Sweden - Americans John C. Mather and George F. Smoot won the 2006 Nobel Prize in physics on Tuesday for work that helped cement the big-bang theory of the universe and deepen understanding of the origin of galaxies and stars.

Mather, 60, works at the NASA Goddard Space Flight Center in Greenbelt, Md., and Smoot, 61, works at the Lawrence Berkeley National Laboratory in Berkeley, Calif.

The scientists discovered the nature of "blackbody radiation," cosmic background radiation believed to stem from the "big bang," when the universe was born.

"They have not proven the big-bang theory but they give it very strong support," said Per Carlson, chairman of the Nobel committee for physics.

"It is one of the greatest discoveries of the century. I would call it the greatest. It increases our knowledge of our place in the universe."

Their work was based on measurements done with the help of NASA's COBE satellite launched in 1989. They were able to observe the universe in its early stages about 380,000 years after it was born. Ripples in the light they detected also helped demonstrate how galaxies came together over time.

"The COBE results provided increased support for the big-bang scenario for the origin of the Universe, as this is the only scenario that predicts the kind of cosmic microwave background radiation measured by COBE," the Royal Swedish Academy of Sciences in Stockholm said in its citation.

The big-bang theory states that the universe was born billions of years ago from a rapidly expanding dense and incredibly hot state.

Reached at his home in Berkeley, Smoot told The Associated Press he was surprised when he got the call from the Nobel committee in the middle of the night.

"I was surprised that they even knew my number. After the discovery I got so many calls I unlisted it," he said.

"The discovery was sort of fabulous. It was an incredible milestone. Now this is a great honor and recognition. It's amazing," he said.

Mather said he was "thrilled and amazed" at receiving the prize.