It is remarkable how often the origin of things is tied to the very same phenomena that ultimately lead to their demise—a fact that is especially evident when we ponder cosmic endings, from the end of the Earth to the end of the universe.
For instance, planetary scientists increasingly suspect that comets (frozen balls of dust and ice) and ice-laden meteorites crashing into the primordial Earth probably provided most of the planet’s water—and perhaps much of the organic material—necessary for life. Organic molecules have been detected in comets such as the Hale-Bopp, and, in a recent study, researchers simulated those cosmic crash landings by using a gas gun to fire metal projectiles at 16,000 miles per hour into blocks of ice containing some of the same chemicals that make up comets. The shock wave and heat generated by the impact created molecules that formed amino acids, the building blocks of proteins.
Yet the very same objects that gave this planet life could also spell its demise. Astronomers predict that a comet or asteroid large enough to cause global devastation will smash into the Earth about every 100 million years or so. Fortunately, if such a comet or asteroid were to arrive sooner than expected, we are constructing observational systems to discover and track near-Earth objects, conceivably providing us with sufficient time to pre-empt catastrophe.
Other cosmic smash-ups, however, cannot be averted, no matter how much advance warning we have. The inexorable tug of gravity that enabled the formation of the Milky Way has also put us on a collision course with our neighboring galaxy, Andromeda. Recent observations confirm that Andromeda is heading straight toward us at about 60 miles per second, and will traverse the 2.5 million light-year distance currently separating our galaxies in about four billion years.
While the collision of two galaxies might conjure up images of mass devastation, the event will be largely imperceptible to our descendants, if any are still around. (They will have had to find another home: By that time, the increasing luminosity of our sun will have rendered Earth uninhabitable.) Galaxies are mostly empty space, so almost no stars or planets will actually collide.
Nonetheless, the Milky Way as we know it will cease to exist. Initially, the two galaxies will slide past each other and draw apart until gravity hits the brakes and pulls them back together. As Andromeda and the Milky Way merge, both will lose their disk-like structure, forming a single elliptical galaxy that some astronomers have dubbed “Milkomeda.”
If the size of the universe were to remain unchanged, the mutual gravitational attraction among galaxies eventually would cause all of them to merge together. But as we’ve known ever since the astronomer Edwin Hubble’s 1929 discovery, the universe is expanding and galaxies, on average, are moving farther apart. Throughout much of the 20th century, the big question in cosmology circles was: Is there sufficient mass in the universe to enable gravity to halt this expansion? Or will distant galaxies continue to move apart, slowing down but never quite stopping?
Then came the 1998 discovery that presented a third, unforeseen option: The expansion of the universe is not slowing down, as any sensible universe should be doing, but speeding up. We now know that about 70 percent of the energy of the universe resides in empty space, though we don’t have the slightest understanding of why. This “dark energy” acts as a kind of cosmic antigravity—empty space appears to produce a repulsive force, which is the opposite of the attractive force exerted by all forms of matter. These two competing forces have affected the expansion of the universe since the aftermath of the Big Bang. But as the universe has expanded, the density of matter has decreased while the amount of dark energy has remained constant. And as the braking effect of gravity has diminished, the expansion has accelerated. If dark energy continues to dominate, the most likely outcome is more dismal than any of the scenarios previously envisaged. The current expansion will continue forever, gaining speed, so that all the galaxies we now observe, 100 billion or so of them, will one day disappear beyond our ability to detect them. Our galaxy will be alone in the visible universe. And then, once the stars burn out, the universe will be truly cold, dark and empty.
If you feel let down by this gloomy conclusion to all things, take solace in knowing that this outcome is only the future as it might be. Until we understand the true nature of dark energy, the fate of the cosmos will remain a mystery. The universe might yet have a surprise ending.