The Big Bang Singularity and its Observational Confirmation

Key Insight

All Friedmann models imply a specific point in the past, between 10 and 20 thousand million years ago, where the distance between all galaxies was zero, a moment referred to as the big bang. At this point, the density of the universe and the curvature of space-time would have been infinite, creating a 'singularity' where the general theory of relativity breaks down. This breakdown means that physical theories cannot describe events at or before the big bang, effectively defining it as the beginning of time for scientific understanding. Despite philosophical and scientific resistance to a universe with a beginning, the concept gained traction, even being officially accepted by the Catholic Church in 1951.

Early attempts to circumvent the big bang included the 'steady state theory,' proposed in 1948 by Hermann Bondi, Thomas Gold, and Fred Hoyle. This theory suggested continuous creation of new matter, forming new galaxies to maintain a consistent universe appearance over time and space, even as existing galaxies moved apart. However, observational evidence soon challenged this. In the late 1950s and early 1960s, Martin Ryle's group found more distant radio sources than nearby ones per unit volume, contradicting the steady state's uniform distribution prediction. Crucially, in 1965, Arno Penzias and Robert Wilson accidentally discovered a uniform microwave radiation coming from all directions. This 'Cosmic Microwave Background' (CMB) radiation, interpreted as the highly red-shifted afterglow of a hot, dense early universe (as predicted by George Gamow), provided undeniable evidence against the steady state theory, leading to its abandonment and solidifying the big bang model. Penzias and Wilson were awarded the Nobel Prize in 1978 for this pivotal discovery.

The theoretical inevitability of a big bang singularity was further solidified by rigorous mathematical proofs. In 1965, Roger Penrose demonstrated that any collapsing star must inevitably form a singularity. Building on this, Stephen Hawking in 1966 showed that, by reversing the direction of time in Penrose's theorem, an expanding universe, if roughly Friedmann-like on large scales, must have originated from a singularity. In 1970, Penrose and Hawking jointly published a theorem, proving that a big bang singularity is a necessary consequence of general relativity, provided the universe contains the observed amount of matter. This landmark result underscores the incompleteness of general relativity, as it predicts its own failure at the universe's origin, indicating the necessity of incorporating quantum mechanics to fully comprehend the extraordinarily small, earliest moments of the universe.