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.
To make matters even stranger, preliminary results from some observations of distant galaxies suggest that not only will the universe not collapse, but the expansion will accelerate over time, with the galaxies flying ever faster apart. At any rate, it looks more and more like the universe will indeed expand forever. This raises a lot of interesting questions. With the Big Crunch idea, there was a definite end, with everything getting hotter and denser until matter itself broke down into a firey subatomic soup. But what happens when you leave the universe to run down over a vast span of time?
For starters, the galactic neighborhood we know today won't be around for all that much longer. Galaxies are constantly interacting and colliding with each other, forming larger, blob-shaped super-galaxies. Several nearby large elliptical galaxies have been caught red-handed swallowing up smaller galaxies, and we'll collide and possibly merge with the Andromeda galaxy in a few billion years.
On an intergalactic scale, we'll see the end of the lovely, clustered spiral galaxies of today replaced by fewer, larger elliptical galaxies. Those leftover galaxies will continue to move farther and farther away from each other, until each one is functionally alone in the void of space. Light from distant galaxies is red-shifted, meaning the light loses energy (and becomes more red) as it travels through the expanding space of the universe. Eventually, light from all the other galaxies in the sky will be red-shifted way down past visible light, through infrared, microwaves, and finally to low-energy radio waves. If people in the far future looked out into space using telescopes like ours, they wouldn't be able to detect a single other galaxy.
That might make things a bit boring for far-future astronomers, but in about 100 trillion years, things get even more dire. Stars shine by burning hydrogen into heavier elements in their nuclear furnaces. After a while, all the hydrogen will get burned up, and unfortunately, there's no new source of hydrogen for the universe. Eventually, all the stars will burn out and die, and all the raw material to make new stars (hydrogen) will be used up. No more starlight to light up the night sky.
All that will be left are the corpses of dead stars: black dwarfs (white dwarf stars that have cooled into cold, dark cinders), neutron stars, and black holes. With no more stars to warm their families of planets, environments like the Earth will disappear for good at this point, but that might not spell death for some advanced civilizations.
Science fiction authors, as well as theoretical astronomers, have begun to postulate how life might survive in this cold, dark era. There's still a lot of energy to be had by tapping into the angular momentum of rapidly spinning neutron stars and black holes. Far-future civilizations, huddling around the spinning remains of our present stars, might well look back at us in wonder. It might seem unbelievable to them that there was ever a time when abundant energy fell freely from the sky like manna from heaven.
Now we start getting into really deep time, as some astronomers are calling it. In about one hundred trillion trillion trillion trillion years (that would be a 1 with 38 zeroes after it), all the remaining mass in the universe will have collapsed into black holes. In all likelihood, there will be only one gigantic black hole left at the center of each super-galaxy, as gravity will have attracted all the smaller black holes and stellar remnants together.
At this point, astronomers don't really know if any matter, as we think of it, will exist in the universe at all. Even the members of the black-hole spinning civilizations have a big problem on their hands: physicists suspect that the fundamental building-blocks of matter, protons and neutrons, may not be able to hold up after this much time. The idea that protons and neutrons eventually decay into stray quarks is still controversial; no one has ever observed a proton decaying.
That's not much of a surprise if protons live trillions and trillions of years, and our universe has only been around for a measly 15 billion. But statistically, if you observe trillions and trillions of protons, some should have shorter life-spans than others, and the search is on for the first proton-decay detection. Stay tuned on that one. But if this theory turns out to be true, even the structure of matter itself has a shelf-life.
So, is that the end of it all? Will the very atoms that make up our bodies today either end up sucked down a black hole or wander the universe in a haze of cold quarks? We're not quite to the end yet, it turns out. Even black holes may not be eternal.
After ten thousand trillion trillion trillion trillion trillion trillion trillion trillion years (a 1 followed by 100 zeroes, this time), even the biggest black holes may evaporate into nothingness. How do you get rid of a black hole? The answer comes from Hawking radiation, named after the famous Cambridge physicist Stephen Hawking, who first proposed it.
The idea seems very strange; black holes, by their very nature, don't radiate any kind of light at all. But on the tiny scales of quantum physics, it turns out to be impossible for black holes to be completely black. The theory depends on empty space itself containing some amount of energy. This "vacuum energy" can manifest itself by creating a particle-antiparticle pair (like an electron and its antimatter twin, a positron), literally out of nothing.
This particle creation happens constantly, all around us. But as the two particles usually annihilate each other almost instantaneously, and we never notice it. If such a particle pair is created right at the mouth of a black hole, it's possible for one particle to fall into the black hole while the other escapes. Now, all of a sudden, there's an electron in the universe that shouldn't really have been there. The energy to create that electron, by some cosmic book-keeping, comes from the black hole. And so, over time, black holes lose energy one particle at a time, and eventually evaporate.
What we're left with at that point isn't too inspiring. As the universe continues to expand, any remaining photons will be stretched out to the point of non-existence, and any leftover particles will keep getting farther and farther apart from each other. And, at the moment the last particle disappears over the cosmic horizon of expanding space, we'll hear that final, tiny whimper.
Michelle Thaller is an astronomer at the California Institute of Technology. A massive-star specialist by trade, she dedicates most of her time to education and public outreach for the Space Infrared Telescope Facility.