Searching For the Exact Density of Mass
Unfortunately, it is very difficult to measure the density of mass in the universe. Cosmologists only know that the average density lies somewhere near the critical value. Nonetheless, they have calculated that the slightest variation from the critical density soon after the birth of the universe would have had a significant effect on its development. At one second after the big bang, for example, the average mass density must have been less than 1.000000000000001 times the critical value, or else the universe would have stopped expanding and collapsed in on itself before reaching its present age. But the density must have been more than 0.999999999999999 times the critical value, or else the matter in the universe would have flown apart so quickly that stars and galaxies would never have formed.
Because the traditional big bang theory could not explain why the universe is so uniform or why the density of the primordial fireball was so extraordinarily close to the critical value, cosmologists realized that the theory would have to be refined. In the 1980's, they developed a new theory that accounted for these two phenomena—the inflationary universe theory. The theory proposes that just an instant after it emerged into being, the infant universe was propelled by a force called inflation. In just an eyeblink of cosmic time, the universe increased in size by a factor of trillions of trillions. The theory was first suggested by me in a form which did not quite work. The first fully successful version of inflation was proposed by Andrei Linde, then at the Lebedev Institute in Moscow, and independently in a publication by Andreas Albrecht and Paul Steinhardt at the University of Pennsylvania.