We study time evolution in a simple model of de Sitter quantum gravity, namely, Jackiw-Teitelboim with a positive cosmological constant. We find that time evolution is isometric rather than unitary. The states that are projected out under time evolution correspond to initial conditions that crunch. Our findings suggest that knowledge of bulk physics, even on arbitrarily large timescales, is insufficient to deduce the de Sitter $S$ matrix.

• Received 16 May 2023
• Revised 11 September 2023
• Accepted 11 October 2023

DOI:https://doi.org/10.1103/PhysRevLett.131.211601

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Published by the American Physical Society