In 1917, Albert Einstein published a paper on relativistic cosmology that described a universe which was static, finite, and unbounded, much like the surface of a sphere. He had added a cosmological constant to his field equations specifically to counteract the attractive force of gravity, ensuring the universe would not collapse under its own weight. This model, known as the Einstein model, was the prevailing view of the cosmos for decades, yet it was fundamentally unstable to even the smallest perturbations. History would later reveal that Einstein himself had made a significant error in judgment, famously calling the introduction of the cosmological constant his greatest blunder after evidence emerged that the universe was actually expanding. The static universe was a mathematical convenience that failed to account for the dynamic reality of the cosmos, a reality that was about to be uncovered by astronomers who were beginning to look beyond the Milky Way.
The Expanding Cosmos Revealed
The 1920s marked a seismic shift in our understanding of the universe, driven by the work of Vesto Slipher and Edwin Hubble. Slipher had interpreted the red shift of spiral nebulae as a Doppler shift indicating they were receding from Earth, but the distances to these objects remained a mystery. In 1929, Hubble provided the observational basis that changed everything, showing that these spiral nebulae were actually galaxies outside our own Milky Way. He discovered a relationship between the redshift of a galaxy and its distance, now known as Hubble's law, which demonstrated that galaxies are receding from Earth in every direction at speeds proportional to their distance. Although Hubble's numerical factor relating recessional velocity and distance was off by a factor of ten due to a lack of knowledge about Cepheid variable stars, the implication was undeniable: the universe was expanding. This discovery forced physicists to abandon the static model and consider theories of an evolving cosmos.The Primeval Atom and The Steady State
As the evidence for expansion mounted, two competing theories emerged to explain the origin and evolution of the universe. Georges Lemaître, a Belgian Roman Catholic priest, independently derived the Friedmann-Lemaître-Robertson-Walker equations and proposed that the universe began with the explosion of a primeval atom, a concept that would later be dubbed the Big Bang. This theory was further developed by George Gamow, who argued that the universe evolved from a hot, dense state. In stark contrast, Fred Hoyle proposed the steady state model, which suggested that new matter was continuously created as galaxies moved away from each other, keeping the universe roughly the same at any point in time. For a number of years, support for these theories was evenly divided, but the discovery of the cosmic microwave background in 1965 by Arno Penzias and Robert Woodrow Wilson lent strong support to the Big Bang model. The steady state theory, which required the continuous creation of matter, could not explain this radiation, and it eventually fell out of favor among the scientific community.