Death by Black Hole: And Other Cosmic Quandaries - Neil deGrasse Tyson (2014)
SECTION 5. WHEN THE UNIVERSE TURNS BAD
Chapter 30. ENDS OF THE WORLD
Sometimes it seems that everybody is trying to tell you when and how the world will 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 blight. While different from one another, they all can bring about the end of the human species (and perhaps selected other life-forms) on Earth. Indeed, clichéd slogans such as “Save the Earth” contain the implicit call to save life on Earth, and not the planet itself. Fact is, humans cannot really kill Earth. Our planet will remain in orbit around the Sun, along with its planetary brethren, long after Homo sapiens has become extinct by whatever cause.
What hardly anybody talks about are end-of-world scenarios that do, in fact, 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, the Sun will enjoy another 5 billion years of relatively stable energy output. By then, if we have not figured out a way to leave Earth, we will be around when the Sun exhausts its fuel supply. At that time, we will bear witness to a remarkable yet deadly episode in a star’s life.
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 the fusion sustains. While more than 90 percent of the Sun’s atoms are hydrogen, the ones that matter reside in the Sun’s core. When the core exhausts its hydrogen, all that will be left there is a ball of helium atoms that require an even higher temperature than does hydrogen to fuse into heavier elements. With its central engine temporarily shut off, the Sun will go out of balance. Gravity wins, the inner regions of the star collapse, and the central temperature rises through 100 million degrees, triggering the fusion of helium into carbon.
Along the way, 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. Earth’s surface temperature will rise until it matches the 3,000-degree rarefied outer layers of the expanded Sun. Our oceans will come to a rolling boil as they evaporate entirely into interplanetary space. Meanwhile, our heated atmosphere will evaporate as Earth becomes a red-hot, charred ember orbiting deep within the Sun’s gaseous outer layers. These layers will impede the orbit, forcing Earth to trace a rapid death spiral down toward the Sun’s core. As Earth descends, sinking nearer and nearer to the center, the Sun’s rapidly rising temperature simply vaporizes all traces of our planet. Shortly thereafter, the Sun will cease all nuclear fusion; lose its tenuous, gaseous envelope containing Earth’s scattered atoms; and expose its dead central core.
But not to worry. We will surely go extinct for some other reason long before this scenario unfolds.
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 reliably 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 several-hundred-billion-star Andromeda galaxy are close enough to each other that the expanding universe has a negligible effect on their relative motions. Andromeda and the Milky Way 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 2.4-million light-year distance that separates us will shrink to zero within about 7 billion years.
Interstellar space is so vast and empty that there is no need to worry about stars in the Andromeda galaxy accidentally slamming 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 could swing close enough to our solar system to influence the orbit of the planets and of the hundreds of billions of resident comets in the outer solar system. 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 get flung into interplanetary space.
Back in Section 4, remember how choosy Goldilocks was with other people’s porridge? If Earth gets stolen by the gravity of another star, there’s no guarantee that our newfound 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.
If, by some miracle of future technology, Earth’s inhabitants manage to prolong the Sun’s life, then these efforts will be rendered irrelevant when Earth is flung into the cold depths of space. The absence of a nearby energy source will allow Earth’s surface temperature to drop swiftly to hundreds of degrees below zero Fahrenheit. Our cherished atmosphere of nitrogen and oxygen and other gases would first liquefy and then drop to the surface and freeze solid, encrusting the Earth like icing on a spherical 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 and geochemical sources, deep beneath the surface, in the cracks and fissures of Earth’s crust. At the moment, humans are not among them.
One way to escape this fate is to fire up the warp drives and, like a hermit crab and snail shells, find another planet elsewhere in the galaxy to call home.
WITH OR WITHOUT warp drives, 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 toward a peculiar oblivion. The latest and best evidence available on the space density of matter and energy 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.
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. As we will see in Section 7, the very early universe was a trillion-degree maelstrom of matter mixed with energy. During the 14-billion-year expansion that followed, the background temperature of the universe has dropped to a mere 2.7 degrees on the absolute (Kelvin) temperature scale. And 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 clouds of interstellar gas, less and less gas remains to comprise the next generation of stars. This precious gas supply will eventually run out, as it already has in nearly half the galaxies in the universe. The small fraction of stars with the highest mass will 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 short list of corpses may sound familiar: black holes, neutron stars (pulsars), and white dwarfs are each a dead end on the evolutionary tree of stars. But what they each have in common is an eternal lock on the material of cosmic construction. 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. Earth, sitting beneath starless skies, 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 apple pie cools on a windowsill. 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 because even Hell will have frozen over.
We may then declare that the universe has died—not with a bang, but with a whimper.