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

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