Georges Lemaître

Georges Lemaître was a Belgian priest and physicist who derived an expanding-universe model from general relativity and proposed that the universe began in a dense state he called the primeval atom.

General relativity: Albert Einstein’s theory of gravity, describing it as the curvature of spacetime produced by matter and energy. Lemaître used its field equations to derive a universe that expands.
Expanding universe: A cosmological model in which distances between sufficiently separated galaxies increase over time. Lemaître’s 1927 solution described a universe whose large-scale distances change with time.
Primeval atom: Lemaître’s proposal that the universe began in an extremely compact state that fragmented as it expanded. This was his distinctive early account of the universe’s origin.
Albert Einstein: A German-born theoretical physicist who formulated general relativity and the mass–energy relation. Einstein initially doubted Lemaître’s expanding solution, then accepted that the equations permitted it.
Friedmann equations: Equations from general relativity that describe how a homogeneous, isotropic universe changes over time. They provided a mathematical route to the expanding solutions Lemaître independently developed.
Hubble–Lemaître law: The relation that a galaxy’s recessional velocity is proportional to its distance at relatively small cosmological distances. Lemaître derived the relation and estimated its proportionality from available observations.
Quantum mechanics: The physical theory describing matter and energy at atomic and subatomic scales. Lemaître drew on ideas about atomic disintegration when imagining cosmic beginnings.
Pope Pius XII: Head of the Catholic Church from 1939 to 1958, who publicly connected modern cosmology with creation. Lemaître urged him not to present the Big Bang as proof of Christian doctrine.
Cosmological principle: The assumption that the universe is homogeneous and isotropic on sufficiently large scales. This simplifying assumption underlies the class of cosmological models Lemaître studied.
Edwin Hubble: An American astronomer whose observations established the extragalactic nature of many nebulae and quantified galaxy recession. Hubble’s later distance–velocity results helped make cosmic expansion widely recognized.

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