Thought Experiments


A Trip to the Deep Future

Where will you be in 10100 years?

Yes, I know, we’ll all be long gone by then. But if you could somehow stick around around to experience the universe ten thousand trillion trillion trillion trillion trillion trillion trillion trillion years from now, what would it be like?

Answering that question is a professional hobby for astronomers Fred Adams and Gregory Laughlin. They divide the life of the universe into five distinct stages, beginning with, well, the beginning—the Big Bang and the short period of explosive expansion that followed, all the way through to the formation of the very first stars about one million years later. That’s followed by the second stage, which Adams and Laughlin dub the “stelliferous era”—the era during which stars generate most of the universe’s energy. We are creatures of the stelliferous era; this is the universe we recognize as home.

But while the stars are hitting their stride during the stelliferous era, dark energy—the mysterious energy that is causing the expansion of the universe to accelerate—is well on its way to cosmic domination. If the acceleration continues at its present rate, in another hundred billion years or so, most of the visible universe will pass beyond our cosmic horizon. Future denizens of the Milky Way will turn their telescopes to the sky and see just one galaxy: their own.

As Lawrence Krauss and Robert Scherrer pointed out in a 2007 paper, these future astronomers will see no evidence of cosmic expansion or the Big Bang. They will probably conclude that their universe is static; that it is as it has always been and always will be. Ironically, the very force that sculpted their universe—dark energy—will have erased its own fingerprints.

This idea troubled Harvard astronomer Avi Loeb, who imagined a future in which astronomers would look back on today’s cosmology textbooks (which would then be 100 billion years old) with the same combination of reverence and skepticism with which we view biblical origin stories today. “You will have all these textbooks, but their claims will be unverifiable,” says Loeb.

Loeb went looking for a way in which future astronomers could tease out the history of their universe. He found the answer in hypervelocity stars, stars traveling so fast that they escape the gravity of their home galaxy. Using their advanced telescopes to monitor these stars, says Loeb, future astronomers just might be able to probe the universe beyond their galactic boundaries.

But even those galactic boundaries will be erased in the course of time. Astronomers estimate that the longest-lived stars will begin to burn out some ten trillion years from now, throwing our universe into an era of cosmic twilight. Here, the universe is lit only by the feeble embers of white dwarfs and neutrons stars, stellar corpses that will give off energy as they “hoover up” dark matter particles, says Adams. Though galaxies and galaxy clusters have managed to hold themselves together until now, a slow and steady stream of stars—the very same hypervelocity stars Loeb saw as cosmic ambassadors—will absent themselves from their galaxies until, over a period of about 1020 years, galaxies will “evaporate” entirely, Adams and Laughlin calculate. The finely-woven tapestry of the universe will come undone.


A computer simulation of the cosmic web of dark matter and ordinary matter. Image credit: NASA, ESA, and E. Hallman (University of Colorado, Boulder)

Given sufficient time, even the protons and neutrons that make up the stuff of universe will fall to pieces. How long will it take? That is still a mystery, though a combination of experiment and theory suggests that it will happen some time between 1033 and 1045 years after the Big Bang.

At that point, all that’s left of the stars and galaxies that once illuminated our universe will be a smattering of black holes. But even the reign of the black holes won’t last forever. As Stephen Hawking showed theoretically, black holes slowly leak out their contents via a process we now call Hawking radiation. Given enough time—as long at 10100 years—even the biggest black holes will evaporate away.

Only now will we enter what Adams and Laughlin dub the “dark” era. The dark era isn’t just very, very dark; it is also very, very boring. Next to nothing actually happens in the dark era. Thanks to the accelerating expansion of the universe, even humdrum particle collisions will become rarities.

Will the lonely monotony of the dark era ever end? Maybe. The same energy that has been driving the accelerating expansion of the universe could suddenly change character, a phenomenon theorists call vacuum energy decay. It happened once before—when the era of inflation ground to a halt soon after the Big Bang—and theorists believe that it should happen again.

“You could imagine a new start” for the universe, says Adams, in which matter gets a second chance to coalesce into stars, planets, even people. Or, the vacuum energy could decay before the universe ever makes it to the dark era. “If that happens,” says Loeb, “we’re back to a situation where once again we can see all those galaxies that we lost.”

Of course, these scenarios are a strong cocktail of science and speculation—and the further we look into the future, the more speculation is poured into the mix. So why study a universe that even our most distant descendants will never live to see?

The numerical models scientists use to project into the distant future can yield new insights into stellar life cycles—like how small, long-lived stars evolve into red giants—that we can’t observe progressing over the course of one (or many) lifetimes, says Adams. It also gives us a way “to gauge the cosmic importance of various aspects of the standard model,” says Loeb, by watching how they play out over time.

“It is part of our worldview to want to know what will happen,” adds Loeb. Yet I don’t think I’m alone in enjoying the fact that the next plot twist is, ultimately, a mystery.

Go Deeper
Editors picks for further reading

Astrobites: Avi Loeb and Freeman Dyson on the future of the universe
Can the universe be saved from the “dark era”? Astronomy blogger Nathan Sanders shares a conversation between Freeman Dyson and Avi Loeb on the prospect of “cosmic engineering.”

FQXi: Predicting the End
Science writer Govert Schilling talks with Fred Adams and Greg Laughlin about how they became the authors of the future-biography of our universe.

The Five Ages of the Universe: Inside the Physics of Eternity
Fred Adams and Greg Laughlin had the bad fortune to publish this book just around the time that dark energy was discovered; their predictions therefore don’t account for dark energy. Most of their conclusions about the distant future remain valid, though.

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Kate Becker

    Kate Becker is the editor of The Nature of Reality, where it is her mission to blow your mind with physics. Kate studied physics at Oberlin College and astronomy at Cornell University, and spent seven years as senior researcher for NOVA and NOVA scienceNOW. Follow her on Twitter and Facebook.

    • Anthony Hita

      Resistance is Futile.

    • Arm

      > So why study a universe that even our most
      > distant *ancestors* will never live to see?

      Wrong use of English – it’s *descendants*, not ancestors

      • Kate Becker

        Thank you, I’ve corrected this line.

    • Paul Rich

      There are plenty of other universes out there … in there? through there?