17 Feb 2016
When winter weather closed Harvard University one day in 2011, astronomer Avi Loeb used the snow day not to sled or start a new novel but to contemplate the future of the universe. In that future, cosmologists like him, who study the universe’s origins and evolution, might not be able to make a living.
Nine years before, he had written a paper outlining the problem: Dark energy makes the universe expand faster and faster every femtosecond. As spacetime—the fabric of the cosmos—stretches, it carries galaxies along with it. The stretching sends each galaxy farther and farther from the others, eventually driving them so far apart that light will never be able to bridge the gap between them.
Far future cosmologists won’t have the same evidence as we do to infer the Big Bang. No image credit.
In that future, our own oasis, the Milky Way, will be completely alone. When future astronomers look up, they will see only our galaxy’s own stars. They won’t find any evidence—even with the powerful telescopes of a trillion years hence—that other galaxies even exist beyond the horizon of their visible universe. Without a view of those other galaxies, they won’t be able to tell that everything was born in a Big Bang, or that the black vacuum of space is expanding at all, let alone that that expansion is speeding up. Ironically, dark energy itself will destroy evidence of dark energy.
Thinking of this emptied universe, Loeb stared out the window at the snowfall, which covered the ground in a blank blanket. “I was pretty depressed that there would be nothing to look at, and that we won’t be able to tell how the universe started by observing it.”
He set out to find a solution.
A Galactic Merger
Currently, cosmic expansion clues us in to the Big Bang. Press fast-forward on the growing universe we see today, and it continues growing, with objects flying ever-farther apart. It doesn’t take much creativity to then press rewind: The universe shrinks, and its ingredients squish together. If you rewind until the very beginning of the tape, everything piles into one infinitesimally small, infinitely dense spot. Press play and it bursts forth: a Big Bang.
Astronomers only discovered that expansion because they could see other galaxies, which all seem to be running away from us. In 1999, using ultra-distant supernova explosions, they figured out that faraway galaxies were retreating faster than they “should” be, and that even more distant galaxies were distancing themselves faster than that. Something—which they later termed dark energy—spurs expansion on, like a car whose pedal never reaches the metal no matter how hard you push.
The real problems won’t show up for a while, until about a trillion years after the Big Bang. By that time, the Milky Way will have long ago crashed into the Andromeda Galaxy. The stars will have spent 3 billion years swirling into stable orbits, before becoming a seamless chimera: a single galaxy called “Milkomeda,” a term Loeb coined in 2008 when he simulated and then forecasted the collision’s specifics.
After their first close pass, the Andromeda Galaxy as well as the Milky Way would be tidally stretched out, as shown in this artist’s conception. NASA / ESA / STScI
Even as that galactic collision takes place, dark energy will be dragging everything else away from us. Little by little over billions of years, everything will pop over the visible horizon, along with any physical evidence of its existence, until only our neighbor stars in Milkomeda remain. “The universe becomes lonely,” says Glenn Starkman, a physicist at Case Western Reserve University. He and astronomer Lawrence Krauss of Arizona State University in Tempe wrote an article titled Life, The Universe, and Nothing: Life and Death in an Ever-Expanding Universe, which also discusses this “lonely astronomer” problem. “The longer you wait, the less you will see and the more of the universe will disappear before your very eyes,” Krauss says.
“Earth’s night sky will change,” Loeb says. Stars that humans (or whoever is around) will get to watch in a few billion years will shift radically. Today, the Milky Way appears as a diagonal swash of fuzzy light, the combined photons of billions of stars too small for our eyes to resolve. But when people in the distant future look up at Milkomeda, they will see those stars distributed evenly across the sky.
If astronomers still live in Milkomeda at that point, they could be thrown into an astronomical dark age. To them, the universe will look like the one we thought we understood before telescopes. Back then, we thought we were the center of the cosmos, and we believed the Milky Way to be the entirety of the universe.
That universe seemed static and without beginning. Alone in Milkomeda, future astronomers may—validly, based on actual evidence—see it that way, too. “Scientists who evolve on such a world will look out and find that the three main pillars of the Big Bang will all be gone,” Krauss says.
Three Missing Pillars
“It’s a gloomy forecast,” Loeb says. “We won’t be able to look at anything. It’s not just galaxies—it’s any relic left from Big Bang.” Right now, telescopes can see a glow of light left over from the Big Bang. This relic radiation, called the cosmic microwave background [CMB], comes from every direction in the sky. The Planck Telescope recently made a high-definition map of it, which is essentially a blueprint of a baby universe. It shows us the seeds that grew into groups of galaxies, tells us what the universe is made of, and tips us off about the very beginning of everything.
But as time passes, the photons that make up cosmic microwave background cool off and lose energy, increasing their wavelengths. Eventually, those waves—which today are on the order of millimeters—will be bigger than the visible universe. There’s no telescope, not even one a trillion-year-old society could build, that can detect that. “They will no longer be able to learn what we know about the early universe,” Starkman says.
The composition of the universe, which now tells scientists that the Big Bang occurred, won’t help in the far future, either. After the Big Bang, the universe began to cool off. Soon, free-range quarks settled down into electrons, protons, and neutrons, which could then intertwine into hydrogen atoms. Those atoms then smacked into each other and stuck together, fusing into larger helium atoms. In just 30 minutes, most of the helium that exists today had formed. A comparatively small amount has been created inside stars in the few billion years since.
“Right now, we know the Big Bang happened because 25% of universe is helium,” Krauss says. “There’s no way stars could have made that.” But by the time the universe is 10 trillion years old, stars will have fused most of the hydrogen into helium. That is, in fact, their job. But in doing it so well, they will knock down the last solid evidence that the universe had a beginning at all. “All relics of Big Bang will be gone from us,” Loeb says. “There will be really nothing.”
It seems that we live at a somewhat strange time in the universe—one in which our sky is filled with evidence of the cosmic narrative. Does that make us lucky? And does it make future observers unlucky? Astronomers generally shy away from suggestions that we are anything other than dead-average. They call it the Mediocrity Principle.
But maybe each eon is a special snowflake in its own way, meaning none of them is really special, just like soccer kids who all get trophies. The far-future folks may have easy access to knowledge we, in our dark-energy-dominated and bright-skied time, can’t grasp. “I suspect that each era is interesting for different reasons,” Krauss says. “There may be cosmological observables that we could see in the far future that we can’t see now.”
We can’t know for sure, nor can we know for sure that this future forecast is correct. Just like perfect weather prediction, it can only happen if we know everything about every subatomic particle. The year 1 trillion CE may not look exactly as we envision it. “That broad picture is what will happen if what we know continues to be the whole truth and nothing but the truth,” Starkman says. “There’s a lot of chutzpah in thinking that’s really so, that we’ve captured everything there is to know about physics.”
As the winter storm swirled outside, Loeb considered the dark, empty (potential) future he’d predicted. He hated that so much knowledge—the science he loved—would disappear, like all the galaxies. He had recently given a public talk on the topic, sharing his sadness, and an audience member’s question had sent him reeling: Would this future convert cosmology into a kind of religion? “You would have books talking about the story of how the universe started, but you wouldn’t be able to verify that,” he says. “I was worried that cosmology would be turned into folklore.”
“There will really be nothing,” he thought again. But then a flash swept through his brain. Nothing—except for one thing. “I realized that not everything is lost,” says Loeb. The key is a type of object called a hypervelocity star.
“The center of our galaxy keeps ejecting stars at high enough speeds that they can exit the galaxy,” Loeb says. The intense and dynamic gravity near the black hole ejects them into space, where they will glide away forever like radiating rocket ships. The same thing should happen a trillion years from now.
“These stars that leave the galaxy will be carried away by the same cosmic acceleration,” Loeb says. Future astronomers can monitor them as they depart. They will see stars leave, become alone in extragalactic space, and begin rushing faster and faster toward nothingness. It would look like magic. But if those future people dig into that strangeness, they will catch a glimpse of the true nature of the universe. “Just like Edwin Hubble observed galaxies—historically trying to infer expansion—they could observe those stars outside the galaxy and figure out the universe is expanding,” Loeb says. Starkman says they could accomplish this synthetically, too. “They could send out probes far enough to notice that the probes accelerated away,” he says.
And then, perhaps, they will imagine pressing fast-forward on this scenario. And, if their imaginations are like ours, they will then think about rewinding it—all the way back to the beginning.
Krauss doesn’t necessarily buy this. Occam’s Razor states that the least complicated answer is usually the correct one, and that principle will lead these future beings astray. It sounds crazy that the very fabric of the universe is growing larger faster all the time, carrying some runaway star with it. It’s not the explanation that comes to the tip of the tongue. But perhaps more importantly, with just Milkomeda in the night sky, astronomers will have no reason to come up with a theory of anything beyond those stars. Just as pre-telescope scientists thought only of what they could see with their eyes, not of an invisible universe outside of that, so too could future astronomers’ imaginations be constrained.
Loeb stands by his solution, although he admits it could remain in his 21st century paper and never occur to someone in the 2.1 trillionth century. “It’s difficult to speculate what will happen in a year or 10 years on Earth, let alone a trillion years,” he says. “We don’t even know if humans will still be around…I’m just talking about what one could learn.”
Which is why Loeb is so intent on forecasting the future cosmos, even though he won’t be around to see it. “Most of my colleagues do not care about the future because they regard themselves as down-to-Earth,” he says. “They only think about things that can be tested or looked at right now. We can’t really observe the future, so they prefer not to think about the future. They often run computer simulations of the universe to the present time and then stop. All I’m saying is ‘Why stop?’ ”
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