Zero - Charles Seife [77]
One possibility was that the universe would collapse under its own gravity. As the universe gets smaller and smaller, it heats up more and more. It burns brightly with radiation, destroying all life and eventually destroying the atoms that make up matter. It would be death by fire. Eventually, the universe would crunch itself into a zero-dimensional point—like a black hole—and would disappear forever.
The other possibility is, if anything, more grim. The universe would expand forever. Galaxies would become ever more distant from one another, and the star stuff that drives all the energetic reactions in the universe would become more rarefied. Stars would burn out as they exhausted their fuel, and galaxies would become darker and darker—and then cold and silent. The cold, dead matter of the stars would decay away, leaving nothing but a smear of radiation that spreads equally throughout the universe. The cosmos would be a cold soup of dimming light. It would be death by ice.
To Einstein, these ideas were abhorrent. Like Aristotle, he implicitly assumed that the universe was static, constant, and eternal. The only way out was to “correct” his equations of general relativity to stave off the impending destruction. He did this by adding a cosmological constant, an as yet undetected force that counteracts the force of gravity. The cosmological constant’s push would balance out gravity’s pull; instead of collapsing, the universe could stay in a steady balance, neither collapsing nor expanding. Postulating the existence of such a mysterious force was a desperate act. “I have…again perpetrated something about gravitation theory which somewhat exposes me to the danger of being confined in a madhouse,” wrote Einstein, but he was so worried about the impending destruction of the universe that he was forced to take such a dramatic step.
Einstein wasn’t bundled off to an asylum. Einstein had proposed stranger things and had been entirely right. However, this time he was not so lucky. The stars themselves destroyed Einstein’s vision of a static, eternal cosmos.
In 1900 the Milky Way was the known universe. Astronomers had little idea that anything lay beyond our own dusty little disk of stars. Though astronomers had spotted some glowing, swirly clouds, there was little reason to believe that they were anything but glowing gas inside our galaxy. In the 1920s that all changed, thanks to an American astronomer named Edwin Hubble.
A special type of star, called a Cepheid variable, had a property that allowed Hubble to measure the distance to faraway objects. Cepheid stars pulsate, getting brighter and dimmer in a very predictable way; the way they pulsate is closely related to how much light they put out. They are standard candles, objects of known brightness, and became a key tool for Hubble. They were like the headlights of a train.
If you watch a train coming at you, you will see that its headlight gets brighter and brighter as it approaches. If you know how much light the headlight puts out—if the headlight is a standard candle—you can tell how bright the headlight will appear at any given distance. The closer it gets, the brighter it seems. The same logic works in reverse; if you know how much light a train’s headlight emits, you can measure its apparent brightness and calculate the train’s distance from you.
Hubble did the exact same thing with Cepheid stars. Most stars he saw were tens or hundreds or thousands of light-years away. But when he found a Cepheid blinking in one of these swirly clouds—the Andromeda nebula, as it was then called—he measured the light and calculated that the nebula was a million light-years away, far beyond the outer reaches of our galaxy. Andromeda was not a cloud of glowing gas; it was a cloud of stars so distant that they looked like a smear rather than individual points of light. Other swirly galaxies were even more distant. Today, astronomers suspect that the universe is about 15 billion light-years across and peppered everywhere with clusters