In theoretical physics, fine-tuning refers to circumstances when the parameters of a model must be adjusted very precisely in order to agree with observations. Theories requiring fine-tuning are regarded as problematic in the absence of a known mechanism to explain why the parameters happen to have precisely the needed values. Explanations often invoked to resolve fine-tuning problems include natural mechanisms by which the values of the parameters may be constrained to their observed values, and the anthropic principle.
The necessity of fine-tuning leads to various problems that do not show that the theories are incorrect, in the sense of falsifying observations, but nevertheless suggest that a piece of the story is missing. For example, the cosmological constant problem (why is the cosmological constant so small?); the hierarchy problem; the strong CP problem, and others.
An example of a fine-tuning problem considered by the scientific community to have a plausible "natural" solution is the cosmological flatness problem, which is solved if inflationary theory is correct: inflation forces the universe to become very flat, answering the question of why the universe is today observed to be flat to such a high degree.
Famous quotes containing the word fine-tuning:
“Our species successfully raised children for tens of thousands of years before the first person wrote down the word “psychology.” The fundamental skills needed to be a parent are within us. All we’re really doing is fine-tuning a process that’s already remarkably successful.”
—Lawrence Kutner (20th century)