This story comes from our special January 2021 issue, "The Beginning and the End of the Universe.” Click here to purchase the full issue.
How will the universe end? Humanity has pondered this question for thousands of years. And now science actually has the knowledge and tools to attempt an answer.
Until rather recently, astronomers thought the cosmos would repeatedly expand and collapse in an infinite cycle of cosmic death and rebirth. But the best evidence points to a distant Armageddon filled with more existential dread than the Book of Revelation. Trillions of years in the future, long after Earth is destroyed, the universe will drift apart until galaxy and star formation ceases. Slowly, stars will fizzle out, turning night skies black. All lingering matter will be gobbled up by black holes until there’s nothing left. Finally, the last traces of heat will disappear.
Rather than meeting its end through fire and brimstone, the cosmos will likely succumb to “heat death.” Astronomers call it the Big Freeze.
Alpha and Omega
The universe didn’t always seem destined to end this way. Roughly a century ago, astronomers thought that our Milky Way Galaxy was the entire universe. Our cosmos appeared static — it had always been, and would always remain, roughly the same. However, as Albert Einstein formulated his theories of relativity, he noticed signs of something strange. His equations implied a universe in motion, either expanding or contracting. So Einstein added a fudge factor — a cosmological constant — that held the universe in a more appealing steady state.
“Einstein was not being stupid; he was feeling the feeling of astronomers,” says Nobel Prize-winning cosmologist John Mather, the head scientist for NASA’s James Webb Space Telescope.
However, around the same time, astronomers began to accept that some of the fuzzy spiral-shaped nebulae they observed through their telescopes were not collections of stars in our galaxy. They were other galaxies entirely. And when Edwin Hubble meticulously measured their motions, he showed these galaxies were indeed moving away from our own. Humanity had discovered that the universe is expanding.
Pressing rewind on that expansion ultimately revealed that the entire universe was born in a violent Big Bang some 13.8 billion years ago. With its foundations firmly fixed, cosmology turned to the next great question: How will the universe end?
There are two main ways for an expanding universe to die: The cosmos could eventually collapse back in on itself, or it could continue inflating forever. To find out which is right, astronomers had to fast-forward the evolution of the universe.