Vacua in locally de Sitter cosmologies
We analyse the response of a comoving local quantum probe, modelled as an Unruh-DeWitt detector coupled to a massive scalar field, in (1+1)-dimensional spatially homogeneous locally de Sitter cosmologies with compact spatial sections, allowing for the scale factor hyperbolic cosine, hyperbolic sine, and exponential evolution laws, as inherited from different local foliations of the de Sitter geometry. For the sinh and exp scale factors, we consider adiabatic vacua in the past, identifying an ambiguity due to a massive zero mode, while for the cosh and sinh expansion laws, we consider large momentum adiabatic vacua and states induced from the Euclidean vacuum; for the latter, we also evaluate the expectation value of the field's stress-energy tensor. Numerical plots are given in selected parameter regimes.