Sometimes it seems that everybody is trying to tell you when and how the world is supposed to end. Some scenarios are more familiar than others. Those that are widely discussed in the media include rampant infectious disease, nuclear war, collisions with asteroids or comets, and environmental decay. While different in origin, each can induce the end of human species (and perhaps selected other life forms) on Earth. Indeed implicit in clichéd slogans such as “Save the Earth” is the egocentric call to save life on Earth, not the planet itself.
In fact, humans cannot really kill Earth. Earth will remain in orbit around the Sun, along with its planetary brethren, long after Homo sapiens have become extinct by whatever cause. But there are less familiar, though just as real, end-of-world scenarios that jeopardize our temperate planet in its stable orbit around the Sun. I offer these prognostications not because humans are likely to live long enough to observe them, but because the tools of astrophysics enable me to calculate them. Three that come to mind are the death of the Sun, the impending collision between our Milky Way galaxy and the Andromeda galaxy, and the death of the universe, about which the community of astrophysicists has recently achieved consensus.
Computer models of stellar evolution are akin to actuarial tables. They indicate a healthy 10 billion year life expectancy for our Sun. At an estimated age of 5 billion years, it has another 5 billion years of relatively stable energy output. By then, if we have not figured out a way to leave Earth, then we will bear witness to a remarkable evolutionary change in the Sun as it runs out of fuel.
The Sun owes its stability to the controlled fusion of hydrogen into helium in its 15 million degree core. The gravity that wants to collapse the star is held in balance by the outward gas pressure that is sustained by the fusion. While more than 90 percent of the Sun’s atoms are hydrogen, the ones that matter are those that reside in the core. When the core is exhausted of its hydrogen, the Sun is left with a central ball of helium atoms that require a higher temperature than does hydrogen to fuse into heavier elements. Now out of balance, gravity wins, the inner regions of the star collapse, and the central temperature rises through 100 million degrees, which triggers the fusion of helium into carbon.
In the process, the Sun’s luminosity grows astronomically, which forces its outer layers to expand to bulbous proportions, engulfing the orbits of Mercury and Venus. Eventually, the Sun will swell to occupy the entire sky as its expansion subsumes the orbit of Earth. This would be bad. The temperature on Earth will rise until it equals the 3,000 degree rarefied outer layers of the expanded Sun. Our atmosphere will evaporate away into interplanetary space and the oceans will boil off as Earth becomes a red-hot, charred ember orbiting deep within the Sun. Eventually, the Sun will cease all nuclear fusion, loose its spherical, tenuous, gaseous envelope, and expose its dying central core. Scenarios such as these will one day force manned space travel to become a global priority.
Not long after the Sun terrorizes Earth, the Milky Way will encounter some problems of its own. Of the hundreds of thousands of galaxies whose velocity relative to the Milky Way has been measured, only a few are moving toward us while all the rest are moving away at a speed directly related to their distances from us. Discovered in the 1920s by Edwin Hubble (after whom the Hubble Space Telescope was named), the general recession of galaxies is the observational signature of our expanding universe. The Milky Way and the three-hundred-billion-star Andromeda galaxy are close enough to each other that the effect of the expanding universe is negligible. We happen to be drifting toward each other at about 100 kilometers per second (a quarter million miles per hour). If our (unknown) sideways motion is small, then at this rate, the 2.2 million light-year distance that separates us will shrink to zero in about seven billion years.
Interstellar space is so vast that there is no need to fear whether stars in the Andromeda galaxy will accidentally slam into the Sun. During the galaxy-galaxy encounter, which would be a spectacular sight from a safe distance, stars are likely to pass each other by. But the event would not be worry-free. Some of Andromeda’s stars are likely to swing close enough to our solar system to influence the orbit of the planets and of the hundreds of billions of resident comets. For example, close stellar flybys can throw one’s gravitational allegiance into question. Computer simulations commonly show that the planets are either stolen by the interloper in a “flyby looting” or they become unbound and are flung forth into interplanetary space.
Remember how choosy Goldilocks was with other people’s porridge? If we are stolen by the gravity of another star, there is no guarantee that our new-found orbit will be at the right distance to sustain liquid water on Earth’s surface—a condition generally agreed to be a prerequisite to sustaining life as we know it. If Earth orbits too close, its water supply evaporates. And if Earth orbits too far, its water supply freezes solid.
By some miracle of future technology, if Earth inhabitants had managed to prolong the life of the Sun, then these efforts will be rendered irrelevant when Earth is flung in space. The absence of a nearby energy source will allow Earth’s surface temperature to drop swiftly to hundreds of degrees below zero Fahrenheit. This would also be bad. Our cherished atmosphere of nitrogen and oxygen and other gases would first liquefy and then freeze solid, encrusting the Earth like icing on a cake. We would freeze to death before we had a chance to starve to death. The last surviving life on Earth would be those privileged organisms that had evolved to rely not on the Sun’s energy but on (what will then be) weak geothermal sources, where the heat of Earth’s interior emerges from the crust. At the moment, humans are not among them. There will be, of course, other planets that we can visit in orbit around healthy stars in other galaxies.
But the long-term fate of the cosmos cannot be postponed or avoided. No matter where you hide, you will be part of a universe that inexorably marches towards a peculiar oblivion. The latest and best evidence available on the space density of matter and the expansion rate of the universe suggest that we are on a one-way trip: the collective gravity of everything in the universe is insufficient to halt and reverse the cosmic expansion.
Currently, the most successful description of the universe and its origin combines the big bang with our modern understanding of gravity, derived from Einstein’s general theory of relativity. The early universe was a trillion-degree maelstrom of matter mixed with energy, affectionately known as the primordial soup. During the fourteen billion year expansion that followed, the background temperature of the universe has dropped to a mere 3° on the absolute (kelvin) temperature scale. As the universe continues to expand, this temperature will continue to approach zero.
Such a low background temperature does not directly affect us on Earth because our Sun (normally) grants us a cozy life. But as each generation of stars is born from the interstellar gas clouds of the galaxy, less and less gas remains to compose the next generation of stars. Eventually the gas supply will run out, as it already has in nearly half the galaxies in the universe. The small fraction of stars with the highest mass collapse completely, never to be seen again. Some stars end their lives by blowing their guts across the galaxy in a supernova explosion. This returned gas can then be tapped for the next generation. But the majority of stars—Sun included—ultimately exhaust the fuel at their cores and, after the bulbous giant phase, collapse to form a compact orb of matter that radiates its feeble leftover-heat to the frigid universe
The complete list of corpses may be familiar: black holes, neutron stars (pulsars), white dwarfs, and even brown dwarfs are each a dead end on the evolutionary tree of stars. What they each have in common is an eternal lock on cosmic construction materials. In other words, if stars burn out and no new ones are formed to replace them, then the universe will eventually contain no living stars.
How about Earth? We rely on the Sun for a daily infusion of energy to sustain life. If the Sun and the energy from all other stars were cut off from us then mechanical and chemical processes (life included) on and within Earth would “wind down.” Eventually, the energy of all motion gets lost to friction and the system reaches a single uniform temperature. This would really be bad. The starless Earth will lie naked in the presence of the frozen background of the expanding universe. The temperature on Earth will drop the way a freshly baked pie cools on a window sill. Yet Earth is not alone in this fate. Trillions of years into the future, when all stars are gone, and every process in every nook and cranny of the expanding universe has wound down, all parts of the cosmos will cool to the same temperature as the ever-cooling background. At that time, space travel will no longer provide refuge. Even Hell will have frozen over. We may then declare that the universe has died—not with a bang, but with a whimper.