STOCKHOLM, Sweden — Americans John Mather and George Smoot won the 2006 Nobel Prize in physics today 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.

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

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