History of the Big Bang theory

The history of the Big Bang theory began with the Big Bang's development from observations and theoretical considerations. Much of the theoretical work in cosmology now involves extensions and refinements to the basic Big Bang theory.

Ancient discussions on Temporal Finitism and anticipations in philosophy and literature

In medieval philosophy, there was much debate over whether the universe had a finite or infinite past (see Temporal finitism). The philosophy of Aristotle held that the universe had an infinite past, which caused problems for medieval Jewish, Christian and Islamic philosophers who were unable to reconcile the Aristotelian conception of the eternal with the Abrahamic view of creation. As a result, a variety of logical arguments for the universe having a finite past were developed by John Philoponus, Al-Kindi, Saadia Gaon, Al-Ghazali and Immanuel Kant, among others.

In 1610, Johannes Kepler used the dark night sky to argue for a finite universe (Olbers paradox). Seventy-seven years later, Isaac Newton described large-scale motion throughout the universe.

The description of a universe that expanded and contracted in a cyclic manner was first put forward in a poem published in 1791 by Erasmus Darwin. Edgar Allan Poe presented a similar cyclic system in his 1848 essay titled Eureka: A Prose Poem, which obviously was not considered as scientific by either the scientific community or the author himself.

Poe described a finite universe which begins as a single "primordial particle", which expands outwards from "divine volition", a repulsive force, which he described as one of the two forces which make up all matter in the universe—repulsion and attraction (gravity). Matter spreads evenly throughout space, but begins to clump together due to gravity, forming stars and star systems. The material universe is then drawn back together by gravity, eventually returning to the Primordial Particle stage in order to begin the process of repulsion and attraction once again.

Early 20th Century scientific developments

Observationally, in the 1910s, Vesto Slipher and later Carl Wilhelm Wirtz determined that most spiral nebulae were receding from Earth. Slipher used spectroscopy to investigate the rotation periods of planets, the composition of planetary atmospheres, and was the first to observe the radial velocities of galaxies. Wirtz observed a systematic redshift of nebulae, which was difficult to interpret in terms of a cosmology in which the Universe is filled more or less uniformly with stars and nebulae. They weren't aware of the cosmological implications, nor that the supposed nebulae were actually galaxies outside our own Milky Way.

Also in that decade, Albert Einstein's theory of general relativity was found to admit no static cosmological solutions, given the basic assumptions of cosmology described in the Big Bang's theoretical underpinnings. The universe was described by a metric tensor that was either expanding or shrinking, a result that Einstein (at first) himself considered wrong and he tried to fix by adding a cosmological constant. The first person to seriously apply general relativity to cosmology without the stabilizing cosmological constant was Alexander Friedmann. Friedmann discovered the expanding-universe solution to general relativity field equations in 1922. Friedmann's 1924 papers included "Über die Möglichkeit einer Welt mit konstanter negativer Krümmung des Raumes" (About the possibility of a world with constant negative curvature) which was published by the Berlin Academy of Sciences on the 7 January 1924. Friedmann's equations describe the Friedmann-Lemaître-Robertson-Walker universe.

In 1927, the Belgian Catholic priest Georges Lemaître proposed an expanding model for the universe to explain the observed redshifts of spiral nebulae and forecast the Hubble law. He based his theory on the work of Einstein and De Sitter, and independently derived Friedmann's equations for an expanding universe.

In 1929, Edwin Hubble provided an observational basis for Lemaître's theory. Hubble discovered that, relative to the Earth, the galaxies are receding in every direction at speeds directly proportional to their distance from the Earth. In 1929 Hubble and Milton Humason formulated the empirical Redshift Distance Law of galaxies, nowadays known as Hubble's law, which, once the redshift is interpreted as a measure of recession speed, is consistent with the solutions of Einstein’s General Relativity Equations for a homogeneous, isotropic expanding space. This led to the concept of the expanding universe. The law states that the greater the distance between any two galaxies, the greater their relative speed of separation. This discovery later resulted in the formulation of the Big Bang theory.

In 1931, Lemaître proposed in his "hypothèse de l'atome primitif" (hypothesis of the primeval atom) that the universe began with the "explosion" of the "primeval atom" —what was later called the Big Bang. Lemaître first took cosmic rays to be the remnants of the event, although it is now known that they originate within the local galaxy. Lemaître had to wait until shortly before his death to learn of the discovery of cosmic microwave background radiation, the now believed remnant radiation of a dense and hot phase in the early Universe.

Given the cosmological principle whereby the universe, when viewed on sufficiently large distance scales, has no preferred directions or preferred places, Hubble's law suggested that the universe was expanding. This idea allowed for two opposing possibilities. One was Lemaître's Big Bang theory, advocated and developed by George Gamow. The other possibility was Fred Hoyle's steady state model in which new matter would be created as the galaxies moved away from each other. In this model, the universe is roughly the same at any point in time. It was actually Hoyle who coined the name of Lemaître's theory, referring to it sarcastically as "this 'big bang' idea" during a radio broadcast on March 28 1949, on the BBC Third Programme. Hoyle repeated the term in further broadcasts in early 1950, as part of a series of five lectures entitled The Nature of The Universe. The text of each lecture was published in The Listener a week after the broadcast, the first time that the term "big bang" appeared in print. As evidence in favour of the Big Bang model mounted and the consensus became widespread, Hoyle himself, albeit somewhat reluctantly, admitted to it by formulating a new cosmological model that other scientists later referred to as the "Steady Bang".

Late 20th Century

Comparison of the predictions of the standard Big Bang model with experimental measurements. The power spectrum of the cosmic microwave background radiation anisotropy is plotted in terms of the angular scale (or multipole moment) (top).

For a number of years the support for these theories was evenly divided, with the slight imbalance in the fact that the Big Bang theory could explain both the formation and the observed abundances of hydrogen and helium, whereas the Steady State could explain how they were formed but not why they should have the observed abundances. However, the observational evidence began to support the idea that the universe evolved from a hot dense state. Young objects such as quasars were only observed at the very edges of the universe, indicating that such objects only existed in times long past, whereas the Steady State predicted that young galaxies should be scattered all over the universe, both near and far. In addition, the discovery of the cosmic microwave background radiation in 1965 was considered the death knell of the Steady State, although, as Big Bang skeptics point out, this prediction was only qualitative, and failed to predict the actual temperature of the CMB. After some reformulation, the Big Bang has been regarded as the best theory of the origin and evolution of the cosmos. Before the late 1960s, many cosmologists thought the infinitely dense and physically paradoxical singularity at the starting time of Friedmann's cosmological model could be avoided by allowing for a universe which was contracting before entering the hot dense state and starting to expand again. This was formalized as Richard Tolman's oscillating universe. In the sixties, Stephen Hawking and others demonstrated that this idea was unworkable, and the singularity is an essential feature of the physics described by Einstein's gravity. This led the majority of cosmologists to accept the notion that the universe as currently described by the physics of general relativity has a finite age. However, due to a lack of a theory of quantum gravity, there is no way to say whether the singularity is an actual origin point for the universe or whether the physical processes that govern the regime cause the universe to be effectively eternal in character.

Future of the theory

Much of the current work in cosmology includes understanding how galaxies form in the context of the Big Bang, understanding what happened at the Big Bang, and reconciling observations with the basic theory. In the past there was much discussion as to whether the Big Bang would need to be completely abandoned as a description of the universe, but such proponents of non-standard cosmology have become fewer in number over the last few decades. Cosmologists continue to calculate many of the parameters of the Big Bang to a new level of precision and hypothesized an expansion of the universe appears to be accelerating.

Huge advances in Big Bang cosmology were made in the late 1990s and the early 21st century as a result of major advances in telescope technology in combination with large amounts of satellite data, such as that from COBE and the Hubble Space Telescope. In 2003, NASA's WMAP takes more detailed pictures of the universe by means of the cosmic microwave background radiation. The image can be interpreted to indicate that the universe is 13.7 billion years old (within one percent error) and that the Lambda-CDM model and the inflationary theory is correct. No other cosmological theory can yet explain such a wide range of parameters, from the ratio of the elemental abundances in the early Universe to the structure of the cosmic microwave background, the observed higher abundance of active galactic nuclei in the early Universe and the observed masses of clusters of galaxies.

See also

• Timeline of cosmology
• Theory of everything