RACHEL MARTIN, HOST:
So all this week we've been contemplating the universe. It's part of our year-end effort to put big things into perspective. And today we come to the end of our series and the end of the universe. Here's NPR's Nell Greenfieldboyce on how our universe will die.
NELL GREENFIELDBOYCE, BYLINE: When I asked astrophysicist Chuck Bennett how the universe will end, he immediately thought of a poet.
CHUCK BENNETT: Robert Frost said that the only question is whether we would die by fire or ice.
(SOUNDBITE OF POEM, "FIRE AND ICE")
ROBERT FROST: (Reading) Some say the world will end in fire - some say in ice.
GREENFIELDBOYCE: That's Frost reading his poem. He favored fire. But...
(SOUNDBITE OF POEM, "FIRE AND ICE")
FROST: (Reading) I think I know enough of hate to say that, for destruction, ice is also great and would suffice.
GREENFIELDBOYCE: And these days, most astrophysicists are guessing ice. Bennett, who works at Johns Hopkins University, says the universe started out hot and dense. It's been expanding and cooling for nearly 14 billion years. And we now know it's actually expanding faster and faster.
BENNETT: And this is like hyperdrive on the cooling. So it's the ice (unintelligible) - is everything would grow dimmer and dimmer. You would stop seeing things in the sky. Everything would grow dark and cold.
GREENFIELDBOYCE: Everything gets farther and farther apart. Each particle of the universe will end up completely alone. It all sounds bleak. But cheer up. Ending with fire is still possible. Janna Levin is a theoretical physicist at Columbia University. She says a mysterious thing called dark energy is pushing the universe to expand faster and faster. We don't know what dark energy is. So it might just go away. Our expanding universe would slow down.
JANNA LEVIN: Maybe even reverses course, for all we know. And then what? Then we go back to kind of a fiery end.
GREENFIELDBOYCE: She says everything would get closer and hotter.
LEVIN: And we go back towards a big crunch, which is like a big bang happening in reverse.
GREENFIELDBOYCE: Fire or ice - either way, the end is coming but not for a while. Sean Carroll is a theoretical physicist at Caltech.
SEAN CARROLL: We think it'll be at least a quadrillion years before the last star burns out.
GREENFIELDBOYCE: That's a thousand trillion years. Our own sun 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 trillions of galaxies.
CARROLL: So we are not significant on the cosmic scale. We are not important to the universe. That's the bad news.
GREENFIELDBOYCE: He says the good news is that, even with our puny brains, we've managed to figure that out. Nell Greenfieldboyce, NPR News.
(SOUNDBITE OF SONG, "TILL THE WORLD ENDS")
BRITNEY SPEARS: (Singing) See the sunlight. We ain't stopping. Keep on dancing until the world ends. If you feel it, let it happen. Keep on dancing until the world ends. Keep on dancing until the world ends. Transcript provided by NPR, Copyright NPR.