Fine-tuning

Fine-tuning is the claim that small changes in physical parameters would substantially alter the universe or prevent complex structures such as stars and life from arising.

Physical constant: A measurable quantity in a physical theory whose value is fixed within that theory. Fine-tuning arguments examine whether changing such quantities would alter the universe.
Big Bang nucleosynthesis: The early-universe process that formed the lightest atomic nuclei from protons and neutrons. Its outcomes depend on early cosmic conditions and the strengths of physical interactions.
Cosmological natural selection: A hypothesis that black holes produce new universes with slightly altered physical parameters. It offers an evolutionary explanation for parameter values rather than treating them as unexplained coincidences.
Cosmological constant problem: The discrepancy between theoretical estimates of vacuum energy and the small value inferred from cosmological observations. It is a sharp case where a measured parameter appears far smaller than some physical estimates predict.
Dimensionless physical constant: A physical constant expressed as a pure number, independent of units. Its value can be compared across unit systems without ambiguity.
Stellar nucleosynthesis: The production of chemical elements through nuclear reactions inside stars. Changes to fundamental parameters could prevent stars from making the elements needed for planets and life.
Multiverse: A proposed collection of universes, potentially with different physical properties. A large ensemble could make life-permitting conditions less surprising if its universes vary in their parameters.
Initial conditions of the universe: The physical state of the universe at the beginning of its expansion. Fine-tuning arguments include questions about whether the universe began in an unusually special state.
Fundamental interaction: A basic interaction described by physics, such as gravity or electromagnetism. Changing interaction strengths can affect matter, stars, and cosmic structure.
Nuclear binding energy: The energy required to separate an atomic nucleus into its constituent protons and neutrons. Its sensitivity to physical parameters affects which nuclei can form and persist.

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