The Beginning and End of Time in Physical Cosmology

It appears that 15 to 18 billion years1 ago there was an immense eruption of energy, and all the bits of matter and energy then present in the universe started to move apart at immense speed. Within minutes, atoms of the chemical elements began to form: hydrogen, deuterium, and helium first; others later. All the time these fragments of matter were rushing outward at an enormous rate which however slowly decreased, since each particle first feels the gravitational attraction of the rest and that attraction (it can be shown from Newton’s theory of gravity) is directed back toward the point where the explosion took place. It is the same as the attraction that slows a ball or a rocket as it leaves the ground. A ball finally falls back again to the ground. A rocket, at greater speed, may take longer to fall back or it may move so fast that it continues on into space and never returns. The outcome depends on the initial speed. In exactly the same way, the force of gravity either will halt the expansion of the universe and bring it back again, once more to a single point, or else it will not, and the motion will continue forever. Which of the two will happen can in principle be decided by scientific measurement and we are likely to know the answer in a few years. The question is difficult only because the observed speeds place us rather close to the dividing line between continued expansion and ultimate collapse so the measurements must be more precise than is now possible.