Observational cosmologists consider the standard Big Bang model to be a good approximation to the Universe
  • in spatial extent from the Earth outwards towards the observational sphere
  • and in time backwards from the present towards some limiting "time=zero"

It might be the case that the spatial size of the Universe is smaller than this sphere and so the whole Universe is observable. See the shape of the universe in the standard big bang model.

Alternatively, theoretical cosmologists have many ideas of "scenarios" of what the Universe might be like on bigger scales:

  • the quasi Steady State model
  • chaotic inflation
  • brane models
  • models including the Hartle-Hawking boundary condition in which the whole of space-time is finite
  • ekpyrotic scenarios of an eternally bouncing (big banging) universe

Some of these scenarios are either qualitatively or quantitatively (algebraically) compatible with one another. Some are concerned with local properties (sometimes assumed the same everywhere), others also deal with global properties. Each involves a different number of elements of theoretical physics and/or heuristic hypotheses.

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