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Many modern cosmological scenarios feature large volumes of spacetime in a deSitter vacuum phase. Such models are said to be faced with a "Boltzmann Brainproblem" - the overwhelming majority of observers with fixed local conditionsare random fluctuations in the de Sitter vacuum, rather than arising viathermodynamically sensible evolution from a low-entropy past. We argue thatthis worry can be straightforwardly avoided in the Many-Worlds (Everett)approach to quantum mechanics, as long as the underlying Hilbert space isinfinite-dimensional. In that case, de Sitter settles into a truly stationaryquantum vacuum state. While there would be a nonzero probability for observingBoltzmann-Brain-like fluctuations in such a state, "observation" refers to aspecific kind of dynamical process that does not occur in the vacuum (which is,after all, time-independent). Observers are necessarily out-of-equilibriumphysical systems, which are absent in the vacuum. Hence, the fact thatprojection operators corresponding to states with observers in them do notannihilate the vacuum does not imply that such observers actually come intoexistence. The Boltzmann Brain problem is therefore much less generic than hasbeen supposed.

Why Boltzmann Brains Don't Fluctuate Into Existence From the De Sitter Vacuum