Georges Lemaître: The Primeval Atom at the Origin of the Cosmos

Brief Biography of Georges Lemaître

Georges Lemaître was born on July 17, 1894, in Charleroi, Belgium. After studying engineering, he turned to mathematics and physics while entering the priesthood: he was ordained a Catholic priest in 1923. Passionate about relativistic cosmology, he went to study with Arthur Eddington (1882-1944) in Cambridge, then at Harvard and MIT, where he obtained a doctorate in physics.

In 1927, Lemaître published an article in French in which he derived a dynamic solution to Einstein's equations, suggesting that the Universe is expanding. He linked this dynamic to observations of the redshift of galaxies. In 1931, he took the idea further by introducing the hypothesis of a "primeval atom," the first physical model of an expanding universe from an initial dense and hot state.

Contrary to popular belief, Lemaître saw no contradiction between science and faith: he insisted on the fundamental distinction between the realm of physics and that of metaphysics, refusing any theological appropriation of his cosmological models. A member of the Pontifical Academy of Sciences, he became its president in 1960. He died on June 20, 1966, in Louvain, a few days after learning of the discovery of the cosmic microwave background, which validated his initial vision.

Genesis of the Universe: The Primeval Atom

In 1931, the Belgian canon Georges Lemaître (1894–1966) proposed a bold idea: the Universe emerged from a "Primeval Atom," an extremely dense and hot initial state, whose decay marked the beginning of space, time, and matter. This hypothesis gave physical meaning to the expansion of the Universe, whose mathematical foundations had already been laid in 1922 by the Russian physicist Alexander Friedmann (1888-1925). Unlike Friedmann, who remained on the theoretical level, Lemaître linked cosmic expansion and observations (redshift) and postulated a beginning. At a time when Albert Einstein (1879-1955) still favored a static cosmos, Lemaître introduced a dynamic vision that foreshadowed the future model of the Big Bang.

Lemaître relied on Einstein's equations, particularly on the expanding universe solutions derived by Friedmann. But where Friedmann limited himself to mathematics, Lemaître dared a physical interpretation: he postulated that all the matter in the Universe was concentrated in a single initial point, an "atom," whose decay produced space, time, and matter. This concept, both scientific and deeply philosophical, fits into a unified vision of the laws of nature.

Opposition and Confirmations

The idea was initially marginalized, even by Einstein himself, who considered the proposal "abominable." However, the observations of the time, notably the redshift of galaxies discovered by Hubble, supported an expanding universe. In 1965 (one year before Lemaître's death), the discovery of the 3 K radiation of the cosmic microwave background by Arno Allan Penzias (1933-2024) and Robert Woodrow Wilson (1936-) constituted a brilliant confirmation of this hypothesis. This fossil radiation is interpreted as the thermal echo of the initial explosion postulated by Lemaître.

The Primeval Atom and the Singularity

Lemaître's "primeval atom" is today interpreted as a cosmological singularity, a point of infinite density and curvature of spacetime within the framework of general relativity. This extreme scenario corresponds to the moment \( t = 0 \) in the so-called Friedmann-Lemaître-Robertson-Walker (FLRW) solution, which describes a homogeneous, isotropic universe in expansion or contraction. The FLRW metric implies that the scale factor \( a(t) \rightarrow 0 \) as \( t \rightarrow 0 \), which causes the energy density \( \rho(t) \sim \frac{1}{a(t)^3} \) and the temperature \( T(t) \sim \frac{1}{a(t)} \) to diverge.

But this singularity does not necessarily mean a real physical point: it rather reflects an incompleteness of the theory. General relativity, although extremely precise on a large scale, does not take into account quantum effects. However, at scales smaller than the Planck length (\( \ell_P \sim 1.6 \times 10^{-35} \, \mathrm{m} \)) and times shorter than the Planck time (\( t_P \sim 5.4 \times 10^{-44} \, \mathrm{s} \)), it becomes imperative to describe gravity in quantum terms.

Theories such as loop quantum gravity or string theory suggest that the singularity could be replaced by a quantum transition or a pre-Big Bang phase, or even a cosmic bounce ("bounce"). In these models, the universe does not emerge from a point without space or time, but from a previous quantum state where classical notions cease to be relevant. Thus, the "primeval atom" remains a founding metaphor, but its physical content evolves with theoretical and observational advances.

Evolution of Models of the Time

Before Lemaître, cosmology often relied on metaphysical postulates or on the eternity of the cosmos (steady-state model). After him, cosmological models integrate the notion of cosmic time and evolution.

N.B.: In Lemaître's model, spacetime did not exist before the primeval atom. The very notion of "before" loses its meaning: time begins with the expansion.

Sources: Nature – The Primeval Atom, G. Lemaître (1931), AIP – Oral History of G. Lemaître, arXiv – Georges Lemaître and the Foundations of Big Bang Cosmology.