How Will The Universe End? (With A Bang or A Whimper?)

By Timothy Ferris

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.

Our ultimate fate, though, remains in doubt, in part because the jury is still out on whether the expansion or gravity will triumph in the end. Most observations point toward the former, but many uncertainties persist. One is the galling "dark matter" issue. Studies of how galaxies are moving around indicate that there's lots of extra gravity out there, suggesting that the stars and nebulae we can see constitute only 1% to 10% of the matter in the universe. The rest is invisible; it emits no light. Nobody yet knows what this dark matter is. One possibility is that it's made of WIMP--weakly interacting massive particles. Until the dark matter is identified and tallied, predicting the future of the universe on the basis of what we can see will be as uncertain as trying to predict an election by polling a few golfers down at the country club.

Meanwhile, ironists and fatalists draw austere satisfaction from the fire-or-ice scenario, which reflects the quintessential human perception that nobody gets out of life alive. And that's just what makes me suspicious of it. The great lesson of scientific cosmology is that the universe does not usually conform to our time-honored ways of thinking--that to understand it, we need to think in new ways. Integral to modern cosmology are mind-bending 20th century concepts like Einstein's curved space, Heisenberg's uncertainty principle and the realization that exotic subatomic particles sail through our bodies by the trillions without laying a glove on us, and I see no reason to suppose that doors won't open onto even stranger notions in the century to come. So perhaps we can glimpse a few shafts of light, shining under doors as yet unopened, that promise to refine our predictions of cosmic destiny.

One unknown has to do with inflation--the theory that the universe began as a bubble of empty space that initially expanded at a velocity much faster than that of light (see "Will We Discover Another Universe?", in this issue). Cosmologists take inflation seriously because it resolves problems that bedeviled older versions of the Big Bang, but inflation also has implications for the study of cosmic destiny. Among them is that the force that drove the inflationary spasm, sometimes tagged with the Greek letter lambda after its designation in Einstein's general-relativity equations, might not have subsided altogether back when the inflationary hiccup ended. Instead, it might still be there, lurking in empty space and urging expansion along, like an usher politely shooing playgoers back into the theater at intermission's end. Some observations of exploding stars in distant galaxies suggest the presence of just such an ongoing inflationary impulse. If so, the tug-of-war over the future of the universe involves not only expansion and gravitational braking but also the subtle turbocharging of lingering inflation, which acts to keep the universe expanding indefinitely.

Perhaps the most intriguing unknown, however, concerns the cosmic role played by intelligent life itself. As the physicist Freeman Dyson notes, "It is impossible to calculate in detail the long-range future of the universe without including the effects of life and intelligence." Much of the earth has been transformed, for better and worse, by the presence here of an intelligent species capable of manipulating its environment for its own benefit.

Similarly, advanced civilizations in the far future might be able to melt down stars and even entire galaxies to make gigantic campfires, or otherwise tilt the long-term odds in their favor. Life in the waning cosmic twilight might be jejune, but it could last a long time. Consider the marshaled resources of all the natural and artificial intelligences in the observable universe over the next, say, trillion years. Which would you bet on to prevail--that level of smarts or a claim, based on 19th century thermodynamics, that they're doomed?

So let's stay tuned, heeding the words of Einstein, who wrote to a child anxious about the fate of the world, "As for the question of the end of it I advise: Wait and see!"

Timothy Ferris is the author of The Whole Shebang: A State-of-the-Universe(s) Report and the PBS special Life Beyond Earth