Flatness problem

The flatness problem is a cosmological fine-tuning problem within the Big Bang model. Along with the monopole problem and the horizon problem, it is one of the three primary motivations for the theory of an inflationary universe. The flatness problem arises because of the observation that the density of the universe today is very close to the critical density required for spatial flatness.. Since the total energy density of the universe departs rapidly from the critical value over cosmic time, the early universe must have had a density even closer to the critical density, leading cosmologists to question how the density of the early universe came to be fine-tuned to this 'special' value.
See more at Wikipedia.org...

The puzzle created by the fact that, out of an infinite number of possibilities that the universe could be an open universe and an infinite number of possibilities that the universe could be a closed universe, observations are suggesting that it is, in fact, the dividing option between the two; the 'flat' universe. If there are an infinite number of other possibilities why should the universe be this one special case? Answers to this problem are provided by the theory of inflationary cosmology