American astronomer Adam Riess initially thought his Nobel-prize winning discovery that the expansion of the universe is speeding up was a mistake.

Riess shared the physics prize on Tuesday with Saul Perlmutter and Brian Schmidt. They were honored for their observations of exploding stars that transformed our view of the world, and of how it may end.

I make two or three mistakes a day on a good day, and so when you see something odd in the results, the opposite of what you expect, you usually have to go, 'Well, I did something stupid,' Riess told a news conference at The Johns Hopkins University in Baltimore, where he is a professor of physics and astronomy.

So I spent weeks looking for that mistake and I couldn't find it, he said of the discovery made over a decade ago.

The astronomers work had shown how the universe that emerged from the Big Bang may fly apart so far, cooling as it goes, and that it will end in ice, the Nobel Committee said.

It also gave birth to the theory of dark energy, a kind of inverse gravity, that causes the expansion to accelerate. Up to three-quarters of the universe seems to be composed of dark energy -- but just what it is a matter of speculation.

Riess was cautious about the prediction that the world would eventually freeze up into ice, without energy.

It's also possible that dark energy is a more complicated phenomenon and that we'll still get some change or some transition, in which case I would say all bets are still off for the universe, he told a teleconference of reporters earlier on Tuesday.

Riess, who was still in his 20s when the groundbreaking research was published, said he told his daughter, 7, that winning the Nobel prize was like getting a great big gold sticker on your homework.

He said he was already awake when he got the early morning call telling him he had won the Nobel prize.

My 10-month-old son was making some yips and yaps because he hasn't been sleeping well and I was listening to that hoping he would fall asleep, thinking it must be 2 or 3 in the morning and then the phone rang, he told the teleconference.

I immediately looked at the clock and I was surprised it was 5:30 a.m., he said. I immediately thought, 'Isn't that when you are supposed to get the famous call?'