Observers and Observations in de Sitter Space
The key word in the title is “IN”. Unlike AdS where observers can be thought of as living on the boundary of space, or even in a laboratory beyond the boundary, in dS the observer is part of the system; he or she lives “in” the de Sitter spacetime. Even worse, observers are mere transient fluctuations that originate on the horizon and eventually fall back into it. The ordinary rules of quantum mechanics need to be understood in such a setting. Chandrasekhan, Longo, Pennington, and Witten have made some important contributions to this problem but their discussion is incomplete.
This is not merely a philosophical problem. Solving it is essential for knowing what and how to calculate in a holographic theory of dS.