Three astronomers won the Nobel Prize on Tuesday for discovering that the universe is apparently being blown apart by a mysterious force that cosmologists now call dark energy, a finding that has thrown the fate of the universe, and indeed the nature of physics, into doubt.

The astronomers are Saul Perlmutter, 52, of the Lawrence Berkeley National Laboratory and the University of California, Berkeley; Brian Schmidt, 44, of the Australian National University in Canberra, and Adam Riess, 41, of the Space Telescope Science Institute and Johns Hopkins University in Baltimore.

“I’m stunned,” Riess said by email, after learning of his prize by reading about it on The New York Times’ website.

The three men led two competing teams of astronomers who were trying to use the exploding stars known as Type 1a supernovas as cosmic lighthouses to delineate the expansion of the universe. The goal of both groups was to measure how fast the cosmos, which has been expanding since its fiery birth in the big bang 13.7 billion years ago, was slowing down, and thus to find out if its ultimate fate was to fall back together in what is called a big crunch or to drift apart into the darkness.

Instead, the two groups found the expansion of the universe was actually speeding up, a conclusion that nobody would have believed if not for the fact that both sets of scientists wound up with the same answer. It was as if, when you tossed your car keys in the air, instead of coming down, they flew faster and faster to the ceiling.