Timeline of cosmological theories
A chronological account of ideas and scientific models proposed to explain the universe’s origin, structure, and evolution.
Ancient Greek cosmology: Ancient Greek accounts of the universe, including models of its structure, motion, and place in nature. Early Greek thinkers offered influential geocentric and atomist pictures of the cosmos.
General relativity: Einstein's theory of gravity, describing it as the curvature of spacetime produced by matter and energy. Its equations allow cosmological models in which space itself expands or contracts.
Aristotle: An ancient Greek philosopher whose natural philosophy described a finite, geocentric cosmos of nested celestial spheres. His influential account made a stationary Earth central to classical cosmology.
Steady-state theory: A cosmological model proposing that the universe expands while maintaining constant average density through continuous matter creation. It rivaled the Big Bang by denying a unique hot, dense beginning.
Stellar parallax: The apparent annual shift in a nearby star's position caused by Earth's orbit around the Sun. Its eventual measurement supported Earth's motion and the vast scale of the stellar universe.
Ptolemaic system: A geocentric astronomical model that used deferents and epicycles to describe planetary motions. It formalized Earth-centered cosmology and remained dominant for centuries.
Friedmann equations: Equations derived from general relativity that govern the expansion of homogeneous, isotropic universes. They describe how cosmic expansion changes with matter, radiation, and curvature.
Nicolaus Copernicus: A Renaissance astronomer who developed a mathematical model placing the Sun near the center of planetary motion. His heliocentric proposal marked a decisive break with Ptolemaic astronomy.
Oscillatory universe: A cosmological scenario in which expansion is followed by contraction and a possible new expansion phase. It offers a cyclic alternative to a single uninterrupted expansion history.
Olbers' paradox: The question of why the night sky is dark if an infinite, eternal, static universe contains stars everywhere. Its resolution helped expose problems with a simple eternal static cosmos.