We Earthlings are newcomers to cosmology, the study of the universe as a whole, and there is no cosmological question about which we have more to learn than the riddle of where it's all ultimately headed. But we have glimpsed at least a few clues to cosmic destiny, some of them hopeful and others bleak.

The good news is that we're not going to be evicted. The universe is likely to remain hospitable to life for at least an additional 100 billion years. That's 20 times as long as the earth has existed, and 5 million times as long as Homo sapiens has lasted so far. If we're not around to shoot off fireworks on New Year's Eve of the year 100,000,000,000, it won't be the universe's fault.

The bad news is that nothing lasts forever. The universe may not disappear, but as time goes by it may get increasingly uncomfortable, and eventually become unlivable. Calculating how and when this will happen is a genuinely dismal science, but not without a certain grim fascination. The classic Big Bang theory, refined over the decades since the astronomer Edwin Hubble discovered the expanding universe in 1929, suggests that cosmic destiny will be decided through a tug-of-war between two opposing forces. One is the expansion of space, which for more than 10 billion years has been carrying galaxies ever farther apart from one another. The other is the mutual gravitational force exerted by those galaxies and all the other stuff in the universe; it acts as a brake, slowing down the expansion rate.

In this simple picture, if the gravitational force is strong enough to bring expansion to a halt, the universe is destined to collapse, ultimately dissolving into a fireball—a Big Crunch that amounts to the Big Bang run in reverse. If it's not, and expansion wins out, then the universe will eventually grow unpleasantly dark and cold. Stars produce energy by fusing light atomic nuclei, mainly hydrogen and helium, into heavier ones. When the hydrogen and helium run low, old stars will sputter out without any new ones to take their place, and the universe will gradually fade to black. Such were the gloomy alternatives that Robert Frost wrote about after being briefed on the theory of the cosmic endgame by the astronomer Harlow Shapley:

Some say the world will end in fire,
Some say in ice...
I think I know enough of hate
To know that for destruction ice
Is also great
And would suffice.
Either fate looks like curtains for life. If the end comes in fire, the Big Crunch would melt down everything, even subatomic particles. If, on the other hand, the universe winds up cold and dark, life might hang on for a long time—say, by extracting gravitational energy from black holes. But trying to make a living once everything has subsided to pretty much the same temperature—a tad above absolute zero—is like trying to run a water mill on a dead-still pond.

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