Cosmological principle

The assumption that the universe is homogeneous and isotropic on sufficiently large scales: its properties are broadly uniform and look the same in every direction.

Homogeneity: The property of being uniform across locations at a chosen scale. It captures the principle’s claim that large regions of space have broadly similar properties.
Friedmann equations: Equations describing how the scale factor of a homogeneous, isotropic universe evolves under gravity. They translate the principle’s symmetries into predictions for cosmic expansion.
Cosmic microwave background: Relic radiation released when the early universe became transparent, now observed across the sky. Its near-uniform temperature is a central observational sign of large-scale isotropy.
Cosmic isotropy: The observed or assumed sameness of the universe’s large-scale properties across directions in the sky. Isotropy alone is weaker than the principle, which also requires spatial homogeneity.
Isotropy: The property of having the same properties in every direction around a point. It expresses the directional uniformity required by the principle.
Friedmann–Lemaître–Robertson–Walker metric: A spacetime geometry representing a homogeneous and isotropic universe with an evolving scale factor. This metric encodes the spatial symmetries assumed by the principle.
Cosmic microwave background anisotropy: Small directional variations in the temperature and polarization of the cosmic microwave background. Its measured fluctuations reveal departures from perfect isotropy and seed later structure.
Cosmic homogeneity: The large-scale uniformity of the universe’s properties from place to place. Homogeneity alone does not guarantee that observations look the same in every direction.
Copernican principle: The idea that Earth and its location are not privileged in the universe. It motivates extending observations from our location to the cosmos as a whole.
Scale factor: A dimensionless quantity describing how distances between comoving points change with cosmic time. It tracks expansion once large-scale uniformity reduces cosmic evolution to one function of time.

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