The Birth of Big Bang Cosmology

The notion of a Big Bang implies that the Universe was once in a state of high density that it no longer occupies today - in short, that it has changed. The idea of a changing Universe (as opposed to a static, eternal one) got a big boost from several sciences during the nineteenth century. Geologists began to realize that the Earth's crust had evolved and that it had taken a very long time to create the surface of the Earth at present geological rates. Paleontologists saw countless species in the fossil record come and go and concluded that the logbook of terrestrial species had profoundly changed. Charles Darwin capped it all by arguing persuasively that terrestrial life and its environment had both evolved massively over a very long period of time.

Although the idea of an evolving Earth was in the air, by the turn of the century astronomers had not yet measured the age of any celestial object, and few in fact had any inkling of a Universe out there beyond our Galaxy (most thought the Universe was the Galaxy). Two events early in the century shattered this landscape and ushered in the era of scientific cosmology.

The first was Einstein's discovery of general relativity (GR) in 1915-1917. GR is central to cosmology because it is a theory of gravity, and gravity is unique among the four forces of nature for its long-range character - stars and galaxies pull on one another by gravity clear across the Universe. Moreover, unlike Newton's theory of gravity, GR links the very geometry of space to the distribution of matter and energy within it. Matter and energy are each a source of gravity in GR, and the local curvature of spacetime is determined by their local densities. The large-scale topology of the Universe is thus determined by its contents, and the two have to be solved for simultaneously. This imposes a degree of self-consistency not present in Newton, making for a much tighter theory.

Gravity is inherently dynamic - it pulls on things and makes them move. A little reflection shows that the only form of motion that is consistent with large-scale uniformity of the Universe is either global expansion or contraction. Strangely, it took twelve years after the promulgation of GR for this idea to sink in. One reason was Einstein himself. In developing the basic equations of GR, he noticed a freedom to insert an ad hoc term that at that time corresponded to no known gravitational force. This famous term, called the cosmological constant (Λ) corresponds to a large-scale repulsive form of gravity (repulsive if the right sign for Λ is chosen). I will return to Λ later, but for now I want to stress that the classical Λ-force of Einstein does not depend on the presence of matter or energy for its source - it just simply is, at the same strength throughout all space and time. Einstein was attracted to Λ because of his belief at the time - the year was 1917 and no contrary evidence was as yet in hand - that the Universe was static and unchanging. The repulsive “anti-gravity” of Λ could then be chosen precisely to balance the attractive force of normal gravity and “prop the Universe up”.

Twelve years later, in 1929, Edwin Hubble announced that the Universe was in fact expanding. This was the second event that marked the beginning of scientific cosmology. Actually, Hubble himself spoke only of “the recession of the nebulae” and avoided the word “expansion” as an unwarranted extrapolation beyond the data. We owe the concept of expansion to the Belgian cleric Georges Lemaître. Lemaître had been playing around with solutions to Einstein's GR equations during the 1920'sand had discovered (with Eddington) that Einstein's static Λ-model was in fact unstable - it was balanced, but on a knife edge. The slightest mismatch in the value of Λ either way would trigger either a dizzying collapse or a wild expansion. In such evolving models, Lemaître noticed that the velocities of galaxies would vary in proportion to their distances, the same law of proportionality discovered two years later for the real universe by Hubble. Perhaps the famous “Hubble law” should really be called “Lemaître's law.”

Primed by his earlier work, Lemaître seized on Hubble's discovery as evidence for a real physical expansion, and in 1931 he produced the first cosmological model based on actual data - Hubble's. His model had a repulsive Λ and was just entering its rapid expansion phase at the present time. It also contained substantial ordinary matter, whose gravity, Lemaître showed, would generate a “singularity” at a finite time in the past. Lemaître termed this initial high-density state the “primeval atom.” He even described the exit from this state as a “bang” although he did not embellish it with the word “big.” (That came later, from the British steady-state cosmologist Fred Hoyle, who invented the term “Big Bang”as a derisive dismissal of it. To him, it was an aesthetically unappealing rival to his steady state cosmology - more on this later.) Label aside, the concept of a Big Bang had entered cosmology. 

Contributed by: Dr. Sandra Faber