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Terrorist attacks, hurricanes, a divisive U.S. election, Brexit — 2016 has not been easy. With the year coming to an end, we thought it was time to get some serious perspective — from the scale of the entire universe.

We're tackling big questions: what scientists know, and what they have yet to learn.

So before you ring in another year, take a moment to contemplate the billions of years that led to 2017 and the billions more yet to come.


"That happens to be my absolute favorite question," says 

, an astrophysicist at Johns Hopkins University.
He points out that the big-bang theory says the universe started out dense and hot, and that it has been expanding and cooling for 13.8 billion years, but, he says, "the 

 theory doesn't actually say what happened right at the beginning."


You can follow our laws of physics back in time, he says, but they break down close to the start, when things were unspeakably fiery and close together. Still, there may be clues from the weird world of quantum physics. In that world, strange stuff can happen, like particles can just appear out of nowhere.

"Even if you take something that's a complete vacuum, you've gotten all of the particles and dust and everything out of the way, in quantum mechanics you still have particles popping in and out of existence all the time," explains Bennett.

So maybe the kernel that became our universe just randomly and spontaneously appeared. "It seems bizarre, but that is kind of the going thinking about this," Bennett says.


And if you want to think about something even more bizarre, consider this point made by Caltech theoretical physicist 

. If the big bang was the first moment in time, that creates a conundrum: "There's no verbs before time itself exists, right? There's no popping into existence, there's no fluctuating, there's no quantum mechanical craziness, there is literally nothing," says Carroll.


You might be tempted to try to answer this question by stepping outside the universe so you can take a gander. But, obviously, that's impossible. "There is no such thing as outside the universe, as far as we can tell," says Carroll.

Even though the universe has been expanding for about 14 billion years, that doesn't mean it's ballooning out into some other realm. "I know it's difficult to wrap our minds around," says Carroll, "but it's just getting more and more of it, even though it's not expanding into anything at all."

So if we can't leave the universe, all we can do is look around inside. Let's say you flew off the Earth, out of our solar system, out of the Milky Way galaxy, out of our cluster of galaxies, and flew on and on. How far could you go?

, a theoretical physicist at Columbia University. "What we see of the universe is vast. We know that the universe is something like 90 billion light-years across." But that's just the part we can see.


Anything beyond that has to remain a mystery, because stuff out there is so far away, its light will never be able to reach us. "It makes logical sense to assume the universe goes on beyond that boundary. It would be kind of magical if we were just happening to be able to see right to some boundary and then something crazy happened beyond that, like galaxies ceased to exist," says Levin. "I mean, that just seems nuts."

So the universe goes on, but is it infinite? "It is somewhat unimaginable but quite possible that our universe simply goes on forever," says Bennett.

To us, the universe seems flat, so maybe it's like an endless sheet of paper. But on the other hand, people used to think the Earth was flat, too, because people saw flat land stretching to a horizon, beyond which they could not see. These days, the idea of a flat Earth seems silly — we know it's really a huge sphere.

"Our universe might be like that," says Bennett, noting that the universe might be curved and might even curve back on itself like a sphere, "but on a scale that is truly enormous."

If so, and you headed off into the universe, going straight in one direction, you would eventually find yourself right back where you started.

You might think this is one of the easier questions about the universe to answer. But you would be wrong. "All the stuff we've ever seen in the laboratory, all the kinds of particles and matter and energy, that only makes up 5 percent of our universe," says Carroll.

Five percent! So what is the rest of the universe made of? Well, one biggie is something called dark matter. About 25 percent of the universe is dark matter, which is quite literally dark. "It just doesn't interact with light at all," says Bennett. "It doesn't give off any light; it doesn't absorb light; it doesn't scatter light; there's no way to see it. The only way we know that it's there is because it has gravitational effects."

Scientists discovered dark matter when they looked at the motion of galaxies and realized that something unseen had to be exerting a gravitational pull. Dark matter may be some kind of particle that we just haven't detected yet.

The rest of the universe — 70 percent — is something even more crazy, called dark energy. It appears to be some kind of energy that's inherent to empty space, and it acts to push the universe apart, speeding up its expansion. Like dark matter, dark energy is another big mystery.

"Other than the fact that we don't quite understand 95 percent of the universe, we're doing really well," jokes Bennett.


All of the world's leading theoreticians, who write whole books about the universe, just have to live with this state of affairs. "You're entitled to say, if you're so smart, why don't you tell me what that dark matter is? And I'll have to confess I don't know," says 

, Albert Einstein professor of science, emeritus and professor of physics, emeritus at Princeton University.


He's not depressed, however, that so much of the universe remains unknown. "I think I'd be depressed if everything were nearly all known," says Peebles, "but I don't feel any danger of that happening."

Let's face it; people tend to be pretty self-centered. "If you look back at the history of astronomy, you know, we used to think that the Earth was the center of the solar system. Everything was about us," says Bennett.

Even when we figured out that Earth went around the sun, and the sun was part of the Milky Way galaxy, we thought our galaxy was the center of the universe. "Then we learned no, it's just one galaxy out of hundreds of billions of galaxies out there," he notes.

With that track record in mind, it's natural to wonder whether our whole universe isn't so special — if it's just one among many. "We don't know yet," says Bennett, "but it's very possible."

Given that scientists believe the seed that started our universe may have spontaneously popped into existence through a kind of quantum weirdness, that presents an obvious question: If that could happen once, why not more than once? "So then you have this kind of array of universes in which ours is not unique," says Bennett.

"A really, really big number," says Carroll.

But since everything we can observe and poke and prod is, by definition, part of our universe, it's unclear how we could ever detect some other universe. This is why some thinkers worry that pondering the so-called multiverse is more like philosophy than science. It's sort of fun to think about whether our universe is solitary, and it's a legitimate question, says Peebles, "but since we'll never be able to answer it, I can't get very excited."


But maybe this idea could be testable. Imagine if you had two universes that were expanding and ran into each other, says Bennett. If another universe bumped into ours, there could be ways to tell. In fact, there have been 

 to search the skies for evidence of that kind of impact, but there's no sign it ever happened. Which might be a good thing, since that kind of event "would be very dangerous at least for people in one of the universes or the other because one of them would probably be destroyed," Bennett says.



"Some say the world will end in fire, some say in ice," wrote Robert Frost in his famous poem 

. He favored fire but, hedging his bet, added that:


These days, most astrophysicists are guessing the universe will end as cold as ice.

The universe, which started out hot and dense, has been expanding and cooling for nearly 14 billion years. We now know it's actually expanding faster and faster. "This is like hyperdrive on the cooling," says Bennett. "So it's the ice solution. Everything would grow dimmer and dimmer; you would stop seeing things in the sky; everything would grow dark and cold."

As everything gets farther and farther apart, each particle of the universe will eventually end up completely alone. It all sounds bleak.

But, cheer up! Ending with fire is still possible.

Since dark energy is pushing the universe to expand faster and faster, and physicists don't know what dark energy is, it's possible that it might just decay or go away, making our expanding universe slow down. "Maybe even reverse its course for all we know, and then what? Then we go back to kind of a fiery end," says Levin. She explains that everything would fly back together toward a big crunch, which is like the big bang happening in reverse.

Fire or ice, either way, the end is coming. But not for a long while. "We think it will be at least a quadrillion years before the last star burns out," says Carroll, noting that this is 1,000 trillion years.

 will burn out way sooner, in about 5 billion years. Though Carroll says that's kind of a parochial concern, when you consider that our Milky Way galaxy has around 100 billion stars and is just one of 

 of galaxies.
"So we are not significant on the cosmic scale. We are not important to the universe. That's the bad news," says Carroll. The good news is that, even with our puny brains, we've managed to figure that out.  Copyright 2019 NPR. To see more, visit https://www.npr.org.

Science

As 2016 winds down, we take a moment to contemplate the billions of years that led to 2017 and the billions more yet to come.

Humans point their telescopes toward the heavens

As a rough year ends, we turn to the cosmos for some perspective

Go Deeper.

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