“
Yes, he admitted, if gravity is always attractive, and never repulsive, then the stars in the universe might be unstable. But there was a loophole in this argument. Assume that the universe is, on average, totally uniform and infinite in all directions. In such a static universe, all the forces of gravity cancel one another out, and the universe becomes stable once again. Given any star, the forces of gravity acting on it from all the distant stars in different directions eventually sum to zero, and hence the universe does not collapse. Although this was a clever solution to this problem, Newton realized there was still a potential flaw to his solution. The universe might be uniform on average, but it cannot be exactly uniform at all points, so there must be tiny deviations. Like a house of cards, it appears to be stable, but the tiniest flaw will cause the entire structure to collapse. So Newton was clever enough to realize that a uniform infinite universe was indeed stable but was always teetering on the edge of collapse. In other words, the cancellation of infinite forces must be infinitely precise or else the universe will either collapse or be ripped apart. Thus, Newton’s final conclusion was that the universe was infinite and uniform on average, but occasionally God has to tweak the stars in the universe, so they do not collapse under gravity. Why Is the Night Sky Black? But this raised another problem. If we start with a universe that is infinite and uniform, then everywhere we look into space our gaze will eventually hit a star. But since there are an infinite number of stars, there must be an infinite amount of light entering our eyes from all directions. The night sky should be white, not black. This is called Olbers’ paradox. Some of the greatest minds in history have tried to tackle this sticky question. Kepler, for example, dismissed the paradox by claiming that the universe was finite, and hence there is no paradox. Others have theorized that dust clouds have obscured starlight. (But this cannot explain the paradox, because, in an infinite amount of time, the dust clouds begin to heat up and then emit blackbody radiation, similar to a star. So the universe becomes white again.) The final answer was actually given by Edgar Allan Poe in 1848. Being an amateur astronomer, he was fascinated by the paradox and said that the night sky is black because, if we travel back in time far enough, we eventually encounter a cutoff—that is, a beginning to the universe. In other words, the night sky is black because the universe has a finite age. We do not receive light from the infinite past, which would make the night sky white, because the universe never had an infinite past. This means that telescopes peering at the farthest stars will eventually reach the blackness of the Big Bang itself. So it is truly amazing that by pure thought, without doing any experiments whatsoever, one can conclude that the universe must have had a beginning.
”
”