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.

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"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 academy 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.

"I can't say I was completely surprised, because people have said we should be awarded, but this is just such a rare and special honor," Mather said in a telephone interview with the Nobel committee.

He said he and Smoot did not realize how important their work was at the time of their discovery.

The COBE project gave strong support for the Big Bang theory because it is the only scenario that predicts the kind of cosmic microwave radiation measured by the satellite.

The academy called Mather the driving force behind the COBE project while Smoot was responsible for measuring small variations in the temperature of the radiation.

With their findings, the scientists transformed the study of the early universe from a largely theoretical pursuit into a new era of direct observation and measurement.

"The very detailed observations that the laureates have carried out from the COBE satellite have played a major role in the development of modern cosmology into a precise science," the academy said.

Phillip F. Schewe, a spokesman for the American Institute of Physics, said he had expected the two to win the honor.

"It's just a really really difficult experimental measurement to make. It's the farthest out we can see in the universe and it's the farthest back in time," he said in a telephone interview.

Since 1986, Americans have either won or shared the physics prize with people from other countries 15 times.

Last year, Americans John L. Hall and Roy J. Glauber and German Theodor W. Haensch won the prize for work that could improve long-distance communication and navigation.

This year's award announcements began Monday with the Nobel Prize in medicine going to Americans Andrew Z. Fire and Craig C. Mello for discovering a powerful way to turn off the effect of specific genes, offering new hope for fighting diseases as diverse as cancer and AIDS.

The winner of the Nobel Prize in chemistry will be named Wednesday. The Bank of Sweden Prize in Economic Sciences in Memory of Alfred Nobel will be announced Oct. 9.

The winner of the peace prize — the only one not awarded in Sweden — will be announced Oct. 13 in Oslo, Norway.

A date for the literature prize has not yet been set.

Alfred Nobel, the wealthy Swedish industrialist and inventor of dynamite who endowed the prizes, left only vague guidelines for the selection committee.

In his will, he said the prize should be given to those who "shall have conferred the greatest benefit on mankind" and "shall have made the most important discovery or invention within the field of physics."

The prizes, which include a $1.4 million check, a gold medal and a diploma, are presented on Dec. 10, the anniversary of Nobel's death in 1896.