Big Bang
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According to the Big Bang, the universe emerged from an extremely dense and hot state (bottom). Since then, space itself has expanded with the passage of time, carrying the galaxies with it.In physical cosmology, the Big Bang is the scientific theory that the universe emerged from a tremendously dense and hot state about 13.7 billion years ago. The theory is based on the observations indicating the expansion of space (in accord with the Robertson-Walker model of general relativity) as indicated by the Hubble redshift of distant galaxies taken together with the cosmological principle.

Extrapolated into the past, these observations show that the universe has expanded from a 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 (for reporting on some of the more notable speculation on this issue, see cosmogony).

The term Big Bang is used both in a narrow sense to refer to a point in time when the observed expansion of the universe (Hubble's law) began - calculated to be 13.7 billion (1.37 × 1010) years ago (±2%) - and in a more general sense to refer to the prevailing cosmological paradigm explaining the origin and expansion of the universe, as well as the composition of primordial matter through nucleosynthesis as predicted by the Alpher-Bethe-Gamow theory.[1]

From this model, George Gamow in 1948 was able to predict, at least qualitatively, the existence of cosmic microwave background radiation (CMB).[2] The CMB was discovered in 1964[3] and further corroborated the Big Bang theory, giving it an additional advantage over its chief rival, the steady state theory.[4]

Contents [hide] 1 History 2 Overview 3 Theoretical underpinnings 4 Observational evidence 4.1 Hubble's law expansion 4.2 Cosmic microwave background radiation 4.3 Abundance of primordial elements 4.4 Galactic evolution and distribution 5 Features, issues and problems 5.1 Horizon problem 5.2 Flatness problem 5.3 Magnetic monopoles 5.4 Baryon asymmetry 5.5 Globular cluster age 5.6 Dark matter 5.7 Dark energy 6 The future according to the Big Bang theory 7 Speculative physics beyond the Big Bang 8 Philosophical and religious interpretations 9 Notes 10 External links and references 10.1 Big Bang overviews 10.2 Religion and philosophy 10.3 Research articles

[edit] History Main article: History of the Big Bang The Big Bang theory developed from observations of the structure of the universe and theoretical considerations. Observationally, it was determined that most spiral nebulae were receding from Earth, but those who made the observation weren't aware of the cosmological implications, nor that the supposed nebulae were actually galaxies outside our own Milky Way.[5] In 1927, Georges Lemaître, a Belgian Roman Catholic priest, independently derived the Friedmann-Lemaître-Robertson-Walker equations from Albert Einstein's equations of general relativity 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.[6]

In 1929, Edwin Hubble provided an observational basis for Lemaître's theory. He discovered that, seen from Earth, light from other galaxies is redshifted in direct proportion to their distance from the Earth. This fact is now known as Hubble's law.[7][8] 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, contradicting the infinite and unchanging static universe scenario developed by Einstein.[9]

Artist's depiction of the WMAP satellite gathering data to help scientists understand the Big Bang.This idea allowed for two main 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.[10] It was actually Hoyle who coined the name of Lemaître's theory, referring to it sarcastically as "this big bang idea" during a program broadcast on March 28, 1949, by the BBC Third Programme. Hoyle repeated the term in further broadcasts in early