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.

Speculative physics beyond the Big Bang

While the Big Bang model is well established in cosmology, it is likely to be refined in the future. Little is known about the earliest universe, when inflation is hypothesized to have occurred. There may also be parts of the universe well beyond what can be observed in principle. In the case of inflation this is required: exponential expansion has pushed large regions of space beyond our observable horizon. It may be possible to deduce what happened when we better understand physics at very high energy scales. Speculations about this often involve theories of quantum gravity.

Related Topics:
Inflation - Quantum gravity

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Some proposals are:

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• chaotic inflation
• brane cosmology models, including the ekpyrotic model in which the Big Bang is the result of a collision between branes
• an oscillatory universe in which the early universe's hot, dense state resulted from the Big Crunch of a universe similar to ours. The universe could have gone through an infinite number of big bangs and big crunches. The cyclic extension of the ekpyrotic model is a modern version of such a scenario.
• models including the Hartle-Hawking boundary condition in which the whole of space-time is finite.

Some of these scenarios are qualitatively compatible with one another. Each involves untested hypotheses.

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