The theory

A  The Big Bang Theory

The big bang theory describes a hot explosion of energy and matter at the time the universe came into existence. This theory explains why the universe is expanding. Recent versions of the theory also explain why the universe seems so uniform in all directions and at all places.

The work of Edwin Hubble, which showed that the universe is expanding, led cosmologists to begin tracking the history of the universe. The dominant idea is that the universe would have been hotter and denser billions of years ago. In the 1940s Russian American physicist George Gamow and his students, American physicists Ralph Alpher and Robert Herman, developed the idea of a hot explosion of matter and energy at the time of the origin of the universe. (This theory of an explosion at the beginning of the universe was given the originally derisive name “big bang” by British astronomer Fred Hoyle in 1950.) Current calculations place the age of the universe at about 13.7 billion years. Gamow and his students realized that some of the chemical elements in the universe today were forged in the hot early stage of the universe’s existence. They also hypothesized that some radiation that remains from the big bang explosion may still be circulating in the universe, though this idea was forgotten for some time.

Current methods of particle physics allow the universe to be traced back to a tiny fraction of a second—1 × 10-43 seconds—after the big bang explosion initiated the expansion of the universe. To understand the behavior of the universe before that point cosmologists would need a theory that merges quantum mechanics and general relativity. Scientists do not actually study the big bang itself, but infer its existence from the universe’s expansion.

In the 1950s American astronomer William Fowler and British astronomers Fred Hoyle, Geoffrey Burbidge, and Margaret Burbidge worked out a series of calculations that showed that the lightest of the chemical elements (those of lowest atomic weight) were formed in the early universe shortly after the big bang. These light elements include ordinary hydrogen, hydrogen’s isotope deuterium, and helium. Heavier elements, according to those calculations, were formed later. Scientists now know that the elements heavier than helium and lighter than iron were formed in nuclear processes in stars, and the heaviest elements (those heavier than iron) were formed in supernova explosions.

B  Steady-State Theory

In the 1940s British scientists Hermann Bondi, Thomas Gold, and Fred Hoyle were philosophically opposed to the requirements that the big bang theory put forth for the extreme conditions in the early universe. The big bang theory was framed in terms of what they called the cosmological principle—that the universe is homogeneous (the same in all locations) and isotropic (looks the same in all directions) on a large scale. Bondi, Gold, and Hoyle suggested an additional postulate, which they called the perfect cosmological principle. This principle stated that the universe is not only homogeneous and isotropic but also looks the same at all times. Since the universe is expanding, though, one might think that the density of the universe would decrease. Such a decrease would be a change that would not fit with the perfect cosmological principle. Bondi, Gold, and Hoyle thus suggested that matter could be continuously created out of nothing to maintain the density over time. The rate at which matter would have to be created was much too low to be observationally testable, however. They called this theory the steady-state theory.

C  Big Bang vs. Steady-State

The only evidence necessary for supporters of the big bang theory to prove that this theory was more acceptable than the steady-state theory was to show that the universe changed over time. Just such a change was found in 1963 when Dutch American astronomer Maarten Schmidt identified quasars while working at the Palomar Observatory in California. As seen from Earth, quasars are bluish astronomical objects that resemble stars. Astronomers believe that quasars are the cores of certain types of galaxies. Quasars are all quite far from Earth, which means they must have originated during the early formation of the universe. They are distant from Earth in both time and space. The lack of quasars near Earth (and therefore nearer in time to Earth) shows that the universe has been evolving. This finding dealt a serious blow to steady-state cosmology.

D  Discovery of Cosmic Background Radiation

In 1965 a piece of evidence was found that almost all scientists agree conclusively rules out the steady-state theory of the universe. At that time, American physicists Arno Penzias and Robert W. Wilson, working at the Bell Laboratories in New Jersey (now part of Lucent Technologies), discovered faint isotropic radio waves. American astronomers James Peebles, David Roll, David Wilkinson, and Robert Dicke at Princeton University had recently predicted that just such radiation would have been emitted as a result of the hot, dense early universe predicted by the big bang theory. These scientists were themselves preparing a radio telescope to search for this radiation. (Scientists only later recalled that Gamow and colleagues had earlier predicted such radiation.) This cosmic background radiation is now widely accepted as proof of the big bang theory. The existence of cosmic background radiation is the third pillar of modern cosmology. The other two pillars are: (1) the uniform expansion of the universe and (2) the match between calculations of the amounts of the lightest chemical elements that would be formed in the first few minutes after a big bang and observations of these elements’ actual relative abundance in space.