RACHEL MARTIN, HOST:
Now we are of course on the cusp of a new year, which is a good reason for us to try to put a whole lot of things into perspective - a really big perspective. Yesterday, we took on questions about where the universe comes from. Today, we'll look at whether the universe is infinite. Here's NPR's Nell Greenfieldboyce.
NELL GREENFIELDBOYCE, BYLINE: Let's say you wanted to step outside of the universe and take a look at it. Well, you can't.
SEAN CARROLL: There is no such thing as outside the universe as far as we can tell.
GREENFIELDBOYCE: Sean Carroll is a theoretical physicist at Caltech. He says the universe has been expanding for about 14 billion years, but it's not ballooning out into some other realm.
CARROLL: I know it's difficult to wrap our minds around, but it's just getting more and more of it, even though it's not expanding into anything at all.
GREENFIELDBOYCE: 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 on and on and on. I asked theoretical physicist Janna Levin at Columbia University, how far could you go?
JANNA LEVIN: We don't a hundred percent know. So what we see of the universe is that we know that the universe is something like 90 billion light years across.
GREENFIELDBOYCE: That's only the part of it we can see. Anything beyond that has to remain a mystery. Stuff out there is so far away the light can never get to us.
LEVIN: 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 at some boundary and then something crazy happened beyond that like galaxies cease to exist. I mean, that just seems nuts.
GREENFIELDBOYCE: So it goes on, but is it infinite? Chuck Bennett is an astrophysicist at Johns Hopkins University.
CHUCK BENNETT: It is somewhat unimaginable, but quite possible that our universe simply goes on forever.
GREENFIELDBOYCE: He says 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. 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.
BENNETT: Our universe might be like that.
GREENFIELDBOYCE: Bennett says the universe might be curved, might even curve back on itself...
BENNETT: But on a scale that's truly enormous.
GREENFIELDBOYCE: If so and you headed boldly off into the universe going straight in one direction, you would eventually find yourself right back where you started. Nell Greenfieldboyce, NPR News.
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