 Alexander Friedmann

Alexander Friedmann derived evolving homogeneous cosmological solutions of the Einstein field equations. The Friedmann equations relate cosmic expansion to energy density, pressure, spatial curvature and the cosmological constant.

The conceptual change is that general relativity permits the scale of a homogeneous universe to evolve, rather than requiring a static geometry. Observational evidence and the choice of matter content are additional inputs needed to identify which mathematical solution describes the Universe.^[ref]

Alexander Friedmann was a Russian physicist and mathematician known for his pioneering work in cosmology during the early 20th century. He is best known for the Friedmann equations, which are central to modern cosmology and describe how the universe expands over time. Friedmann's equations emerged from his analysis of Einstein's general theory of relativity and addressed the dynamics of a homogeneous and isotropic universe. His work laid the groundwork for the Big Bang theory of the universe's origin.