Since the link came up...
http://arxiv.org/PS_cache/hep-th/pdf/0106/0106109.pdf [Broken]Edward Witten
Dept. of Physics, Cal Tech, Pasadena, CA and CIT-USC Center For Theoretical Physics, USC, Los Angeles CA
We discuss some general properties of quantum gravity in de Sitter space. It has been argued that the Hilbert space is of finite dimension. This suggests a macroscopic argument that General Relativity cannot be quantized – unless it is embedded in a more complete theory that determines the value of the cosmological constant. We give a definition of the quantum Hilbert space using the asymptotic behavior in the past and future, without requiring detailed microscopic knowledge. We discuss the difficulties in defining any precisely calculable or measurable observables in an asymptotically de Sitter spacetime, and explore some meta-observables that appear to make mathematical sense but cannot be measured by an observer who lives in the spacetime. This article is an expanded version of a lecture at Strings 2001 in Mumbai. June, 2001 [continued]
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