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 moments of the Universe's history. The Penrose–Hawking singularity theorems require the existence of a singularity at the beginning of cosmic time. However, these theorems assume that general relativity is correct, but general relativity must break down before the Universe reaches the Planck temperature, and a correct treatment of quantum gravity may avoid the singularity.
Some proposals, each of which entails untested hypotheses, are:
- models including the Hartle–Hawking no-boundary condition in which the whole of space-time is finite; the Big Bang does represent the limit of time, but without the need for a singularity.
- Big Bang lattice model states that the Universe at the moment of the Big Bang consists of an infinite lattice of fermions which is smeared over the fundamental domain so it has both rotational, translational and gauge symmetry. The symmetry is the largest symmetry possible and hence the lowest entropy of any state.
- brane cosmology models in which inflation is due to the movement of branes in string theory; the pre-Big Bang model; the ekpyrotic model, in which the Big Bang is the result of a collision between branes; and the cyclic model, a variant of the ekpyrotic model in which collisions occur periodically. In the latter model, the Big Bang was preceded by a Big Crunch and the Universe endlessly cycles from one process to the other.
- chaotic inflation, in which universal inflation ends locally here and there in a random fashion, each end-point leading to a bubble universe expanding from its own big bang.
Proposals in the last two categories see the Big Bang as an event in a much larger and older Universe, or multiverse, and not the literal beginning.