Rethinking the Big Bang Theory
Despite the success of the big bang theory in accounting for the present state of the universe, there is strong evidence that the theory is incomplete. Because the theory describes only the aftermath of the primordial explosion, and not the explosion itself, there are mysteries that the big bang theory leaves unresolved. Most importantly, the theory gives no explanation for two remarkable properties of the observable universe: its uniformity and its average density of mass.
If the big bang had been an ordinary explosion, like a blast of dynamite or a nuclear bomb, we would not expect the universe to be uniform. Objects would be distributed randomly and unevenly. However, that is not the case—the universe appears to be remarkably uniform. If we could imagine stepping back and looking at the very big picture, we would find that the distribution of stars and galaxies is very much the same throughout the universe. The most striking evidence for the uniformity of the universe comes from the cosmic background radiation. Precise measurements have shown that the intensity of the radiation is almost exactly the same from every direction in the sky.
Cosmologists usually discuss the average density of mass in the universe in terms of its effect on the fate of the universe. Recall that the universe is expanding and that the expansion is being slowed by the gravitational attraction that exists between any two masses. If the average mass density is greater than a certain value, called the critical density, the force of gravity will eventually halt the expansion of the universe and cause it to collapse into what is sometimes called the “big crunch.” If the average density is less than or equal to the critical density, the universe will continue to expand forever.