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In theoretical physics
Theoretical physics

Theoretical physics employs mathematical models and abstractions of physics in an attempt to explain experimental data taken of the natural world....
, 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
Anthropic principle

In physics and cosmology, the anthropic principle is the collective name for several ways of asserting that physical and chemistry theories, especially astrophysics and cosmology, need to take into account that there is life on Earth, and that one form of that life, Homo sapiens, has attained sapience....
.

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 indicate that a piece of the story is missing.






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In theoretical physics
Theoretical physics

Theoretical physics employs mathematical models and abstractions of physics in an attempt to explain experimental data taken of the natural world....
, 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
Anthropic principle

In physics and cosmology, the anthropic principle is the collective name for several ways of asserting that physical and chemistry theories, especially astrophysics and cosmology, need to take into account that there is life on Earth, and that one form of that life, Homo sapiens, has attained sapience....
.

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 indicate that a piece of the story is missing. For example, the cosmological constant problem (why is the cosmological constant
Cosmological constant

In physical cosmology, the cosmological constant was proposed by Albert Einstein as a modification of his original theory of general relativity to achieve a Einstein's universe....
 so small?); the hierarchy problem
Hierarchy problem

In theoretical physics, a hierarchy problem occurs when the fundamental parameters of some Lagrangian mechanics are vastly different from the parameters measured by experiment....
; 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
Flatness problem

The flatness problem is a physical cosmology fine-tuning problem within the Big Bang model; i.e., the observation that the current density of the universe is very close to critical density at which space is perfectly flat....
, 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.

See also

  • Fine-tuned universe
    Fine-tuned universe

    The fine-tuned Universe is the idea that the conditions that allow life in the Universe can only occur when certain universal physical constants lie within a very narrow range, so that if any of several fundamental constants were only slightly different the universe would be unlikely to be conducive to the establishment and development of mat...
  • Naturalness (physics)