The flatness problem is a cosmological fine-tuning problem within the Big Bang model. The problem is that the density of energy in the universe today is very close to the critical density, and extrapolating backwards in time, the energy density of the universe is increasingly closer to the critical density, without any reason why the energy density should be close to the critical density at the time of the Universe's creation. Along with the Monopole problem and the Horizon problem, it is one of the three primary motivations for the theory of an inflationary universe.