Big Bang

In physical cosmology, the Big Bang is the scientific theory that the universe emerged from an enormously dense and hot state nearly 14 billion years ago. The Big Bang is a consequence of the observed Hubble's law velocities of distant galaxies that when taken together with the cosmological principle implies that space is expanding according to the Friedmann model of general relativity. Extrapolated into the past, these observations show that the universe has expanded from a primeval state, in which all the matter and energy in the universe was at an immense temperature and density. Physicists do not widely agree on what happened before this, although general relativity predicts a gravitational singularity.

History

The Big Bang theory developed from observations and theoretical considerations. Observationally, it was determined that most spiral nebulae were receding from Earth, but they weren't aware of the cosmological implications, nor that the supposed nebulae were actually galaxies outside our own Milky Way. In 1927, the Belgian Catholic priest Georges 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.

Related Topics:
Milky Way - 1927 - Belgian - Catholic - Georges Lemaître - Atom

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In 1929, Edwin Hubble provided an observational basis for Lemaître's theory. He discovered that, relative to the earth, the galaxies are receding in every direction at speeds directly proportional to their distance from the earth. This fact is now known as Hubble's law (see Edwin Hubble: Mariner of the Nebulae by Edward Christianson). 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.

Related Topics:
1929 - Edwin Hubble - Hubble's law - Cosmological principle

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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 1949 BBC radio program, The Nature of Things, the text of which was published in 1950.

Related Topics:
George Gamow - Fred Hoyle - Steady state model - 1949 - BBC - 1950

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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. Since the discovery of the cosmic microwave background radiation in 1965 it has been regarded as the best theory of the origin and evolution of the cosmos. Virtually all theoretical work in cosmology now involves extensions and refinements to the basic Big Bang 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.

Related Topics:
Cosmic microwave background radiation - 1965

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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, the Hubble Space Telescope and WMAP. These data have allowed cosmologists to calculate many of the parameters of the Big Bang to a new level of precision and led to the unexpected discovery that the expansion of the universe appears to be accelerating. (See dark energy.)

Related Topics:
1990s - 21st century - Telescope - COBE - Hubble Space Telescope - WMAP - Dark energy

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See also: Timeline of cosmology

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