For quite some time it has been known that the mean density of our Universe agrees with the critical density to within better than a factor of ten. Even with such large margin of error this agreement is remarkable. Establishing initial conditions so that the mean density remains close to the critical density for more than a fleeting moment is much like trying to balance a pencil on its point. A Universe initially with slightly subcritical density rapidly becomes increasingly subcritical and soon virtually indistinguishable from an empty Universe. Similarly, an ever so slightly supercritical Universe rapidly collapses into a Big Crunch, never reaching the old age of our Universe – somewhere around twelve billion years. To obtain a Universe like ours seems to require fine tuning of the initial density to agree with the critical density to an accuracy around one part in 1060!For a long time it was regarded simplest and aesthetically most pleasing to postulate that our Universe is now of exactly critical density. The versions of inflation developed in the early 1980s provided a mechanism for setting the density of the Universe near the critical density with nearly unlimited precision. For many years an exactly critical Universe was touted as one of the few firm predictions of inflation.

# The Critical Density

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