Molly Kaplan (UCSB) “Limitations of the QFT approximation in Global de Sitter: How Useful is a Quantum Observer?”

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Abstract: In de Sitter (dS), observers cannot be treated as separate from the rest of spacetime, and hence observables are inextricably linked to their observers. In this talk, we emphasize that observers must also be treated using QFT.  We then explore how well spaces of states and observables, each defined relative to a QFT observer, approximate standard non-gravitating QFT in a fixed dS background. We take into account that perturbative gravity in global dS requires states to be invariant under the dS isometries. This, however, leaves too few states, and so we build a new Hilbert space using group averaging. We study 1+1 dS with a one-particle observer in the static patch, and find that QFT in curved space is only a good approximation in a spacetime volume proportional to 1/G. Adding additional particles along a timelike path does not seem to increase the size of this “good” region.