The BIrth of Cosmology
The Belgian Priest and the Origin of the Universe
Interestingly, the Belgian physicist and priest Abbe Georges Lemaître had no qualms about leaving the cosmological constant in the field equations as he believed it could be at least partially responsible for the observed expansion. He differed from Einstein in that he did not believe Λ was merely a constant that helped describe the geometry (curvature) of spacetime due to the presence of mass. Rather, Lemaître wrote the field equations with Λ as a multiplicative constant on the stress-energy tensor. In this manner, he sought to tie the cosmological constant to a physical force related to mass-energy that drives the expansion of the universe. Given the discovery in 1998 that the universe's rate of expansion is accelerating, Abbe Lemaître's interpretation of the cosmological constant and its role as a driving mechanism of expansion may been seen as evidence of his foresight and keen intuition about the physical phenomena that are described by the mathematics of a theory.
When first published in 1927, Lemaître's interpretation of the field equations of general relativity did not hint at a creation event that started the universe. He simply believed that the universe was expanding, but that expansion was from an initially static state. That static state was assumed to be infinitely old. Further thought and calculations lead Lemaître to conclude that an infinitely old static state was instable, however, and that the universe had to have a recent beginning. He estimated that beginning—the day without yesterday—happened 4 billion years ago; an estimate that fit the geologic record of Earth and estimates of the Sun's age reasonably well at that time. Unfortunately, Hubble's work—while giving general support to the idea of an expanding cosmos—gave an estimate of the universe's age that was considerably too small. Initially, the age of the universe appeared to be smaller than the age of Earth!
By the early 1930s, Lemaître had become interested in the initial conditions of the young universe and turned his attention to the developing field of quantum mechanics. Realizing that ordinary physics could not describe the those initial conditions, the Abbe reasoned the universe had to start from conditions very similar to those of an atomic nucleus. In 1931 Lemaître published his theory in a paper describing the initial conditions as a "Primeval Atom." One facet of the theory stated that cosmic rays may be the residual evidence of that initial state.
The paper went unnoticed by the astronomical community until Sir Arthur Eddington proposed a similar model in 1933. A former student of Eddington's, Lemaître wrote his former teacher about his paper and Sir Arthur graciously republished the paper and credited the priest with the original theory.
While there was still a tremendous discrepancy between the estimated age of the universe in Lemaître model and the calculated age based on the Hubble expansion, subsequent measurements of Cepheid variables and a revised value of the Hubble constant in the 1940s brought estimates of the universe's age into agreement with Lemaître's Primeval atom model. By that time, however, the nature of cosmic rays had been determined by Arthur Compton and they could not be left over from the early universe (they are created from the catastrophic explosions of massive stars called supernovae).
While the details of the Primeval atom proved to be incorrect, the developing field of nuclear physics was able to provide a detailed description of how atomic nuclei are created through fusion of successively more massive nuclei (a process called nucleosynthesis). Applying nucleosynthesis to the initial conditions of a hot, dense early universe, a modified theory emerged that came to be known as the Big Bang. A name that was given to the theory by its most vocal critic—the irascible, brilliant astronomer Sir Fred Hoyle. Ironically, it was Sir Fred's dismissive that stuck while Hoyle's own Steady State theory did not withstand the scrutiny of experimental verification.