RACHEL MARTIN, HOST:
We're putting things into perspective in this week before the New Year - really big things. Today, we consider whether our universe is the only one that exists. Here's NPR's Nell Greenfieldboyce.
NELL GREENFIELDBOYCE, BYLINE: Let's face it - people tend to be pretty self-centered.
CHUCK BENNETT: 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.
GREENFIELDBOYCE: Astrophysicist Chuck Bennett is at Johns Hopkins University. He says, even when we figured out that the 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.
BENNETT: Then we learned, no, it's just one galaxy out of hundreds of billions of galaxies out there.
GREENFIELDBOYCE: So with that track record, let's consider our whole universe. Maybe it's nothing special, just one among many.
BENNETT: We don't know yet, but it's very possible.
GREENFIELDBOYCE: Scientists believe the seed that started our universe may have spontaneously popped into existence through a kind of quantum weirdness. Bennett says, if that happened once, why not more than once?
BENNETT: So then you have this kind of array of universes in which ours is not unique.
GREENFIELDBOYCE: When I talked to another physicist, Sean Carroll at Caltech, he said he thought it was very likely that there were multiple universes. How many?
SEAN CARROLL: A really, really, really big number.
GREENFIELDBOYCE: Here's the thing, though - since everything we can observe and poke and prod is, by definition, part of our universe, how could we ever detect some other universe? This is why some thinkers worry that pondering the so-called multiverse is more like philosophy, not science. Jim Peebles is a cosmologist with Princeton University. He says, yeah, it's sort of fun to think about whether our universe is solitary.
JIM PEEBLES: It's a legitimate question. But since we'll never be able to answer it, I can't get very excited.
GREENFIELDBOYCE: But maybe this could be testable. Imagine if you had two universes that were expanding and ran into each other. Chuck Bennett at Johns Hopkins thinks if another universe bumped into ours, there could be ways to tell. In fact, there's been efforts to search the skies for evidence of that kind of impact.
BENNETT: Such an event, by the way, would be very dangerous, at least for people in one of the universes or the other because one of them would probably be destroyed.
GREENFIELDBOYCE: So far, there's no sign it's ever happened. And as far as we know, our universe is still here.
Nell Greenfieldboyce, NPR News.
MARTIN: Tomorrow, we will end our series with the end of the universe. And I really hope it's not as bad as it sounds.
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