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Cosmic time
Cosmic time, also called cosmological time, is the time coordinate of the FLRW metric, the proper time measured by observers at rest with the universe's expansion.
Friedmann equations
The Friedmann equations are the two general-relativity equations governing the expansion of a homogeneous, isotropic universe, linking the scale factor to energy density, pressure, curvature, and the cosmological constant.
Friedmann–Lemaître–Robertson–Walker metric
The Friedmann–Lemaître–Robertson–Walker (FLRW) metric is the spacetime metric describing a homogeneous and isotropic universe, first studied by Alexandr Friedman in 1922 and central to modern cosmology.