Quantum Theory Cannot Hurt You_ A Guide to the Universe - Marcus Chown [66]
A static universe was appealing because it remained the same for all time. There was no need to address sticky questions about where the Universe came from or where it was going. It had no beginning. It had no end. The reason the Universe was the way it was was because that was the way it had always been.
According to Newton, for the Universe to be static, one condition had to be satisfied: matter had to extend infinitely in all directions. In such a neverending cosmos, each body has just as many bodies on one side, pulling it one way with their gravity, as on the opposite side, pulling it the other way. Like a rope being pulled by two equally strong tug-of-war teams, it therefore remains motionless.
However, according to Einstein’s theory of gravity, the Universe was finite, not infinite. Its space-time curved back on itself—the four-dimensional equivalent of the two-dimensional surface of a basketball. In such a Universe the gravitational tug-of-war is at no point perfectly balanced. Because every body tries to pull every other body toward it, the Universe shrinks uncontrollably.
To salvage the idea of a static Universe, Einstein had to resort to mutilating his elegant theory. He added a mysterious force of cosmic repulsion, which pushed apart the objects in the Universe. He hypothesised that it had a significant effect only on bodies that were enormously far apart, explaining why it had not been noticed before in Earth’s neighbourhood. By precisely counteracting the force of gravity that was perpetually trying to drag bodies together, the cosmic repulsion kept the Universe forever static.
THE EXPANDING UNIVERSE
Einstein’s instincts turned out to be wrong. In 1929, Edwin Hubble—the American astronomer responsible for discovering that the Universe’s building blocks were galaxies—announced a dramatic new discovery. The galaxies were flying apart from each other like pieces of cosmic shrapnel. Far from being static, the Universe was growing in size. As soon as Einstein learned of Hubble’s discovery of the expanding universe, he renounced his cosmic repulsion, calling it the biggest blunder he ever made in his life.
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Einstein’s mysterious repulsive force could never have kept the galaxies hanging motionless in space. As Arthur Eddington pointed out in 1930, a static cosmos is inherently unstable, like a knife balanced on its point. The merest nudge would be enough to set it expanding or contracting.
Others did not make the same mistake as Einstein. In 1922 the Russian physicist Aleksandr Friedmann applied Einstein’s theory of gravity to the Universe and correctly concluded that it must either be contracting or expanding. Five years later the same conclusion was reached independently by the Belgian Catholic priest Georges-Henri Lemaître.
As John Wheeler has said: “Einstein’s description of gravity as curvature of space-time led directly to that greatest of all predictions: The Universe itself is in motion.” It is ironic that Einstein himself missed the message in his own theory.
THE BIG BANG UNIVERSE
Since the Universe is expanding, one conclusion is inescapable: it must have been smaller in the past. By imagining the expansion running backwards, like a movie in reverse, astronomers deduce that 13.7 billion years ago all of Creation was squeezed into the tiniest of tiny volumes. The lesson of the receding galaxies is that the Universe, though old, has not existed forever. There was a beginning to time. A mere 13.7 billion years ago, all matter, energy, space, and time fountained into existence in a titanic explosion—the Big Bang.
The cosmic expansion turns out to obey a remarkably simple law: Every galaxy is rushing away from the Milky Way with a speed that is in direct proportion to its distance. So a galaxy that is twice as far away as another