Anthropic principle

The principle that observations of the universe are constrained by the conditions necessary for observers to exist. It distinguishes what observers can observe from what may exist independently of observation.

Weak anthropic principle: The principle that observed cosmic conditions must permit observers to exist at the epoch and location of observation. It expresses the basic selection effect without claiming the universe was designed for observers.
Selection bias: A systematic distortion arising when the process that selects observations makes some outcomes more likely to be included. Observer-dependent sampling is a special case of selection bias.
Fine-tuning (physics): The apparent sensitivity of physical phenomena, including life, to the values of fundamental constants or initial conditions. Anthropic reasoning is often invoked to explain why observed constants fall within life-permitting ranges.
Fine-tuning argument: An argument that the apparent life-sensitivity of physical parameters calls for explanation. It frames the explanatory question that anthropic reasoning is often used to address.
Strong anthropic principle: A family of claims that the universe must have properties allowing observers to arise at some stage. Its stronger formulations go beyond the limits on observation asserted by the weak version.
Observer selection effect: A bias in observed data caused by the requirement that observers be present to make observations. This is the statistical mechanism behind anthropic reasoning.
Cosmological constant problem: The discrepancy between the observed vacuum energy density and the much larger values suggested by theoretical estimates. The small observed value has inspired anthropic explanations based on limits for galaxy formation.
Design argument: An argument that features of the universe support the existence or activity of an intelligent creator. Anthropic explanations are sometimes contrasted with design claims about life-permitting conditions.
Participatory anthropic principle: John Archibald Wheeler’s proposal that observers are necessary to bring the universe into being in some sense. It gives observers a constitutive role rather than treating them only as selection effects.
Self-sampling assumption: The assumption that an observer should reason as though they are randomly selected from a specified class of observers. It formalizes how an observer might reason about their place in a population of possible observers.

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