An extension of the cosmological principle that assumes the universe is homogeneous and isotropic not only in space but also in time, meaning it has always looked essentially the same. This principle was the foundation of steady-state cosmology.
Extended from 'cosmological principle' by adding 'perfect' (Latin 'perfectus' meaning completed/finished) to emphasize temporal uniformity. Proposed by Hermann Bondi, Thomas Gold, and Fred Hoyle in 1948 as the basis for steady-state theory.
The perfect cosmological principle led to the wild idea that new matter must spontaneously appear from nothing to maintain constant density as the universe expands—about one hydrogen atom per cubic meter every billion years! While this steady-state theory was eventually disproven by the cosmic microwave background, it forced cosmologists to think more rigorously about the universe's evolution.
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