The Big Bang theory is the prevailing cosmological model that explains the early development of the Universe. According to the Big Bang theory, the Universe was once in an extremely hot and dense state which expanded rapidly. This rapid expansion caused the young Universe to cool and resulted in its present continuously expanding state. According to the most recent measurements and observations, this original state existed approximately 13.7 billion years ago, which is considered the age of the Universe and the time the Big Bang occurred. After its initial expansion from a singularity, the Universe cooled sufficiently to allow energy to be converted into various subatomic particles. It would take thousands of years for some of these particles (protons, neutrons, and electrons) to combine and form atoms, the building blocks of matter. The first element produced was hydrogen, along with traces of helium and lithium. Eventually, clouds of hydrogen would coalesce through gravity to form stars, and the heavier elements would be synthesized either within stars or during supernovae.

The Big Bang is a well-tested scientific theory which is widely accepted within the scientific community because it is the most accurate and comprehensive explanation for the full range of phenomena astronomers observe. Since its conception, abundant evidence has arisen to further validate the model.Georges Lemaître first proposed what would become the Big Bang theory in what he called his "hypothesis of the primeval atom." Over time, scientists would build on his initial ideas to form the modern synthesis. The framework for the Big Bang model relies on Albert Einstein's general relativity and on simplifying assumptions (such as homogeneity and isotropy of space). The governing equations had been formulated by Alexander Friedmann. In 1929 Edwin Hubble discovered that the distances to far away galaxies were generally proportional to their redshifts—an idea originally suggested by Lemaître in 1927. Hubble's observation was taken to indicate that all very distant galaxies and clusters have an apparent velocity directly away from our vantage point: the farther away, the higher the apparent velocity.

If the distance between galaxy clusters is increasing today, everything must have been closer together in the past. This idea has been considered in detail back in time to extreme densities and temperatures, and large particle accelerators have been built to experiment on and test such conditions, resulting in significant confirmation of this model. On the other hand, these accelerators have limited capabilities to probe into such high energy regimes. There is little evidence regarding the absolute earliest instant of the expansion. Thus, the Big Bang theory cannot and does not provide any explanation for such an initial condition; rather, it describes and explains the general evolution of the universe going forward from that point on. The observed abundances of the light elements throughout the cosmos closely match the calculated predictions for the formation of these elements from nuclear processes in the rapidly expanding and cooling first minutes of the universe, as logically and quantitatively detailed according to Big Bang nucleosynthesis.

Fred Hoyle is credited with coining the term Big Bang during a 1949 radio broadcast. It is popularly reported that Hoyle, who favored an alternative "steady state" cosmological model, intended this to be pejorative, but Hoyle explicitly denied this and said it was just a striking image meant to highlight the difference between the two models. After the discovery of the cosmic microwave background radiation in 1964, and especially when its spectrum (i.e., the amount of radiation measured at each wavelength) was found to match that of thermal radiation from a black body, most scientists were fairly convinced by the evidence that some version of the Big Bang scenario must have occurred.

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