History of the Big Bang theory
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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.
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[edit] Prior to the 20th Century
In 1610, Johannes Kepler used the dark night sky to argue for a finite universe. Seventy-seven years later, Isaac Newton described large-scale motion throughout the universe. In 1791, Erasmus Darwin gave the first description of a universe that expanded and contracted in a cyclic manner.
Though not recognized as scientific by either the scientific community or the author himself, who wished his work to be considered a "prose poem", Edgar Allan Poe proposed a system very similar to the Big Bang theory in his 1848 essay titled Eureka: A Prose Poem. He proposed a finite universe which begins as a single "primal particle", which expands outwards from "divine volition", a repulsive force which Poe 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 Primal Particle stage in order to begin the process of repulsion and attraction once again.
[edit] Early 20th Century
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 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 Brussels 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 made one of the first modern proposition of the occurrence the Big Bang theory for the origin of the universe, although he called it his "hypothesis of the primeval atom". He based his theory, published between 1927 and 1933, on the work of Einstein, among others, as well as ancient cosmological-philosophical traditions. Einstein, however, believed in a steady-state model of the universe. Lemaître independently derived the Friedmann-Lemaître-Robertson-Walker equations and proposed, on the basis of the recession of spiral nebulae, that the universe began with the "explosion" of a "primeval atom"—what was later called the Big Bang. Lemaître took cosmic rays to be the remnants of the event, although it is now known that they originate within the local galaxy.
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.
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 Things. 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.[1]
[edit] Late 20th Century
For a number of years the support for these theories was evenly divided. However, the observational evidence began to support the idea that the universe evolved from a hot dense state. The discovery of the cosmic microwave background radiation in 1965 , 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.
[edit] 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.
