On the temperature of a causal diamond in algebraic quantum field theory.
Tomita-Takesaki theory; massless scalar AQFT; KMS condition; double cone.
In this thesis, we study the temperature of an abstract spacetime region called a double cone, which is also referred to as a diamond. The framework under consideration is free massless scalar field theory treated within the algebraic quantum field theory approach. Before discussing the main result we review, somewhat rigorously, the von Neumann algebras, their type classification, the Tomita-Takesaki (TT) modular theory and KMS condition, which are indispensable parts of the model used in our analysis. We also provide an extensive discussion on some known results, in particular, geometrical transformations and the corresponding modular operators for interconnected spacetime regions: a right wedge, a forward lightcone and a diamond. This review serves as a stepping stone of our studies. As a main result, we present an intrinsic definition of temperature in terms of an inverse temperature vector field that can be computed without referring to a particular TT trajectory. This vector field reproduces the Unruh temperature for a right wedge. Later it is applied to compute the temperature of a diamond. We also consider some counterintuitive limits in which the modular flow of a diamond resembles that of Minkowski spacetime or a wedge. While in the former case the stipulated behavior is found in the center of a diamond, away from the boundaries, in the latter case it is close to the boundaries, more specifically, close to left and right corners.