In the face of disaster, optimists tend to be grateful because they easily imagine how much worse things could have been. Count astrophysicists among them. When we hear about earthly problems, many of us think to ourselves, "You have no idea." Worried about something falling on your head as you walk down the street? We've got something better. Thousands of asteroids the size of baseball stadiums—and larger—orbit the sun with trajectories that intersect Earth's path. Eventually, Earth will collide with every one of them—at impact speeds of at least seven miles per second.
The smallest of that set are large enough to cause deadly tsunamis and trillions of dollars of property damage. A medium-size asteroid will devastate our culture as it disrupts food chains, transportation systems, the electrical grid, and the overall stability of what we call civilization. The largest among them—the size of mountains—will launch an assault on the tree of life that will extinguish more than half of all land and oceanic species.
Worried about a hole in the ozone layer cased by pollutants? How about no ozone layer at all? Not too far away, in a neighboring galaxy, lies a prodigious stellar nursery, birthing stars of all sizes—small ones and large ones. At the top end of this range are stars that die spectacular deaths—exploding their guts at staggering speeds across the galaxy.
A particularly deadly subset of these will focus nearly all their explosive energy into a narrow beam that's bright enough to be seen across the entire universe. The beam is so intense with life-hostile UV and X-rays that if it happens to aim at Earth, the leading edge of this radiation will deplete our protective ozone layer entirely. Without this line of atmospheric defense, the radiation that follows will pass straight through the atmosphere, sterilizing Earth's surface.
Worried about falling into a hole in the ground? How about a black hole in space? If you fell into one of these, you'd never come out. The very fabric of space and time closes back on itself, preventing all escape. And as you fell—feet-first, let's say—the gravity at your feet would rapidly become much greater than the gravity at your head, forcing your body to stretch beyond comfort—beyond your body's capacity to remain whole.
Your body would snap, as your lower half separated from your torso. Each of those two body segments then would snap into two more pieces, and so forth. But it gets worse. During your fall, the fabric of space and time gets narrower, effectively extruding your body parts like toothpaste from a tube. We call this form of death "spaghettification."
Worried about a fender bender on the highway? Consider the impending collision between our beloved 100-billion-star Milky Way galaxy and our nearby cousin, the Andromeda Galaxy. These are two beautiful spiral galaxies currently minding their own business, yet they are hurtling toward each other through the vacuum of intergalactic space, with a closing speed of about 400 miles per second. We collide in about 6 or 7 billion years. Stars will not likely hit each other directly—space is too empty for that—but a gravitational free-for-all will ensue, with stars, and whatever planets orbit them, cast hither and yon in the cosmic equivalent of a train wreck.
Worried about global warming redrawing Earth's coastlines? How about no coastlines at all? In about 5 billion years, the sun will exhaust its stable supply of hydrogen fuel. In response, its inner regions will collapse, raise the core temperature, and ignite helium as the next fuel source.
In the meantime the sun's outer layers will expand prodigiously, engulfing the entire orbits of Mercury and Venus. As the sun continues to grow—as the sun's luminous surface gets closer and closer—Earth will get hotter and hotter. The oceans will come to a rolling boil, evaporate into the atmosphere, and lay bare the ocean floor. Our heated atmosphere will escape into space, as Earth's surface becomes a scorched wasteland.
Worried about Earth running out of fuel? The cosmos shares a similar problem. As the universe expands, the concentration of energy within it gets weaker and weaker. Eventually all gas clouds that make stars will have made all the stars they can. All stars, beginning with the most luminous ones, run out of fuel entirely. With nothing to replace them, the stars you see at night begin to blink off—one by one—as the universe becomes cold and dark and desolate. The cosmos will indeed end. Not with a bang, but with a whimper.
Have a nice day.
