The most widely accepted theory of the origin and evolution of the Universe. According to the Big Bang theory, the Universe originated from an initial state of high temperature and density and has been expanding ever since. The best current measurements place the occurrence of the Big Bang at 13.80 ± 0.02 billion years ago±; in other words, this is the age of the Universe.
The theory of general relativity predicts the existence of a singularity at the very beginning, where the temperature and density were infinite. Most cosmologists interpret this singularity as meaning that general relativity breaks down at the Planck era under the extreme physical conditions of the very early Universe, and that the very beginning must be addressed using a theory of quantum cosmology. With our present knowledge of high-energy particle physics, we can run the clock back through the lepton era and hadron era to about a millionth of a second after the Big Bang, when the temperature was 1013 K. Using more speculative theory, cosmologists have tried to push the model to within 10−35 s of the singularity, when the temperature was 1028 K.
The Big Bang theory accounts for the expansion of the Universe; the existence of the cosmic microwave background; and the abundances of light nuclei such as helium, helium-3, deuterium, and lithium-7, which are predicted to have been formed about 1 second after the Big Bang when the temperature was 1010 K. The cosmic microwave background provides the most direct evidence that the Universe went through a hot, dense phase. In the Big Bang theory, the microwave background is accounted for by the fact that, for the first million years or so (i.e. before the decoupling of matter and radiation), the Universe was filled with plasma that was opaque to radiation and therefore in thermal equilibrium with it. This phase is usually called the primordial fireball. When the Universe expanded and cooled to about 3000 K, at a time known as the recombination epoch, it became transparent to radiation, which we now observe, much cooled and diluted, as thermal microwave radiation.
The discovery of the microwave background in 1965 resolved a long-standing battle between the Big Bang and its then rival, the steady-state theory, which cannot explain the black-body form of the microwave background. Ironically, the term Big Bang was initially intended to be derogatory and was coined by F. Hoyle, one of the strongest advocates of the steady state.
Big Bang Chronology
Era | Time after Big Bang | Temperature |
|---|
Planck era | 0 to 10−43 s | ? to 1034 K |
radiation eraa | 10−43 s to 30 000 years | 1034 to 104 K |
matter erab | 30 000 years to present | 104 to 3 K |
a The time from about 10−6 or 10−5 s to about 1 s or so is subdivided into the hadron and lepton eras.
b Includes the recombination epoch, which took place about 300 000 years after the Big Bang, at a temperature of about 3000 K.