives the system away from equilibrium and modifies the known equilibrium laws. In this context our new results concern the first nonequilibrium correction to particle density and to the Stefan-Boltzmann law.
This thesis is organized as follows. A concise introduction to the theory is developed in second and third chapters, spanning from definition of a Gibbs state to a formulation of perturbative expansion in the coupling constant when an interaction is present; in the second part (chapters 4 and 5) the reader can find calculations obtained in this work. Along with them, a brief discussion of each result can be found.
Finally, in Chapter 6 are discussed possible future developments and some fields of applicability of this work.