Existing theories of physics cannot tell us about the moment of the Big Bang. Extrapolation of the expansion of the universe backwards in time using only
classical general relativity yields a
gravitational singularity with
infinite density and
temperature at a finite time in the past. However this classical gravitational theory is expected to be inadequate to describe physics under these conditions. Thus the meaning of this singularity in the context of the Big Bang is unclear.
In popular media On the web and in popular science media, the Big Bang model is associated with an initial singularity. The reason seems to be simple
extrapolation.
Matt Strassler provides an example to illustrate why this extrapolation is illogical. Imagine extrapolating your life backwards in time: you become a child, then a baby, then a fetus. However, you don't reverse to a singularity because the process of
cellular division does not extrapolate to zero size. Similarly extrapolating the Big Bang model to zero time enters a region where the model does not apply.
History In 1922,
Alexander Friedmann derived the
Friedmann equations from
Albert Einstein's
general relativity. This paper indicated a chaotic dynamics near the singularity known as the
Mixmaster universe, a model developed by
Charles W. Misner the same year. In 1970,
Stephen Hawking and
Roger Penrose developed the
Penrose–Hawking singularity theorems for
black holes and cosmological models, showing that the Big Bang singularity is inevitable under more general assumptions. The BGV theorem however does not necessarily indicate a global singularity for all possible observers. ==Lack of quantum mechanics==