[Revised entry by Simon Friederich on February 26, 2026.
Changes to: Main text, Bibliography]
The term fine-tuning refers to situations where certain phenomena of interest depend sensitively on parameter values falling within extremely narrow ranges. The term is often applied to theories or models in physics, in particular cosmology: a theory or model is said to be fine-tuned when it reproduces observed phenomena only if its parameters take values within an extremely narrow range among those that the theory in principle allows. In such cases, small changes in those parameters would lead the theory to predict radically…

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