Maggiani, Marco
(2025)
Effective Realism and the Problem of Boltzmann Brains.
[Preprint]
Abstract
The Boltzmann brain problem threatens to undermine our use of cosmological evidence. If our best-fit cosmological model, ΛCDM, holds indefinitely into the future, most observers with our evidence would be random fluctuations rather than ordinarily evolved observers like ourselves. This seems to erode the connection between our data and ΛCDM itself. Existing responses to the problem, such as denying our typicality, appealing to externalist evidence, or assigning zero priors to cognitively unstable theories, all face serious difficulties. I propose a different solution: ΛCDM should be interpreted as an effective theory whose domain of applicability does not extend to the extreme time scales that lead to a numerical domination by Boltzmann brains over ordinary observers. This is analogous to the case of quantum field theory, which is highly successful within its own domain of applicability, but breaks down at sufficiently short length scales. On this interpretation, the skeptical issue is resolved not by new epistemic rules but by recognizing that ΛCDM, like other effective theories, must be realized by a more fundamental description. The existence of many possible such descriptions compatible with our evidence, and the way they respond to evidence under self-locating uncertainty, already dissolves the skeptical problem.
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