Three Universes which begin to diverge noticeably after about 1 billion years, slightly below, at, and above the critical density. But at 1 nanosecond, the density of these three universes differed by only 1 gm/cm3, above and below 500 million billion tons/cm3, with the middle (black) value being exactly the critical density at that time. Clearly, to be reasonably close to critical density after several billion years, the Universe must be incredibly close to the critical density at early times.
Figure adapted from one by Ned Wright (and used in Whittle's upcoming undergraduate text).