Vacua in locally de Sitter cosmologies, and how to distinguish them

Abstract: (1+1)-dimensional locally de Sitter Friedmann-Robertson-Walker cosmologies with compact spatial sections allow cosh, sinh and exponential evolution laws, each with a freely-specifiable spatial circumference parameter, and the value of this parameter has an invariant geometric meaning for the cosh and sinh evolution laws. We identify geometrically preferred states for a quantised massive scalar field on these cosmologies, some singled out by adiabatic criteria in the distant past, with an ambiguity remaining due to a massive zero mode, and others induced from the Euclidean vacuum on standard (1+1)-dimensional de Sitter space by a quotient construction. We show that a comoving quantum observer, modelled as an Unruh-DeWitt detector, can distinguish these states from the Euclidean vacuum on standard de Sitter space. Numerical plots are given in selected parameter regimes. We also evaluate the field's stress-energy tensor expectation value for those states that are induced from the Euclidean vacuum by a quotient construction.