Part Three: The Cosmological Case

The universe had a beginning. This is no longer a philosophical position—it is the scientific consensus, confirmed by multiple independent lines of evidence. What that beginning implies has been pursued by cosmologists, physicists, and philosophers with increasing rigor. The entries in this section document the evidence for the beginning and what the physics of that beginning requires.

Chapter 28 — The Universe Had a Beginning

In 1929 Edwin Hubble published measurements showing that distant galaxies are receding from Earth at speeds proportional to their distances. Every direction of the sky shows the same pattern: the farther away a galaxy is, the faster it is moving away. Space itself is expanding. The universe is not static.

An expanding universe implies a beginning. Run the expansion backward and all matter, energy, space, and time converge on a single point of origin. The mathematics of general relativity, applied to the expanding universe first by Alexander Friedmann in 1922 and Georges Lemaître in 1927, produces a singular beginning—a point at which the density and temperature of the universe are effectively infinite and at which the equations of physics cease to describe anything. Lemaître, a Belgian Catholic priest and physicist, called this the "hypothesis of the primeval atom." Einstein initially resisted; Hubble's observations removed the grounds for resistance.

The term "Big Bang" was coined by Fred Hoyle in 1949 as a dismissive description of what he regarded as a scientifically inferior model. The name stuck. Hoyle's preferred alternative—the steady-state model, in which the universe has always existed—was ruled out by the evidence.

Hubble, E. "A Relation between Distance and Radial Velocity among Extra-Galactic Nebulae." Proceedings of the National Academy of Sciences 15, no. 3 (1929): 168–173. Lemaître, G. "Un univers homogène de masse constante et de rayon croissant." Annales de la Société Scientifique de Bruxelles 47 (1927): 49–59.

Chapter 29 — The Proof That Cannot Be Escaped

In 2003 cosmologists Arvid Borde, Alan Guth, and Alexander Vilenkin published a theorem that has become foundational in cosmology. The Borde-Guth-Vilenkin theorem states: any universe that has been, on average, expanding throughout its history cannot be infinite in the past. It must have a beginning.

The significance of the BGV theorem is its scope. It applies not just to the standard Big Bang model but to all known inflationary multiverse models. Cosmic inflation—proposed by Guth in 1981—was sometimes thought to provide a path to a past-eternal universe. The BGV theorem closes this escape route. Even eternally inflating multiverses must have a beginning, because the average expansion rate throughout their history is positive.

Alexander Vilenkin, one of the theorem's authors and a cosmologist who holds no religious commitments, stated: "It is said that an argument is what convinces reasonable men and a proof is what it takes to convince even an unreasonable man. With the proof now in place, cosmologists can no longer hide behind the possibility of a past-eternal universe. There is no escape: they have to face the problem of a cosmic beginning." The problem of a beginning is the problem of a cause. The cause of the universe cannot itself be part of the universe.

Borde, A., Guth, A.H., and Vilenkin, A. "Inflationary Spacetimes Are Not Past-Complete." Physical Review Letters 90, 151301 (2003). Vilenkin, A. Many Worlds in One. Hill and Wang, 2006. p. 176.