FriedmannLemaîtreRobertsonWalkermetrikkene

The Friedmann–Lemaître–Robertson–Walker (FLRW) metric is a fundamental concept in modern cosmology. It describes a homogeneous, isotropic, and expanding universe, which is a good approximation for the universe on large scales. The metric is a solution to Einstein's field equations of general relativity.

It was independently developed by Alexander Friedmann, Georges Lemaître, and later refined by Howard Robertson and Arthur Walker. Friedmann's initial work in 1922 and 1924 derived the metric and showed that the universe could be expanding. Lemaître, in 1927, independently arrived at a similar solution and used it to interpret Edwin Hubble's observations of galactic redshifts, proposing the first dynamical model of the universe. Robertson and Walker, in 1935 and 1937 respectively, provided a more rigorous and general mathematical framework for the metric, solidifying its role in cosmological theory.

The FLRW metric is characterized by a scale factor, denoted by a(t), which describes the expansion or contraction of the universe over time. The geometry of space at any given time is either flat, positively curved (spherical), or negatively curved (hyperbolic). The specific geometry and the evolution of the scale factor depend on the energy content of the universe, such as matter, radiation, and dark energy. The FLRW metric forms the basis for the standard Big Bang model of cosmology, providing the mathematical framework to understand the universe's expansion from an initial hot, dense state.