This year will be a doozy for doomsayers. Depending on the prophecy, the world is predestined to expire by means of a solar storm, asteroid strike, rogue-planet collision, plague, falling stars, earthquake, debt crisis, or some combination thereof. Of course, nobody seems to be preparing for any of these impending 2012 apocalypses, with the exception of a porn studio reportedly building a clothing-optional underground bunker.
And why should we? Scientifically speaking, the prophecies are strictly ballyhoo. Physicists can do a lot better. When it comes to end-times scenarios, cosmological data-crunchers have at their disposal far more meaningful prognostication tools that can tell us how it's really going to end—not just Earth, but the whole universe. Best of all, they can tell us how to survive it.
Science, oddly, is a lot better at predicting things like the death of stars than next week's weather. The same laws of physics that enable scientists to study the Big Bang that occurred 13.7 billion years ago also allow them to gaze into the future with great precision. And few people have peered farther than University of California, Santa Cruz, astronomer Greg Laughlin, science's leading soothsayer. As a graduate student in 1992, he was plugging away at a simple computer simulation of star formation when he broke for lunch and accidentally left the simulation running. When he returned an hour later, the simulation had advanced 100 million billion years, much further into the future than most scientists ever think (or dare) to explore.
The program itself didn't reveal anything terribly startling—the simulated star had long since gone cold and died—but Laughlin was intrigued by the concept of using physical simulations to traverse enormous gulfs of time. "It opened my eyes to the fact that things are going to evolve and are still going to be there in timescales that dwarf the current age of the universe," he says.
Four years later, still fascinated, Laughlin teamed up with Fred Adams, a physics professor at the University of Michigan, to investigate the future of the universe more rigorously. Working in their spare time, the two researchers coauthored a 57-page paper in the journalReviews of Modern Physics that detailed a succession of future apocalypses: the death of the sun, the end of the stars, and multiple scenarios for the fate of the universe as a whole.
The paper made a surprising splash in the popular press, even grabbing the front page of The New York Times.Soon Laughlin and Adams found themselves in great demand on the lecture circuit, joining like-minded colleagues in discussions about such weighty topics as the physics of eternity and possible survival strategies for unthinkably grim cosmic events. (One future projection calls for a violent rip in the fabric of space-time that annihilates all matter within 30 minutes.) "Nobody makes it his life's work," says Glenn Starkman, a theoretical physicist at Case Western Reserve University in Cleveland who has coauthored papers such as "Life and Death in an Ever-Expanding Universe," among other lighthearted fare. "There are more pressing problems," he says, "but it is fun stuff to think about."
FLIGHT FROM PLANET EARTH
For Starkman and other futurists, the fun begins a billion years from now, a span 5,000 times as long as the era in which Homo sapiens has roamed Earth. Making the generous assumption that humans can survive multiple ice ages and deflect an inevitable asteroid or comet strike (NASA predicts that between now and then, no fewer than 10 the size of the rock that wiped out the dinosaurs will hit), the researchers forecast we will then encounter a much bigger problem: an aging sun.
Stable stars like the sun shine by fusing hydrogen atoms together to produce helium and energy. But as a star grows older, the accumulating helium at the core pushes those energetic hydrogen reactions outward. As a result, the star expands and throws more and more heat into the universe. Today's sun is already 40 percent brighter than it was when it was born 4.6 billion years ago. According to a 2008 model by astronomers K.-P. Schröder and Robert Connon Smith of the University of Sussex, England, in a billion years the sun will unleash 10 percent more energy than it does now, inducing an irrefutable case of global warming here on Earth. The oceans will boil away and the atmosphere will dry out as water vapor leaks into space, and temperatures will soar past 700 degrees Fahrenheit, all of which will transform our planet into a Venusian hell-scape choked with thick clouds of sulfur and carbon dioxide. Bacteria might temporarily persist in tiny pockets of liquid water deep beneath the surface, but humanity's run in these parts would be over.
Such a cataclysmic outcome might not matter, though, if proactive Earthlings figure out a way to colonize Mars first. The Red Planet offers a lot of advantages as a safety spot: It is relatively close and appears to contain many of life's required ingredients. A series of robotic missions, from Viking in the 1970s to the Spirit rover still roaming Mars today, have observed ancient riverbeds and polar ice caps storing enough water to submerge the entire planet in an ocean 40 feet deep. This past August the Mars Reconnaissance Orbiter beamed back time-lapse photos suggesting that salty liquid water still flows on the surface.
The main deterrent to human habitation on Mars is that it is too cold. A brightening sun could solve that—or humans could get the job started without having to wait a billion years. "From what we know, Mars did have life and oceans and a thick atmosphere," says NASA planetary scientist Christopher McKay. "And we could bring that back."
McKay is a leading scientist in the study of transforming Mars into an Earth-like world through a process calledterraforming. Drawing on lab experiments and climate models, he has demonstrated that manufacturing and releasing more than 3 billion tons of perfluorocarbons and other intense greenhouse gases there would warm the planet. Natural processes on Mars would then take over: Ice caps would melt, releasing water and carbon dioxide and speeding up the warming process until the planet had a thick, sustainable atmosphere. In McKay's mind, 1 billion years is plenty of time to custom-build a Martian outpost and a spacecraft to take us there. Existing technology, he notes, could theoretically blast astronauts to Mars in three months. One hopes we could improve on that over the next eon.
For now, let's assume we do, and humanity transitions successfully to Mars. By Laughlin's calculations, life there could proceed relatively comfortably for another 4.5 billion years after Earth becomes uninhabitable and before the sun's bloat once again forces a move. According to standard models of stellar evolution, around that time the sun will largely deplete the hydrogen reserves in its core and begin to balloon as its fusion reactions migrate outward. Through their telescopes astronomers have watched this scenario play out with many other stars, so they know with considerable certainty what happens next: In a dramatic growth spurt, the sun will swell to become a red giant star, 250 times as large and 2,700 times as bright as it is now, stretching farther and farther out into the solar system. It will vaporize Mercury, Venus, and Earth and turn Mars into a molten wasteland.
So where to next? Martian colonies could pack up the spaceship and relocate to Jupiter's moon Europa, where scientists believe a large ocean of liquid water hides beneath an icy crust. Heated by a brightening sun, Europa could turn into a lush ocean planet. When Europa overheats, Saturn's moon Titan—which already has a thick atmosphere rich in organic compounds—could be humanity's next rest stop. But eventually the sun will fry that outpost and every other one in the solar system as well. Even the miserably cold Pluto (–400 degrees Fahrenheit at present) will be too hot for habitability. Finally, about 130 million years after the red giant phase, the sun will go through a final spasm and eject its outer layers into space, leaving behind a white dwarf: a hot, dense lump of carbon and oxygen no larger than Earth. Moving within the solar system during all that drama would be a bit like relocating the beach house an inch inland.
ON TO PROXIMA CENTAURI
Under these circumstances, Laughlin believes the continued survival of our species will depend on the development of high-occupancy starships propelled by nuclear fusion or matter-antimatter annihilation that can transport people rapidly to planets orbiting other stars. (Current chemical rockets are far too slow; they would take 100,000 years just to reach the closest stars.) Astronomers have already identified over 600 planets around other stars, some of them roughly the size of Earth, and believe many billions more exist within our galaxy.
For a long-term solution, Laughlin recommends colonizing a planet with a much more stable sun. Take Proxima Centauri, the very closest neighboring star—only 4.2 light-years from Earth. It is a red dwarf, considerably smaller and cooler than our current sun but with a life span of 4 trillion years, roughly 400 times as long. Astronomers have not found any planets orbiting it yet, but they have discovered planets orbiting similar stars. Red dwarfs also happen to be the most common type of star in the galaxy, so even though Proxima Centauri will not always be close, we'll still have plenty of housing options.
If the human population can successfully colonize planets orbiting Proxima Centauri or another red dwarf, we can enjoy trillions of years of calamity-free living. Says Laughlin, "The future lies with red dwarfs."
BUILDING A STAR-IN-A-BOX
That is, until the red dwarfs die. When Proxima Centauri perishes, humanity can relocate to another red dwarf and then another, but that strategy won't work forever. Stars need fuel, and as vast as the universe is, there is only so much to go around. Newly forming stars are gradually depleting the cosmic supply of hydrogen. Roughly 100 trillion years into the future, they will have exhausted the universe's hydrogen stockpile. As existing stars consume their last drops of fuel, they will wink out one by one, and the light of the universe will almost entirely disappear.
What then? How could humanity possibly survive without light and warmth? Laughlin says the answer lies in the universe's secret fuel reserves: brown dwarfs, Jupiter-size balls of hydrogen too massive to be considered planets but that never achieved the heft to become full-fledged stars. In 2009 nasa launched the 1,433-pound wise satellite, carrying a wide-field infrared telescope designed in part to detect such stunted stars; it has since turned up 100 of them within 20 light-years of Earth. Judging from this sample, the galaxy may hold billions more. Laughlin envisions that those cold balls of gas could keep civilization thrumming even after the red dwarfs die out. When brown dwarfs occasionally collide, he explains, they can trigger the birth of a new life- sustaining star. "For a long time there will always be about 10 to 15 stars shining in the galaxy, each lasting trillions of years," Laughlin says. "Brown dwarf collisions should continue for another 10 billion billion years." That would keep us going a thousand times as long as red dwarfs.
But we may not need to rely on chance collisions. Glenn Starkman, the physicist at Case Western, considered the starless era as well and came up with a contingency plan. By the time nature can no longer produce new stars, he says, we may know how to create our own. Starkman holds out hope that someone somewhere will figure out a way to mine the remains of dead stars to generate energy. "We can do really well going from star to star, slowly consuming them," he says.
Neutron stars, the collapsed remnants of giant stars, would provide the most bang for the buck. They are among the densest objects in the universe, packing a mass several times that of the sun into a sphere just 10 to 15 miles across. "Each one would power a civilization for huge amounts of time," Starkman says. How to harness all that energy is another question entirely.
LAST DAYS OF THE UNIVERSE
When physicists project forward 100 trillion years, they see potential threats much more dire than a ballooning sun or even the dying of all the stars. Starkman says we must also consider the potentially game-ending influence of an all-pervasive cosmic force known as dark energy. "To put it simply," he says, "dark energy is very bad for life."
Scientists don't know what dark energy is, but they do know that it exerts a repulsive effect that makes the universe expand faster and faster. At first glance that may seem like a good thing. An expanding universe creates more space, and by extension, an ever-growing frontier for life to explore and exploit. But dark energy has an unfortunate downside: It pulls galaxies away from each other.
In about 100 billion years, as future humans are enjoying an extended stay near Proxima Centauri, some physicists like Starkman believe that dark energy will drastically stretch out the vast amounts of empty space between the Milky Way and other galaxies, creating an impassable gulf between them. In this way, every galaxy outside our own would eventually become invisible and inaccessible; trying to travel between galaxies would be as futile as jogging in place. Even if future generations achieve the sci-fi dream ofStar Trek–esque starships darting at the speed of light, they could never reach extragalactic destinations.
The result of this phenomenon, known to physicists as the Big Chill, would be a catastrophic limit on resources. With all matter and energy beyond the Milky Way fundamentally out of reach, we would no longer even be able to see stars and planets in other galaxies. "There will be no new worlds to conquer," Starkman wrote in his 2000 paper exploring life's ultimate fate. "We will truly be alone in the universe." In that scenario, humans would have to make the most of every remaining neutron star and brown dwarf in our galaxy. But once we consume every last parcel of matter and energy, there will be nothing more. Humanity will go extinct.
The universe would live on forever, though only as a shadow of its former vibrant self. It would gradually become darker, colder, and emptier as the scant remaining matter decays or gets sucked up by the giant black holes at the core of every galaxy. Once they have gobbled up every semblance of matter, in about 10100 years, even the black holes will evaporate and disappear.
That is a bleak scenario, but it's not the bleakest, says Dartmouth College physicist Robert Caldwell. According to his calculations, the Big Chill would be a happy ending compared with something he and his colleague Marc Kamionkowski have dubbed the Big Rip. In his 2003 paper "Phantom Energy and Cosmic Doomsday," Caldwell explored the possibility that in the future dark energy will grow even stronger. At present it makes itself felt only over huge distances, such as the gaps between clusters of galaxies, but Caldwell says that some theories indicate that dark energy might just be kicking into gear. If that is the case, then within 20 billion years—fairly early in our sojourn around a red dwarf—dark energy could start to wreak havoc on much smaller objects.
Stars would be yanked away from galaxies. Then planets would be pulled from their stars. And in one extraordinary half hour, dark energy would progressively tear even the smallest pieces of the universe apart. Layer by layer, humanity's home planet would be dismantled—first the atmosphere, then the crust, all the way down to the core—in a fantastic explosion. "Anything resting on the planet will just—whoosh—float off," Caldwell says. In the final 10-19 second, dark energy would rip individual atoms apart. Finally, it will tear the very fabric of space-time at the seams, marking the official end of the universe. The only solace is that life's extinction would be quick and painless.
Scientists know too little about dark energy to determine with any certainty whether the universe's fate is a Big Chill, a Big Rip, or neither. Caldwell and other cosmologists are studying distant supernovas to measure the universe's expansion and explore the trend of dark energy's influence over time. "We're right on the dividing line between the Big Chill and the Big Rip," Caldwell says. "The window of uncertainty includes both possibilities."
THE LAST ESCAPE
Even in the most optimistic forecast, dark energy will eventually starve us of resources in a Big Chill, but that leaves us 10,000,000,000,000,000,000 years to perfect the most extreme survival strategy of all: escaping the universe before it chills, rips, crunches, bounces, or snaps into nothingness (yes, those are all scenarios that physicists have considered).
Many cosmologists now believe there are other universes hidden from our view—as many as 10500, according to string theory, a leading approach to unifying all the universe's physical laws into one elegant solution. This past August, Greek and German physicists used string-theory equations to demonstrate that it may be possible to develop wormholes connecting our universe to another. With 10500 to choose from, at least one should be suitable for life.
Just don't look to Starkman for how-to advice. Tunneling through wormholes to other universes apparently crosses his delicate line separating scientific prognostication from 2012 theology. "Now we're really getting speculative," he says.