The end of the universe in two poems
Some say the world will end in fire,
Some say in ice.
-Robert Frost, 1920
This is the way the world ends
Not with a bang but a whimper
-T.S. Eliot, 1925
Fire and Ice surely ranks as one of the best-known poems of Robert Frost. What is not widely known about the poem, however, is that Harlow Shapley, one of the founding fathers of modern astronomy, claimed to have inspired it.
Shapley had a favorite anecdote about a conversation with Frost at a Harvard faculty dinner sometime a few years before the publication of Fire and Ice. According to Shapley, Frost sought him out and pressed him with the simple, blunt question: How is the world going to end? Shapley answered as any astronomer would: In about five billion years, the Earth will either be incinerated when our sun swells up to be a red giant star, or it will swing far away from the sun and fall into a deep, permanent ice age. Fire and Ice.
More recently, this poem has been quoted by astronomers talking not just about the end of the Earth, but the end of the entire universe. You see, the Big Bang theory introduced an entirely new element into our view of the universe. Put simply, a beginning implies an end. But what kind of end are we in for?
For a long time, the answer to this question hinged on how, exactly, the expanding universe really works. Would our universe keep expanding forever, or would the gravitational attraction of all the mass in the universe eventually stop the expansion, or even bring the universe collapsing down on itself? Getting back to the Fire and Ice analogy, the universe will either expand until all matter is very very far apart and very very cold, or gravity will bring all the matter in the universe back together into an unimaginably dense and hot ball, similar to the state it was in just before the Big Bang (astronomers often call this theory the "Big Crunch").
The factor separating the two scenarios is density. Is there actually enough matter in the universe for gravity to overcome the expansion? How can we tell which fate awaits us? To answer that, we would have to understand the behavior of the universe as a whole.
Amazingly, astronomers are getting more confident that they actually can do precisely that. A number of research teams are all coming up with the same result: The universe does not have the density needed to stop the expansion. And the view they're putting together of our far future now has astronomers quoting the last stanza of T.S. Eliot's poem, the Hollow Men. The universe, it seems, will end not with a bang, but a whimper.
Conventional estimates of the total mass of all the galaxies have long come up short of the density needed for re-collapse, and even with the addition of dark matter (which may comprise 90 percent of all the matter in the universe), the most you can do is slow the expansion down a little. Recent observations of the cosmic microwave background (which appears to show us the universe as it was about 100,000 years after the Big Bang) agree with this assessment.
