Nasa launched the Cosmic Background Explorer in 1989

They have been honoured "for their discovery of the blackbody form and anisotropy of the cosmic microwave background radiation (CMB)".

The CMB is the "oldest light" in the Universe - it is all around us and comes from a time 380,000 years after the Big Bang.

Scientists say features in the CMB tell them about the evolution of the cosmos.

Mather, 60, is a senior astrophysicist at US space agency Nasa's Goddard Space Flight Center in Maryland. Smoot, 61, is a professor of physics at the University of California at Berkeley.

The pair worked on Nasa's Cobe satellite which was launched in 1989. It made the first precise measurements of the CMB.

Cosmic clues

The CMB has been called the "echo" of the Big Bang - the event that created the Universe less than 14 billion years ago.

It is the radiation that formed when the Universe had cooled to such a degree that hydrogen atoms could exist. Before that time, scientists say, the Universe would have been so hot that matter and radiation would have been "coupled" - it would have been opaque.

THE CMB - OLD AND COLD Theory holds that 380,000 years after Big Bang, matter and radiation "decoupled" Matter went on to form stars and galaxies; the radiation spread out and cooled The radiation - the CMB - now shines at weak radio (microwave) wavelengths Cobe showed the CMB's profile to follow a predicted distribution - a so-called blackbody curve Cobe mapped tiny temperature fluctuations (mottled colours top) in CMB These fluctuations (anisotropy)correspond to the early distribution of matter The data informs scientists about the age, geometry and fate of the cosmos Images from the more advanced WMap probe show finer detail (bottom)

Cobe - the COsmic Background Explorer satellite - measured the temperature of this background radiation - a frigid 2.725 degrees above absolute zero; it shines in the microwave portion of the electromagnetic spectrum.

Mather's team on Cobe showed the CMB's temperature profile to follow a very precise pattern in the energy spectrum - a so-called blackbody curve. The existence of such a profile was a major prediction of Big Bang theory.

The satellite also detected temperature fluctuations in the CMB - the result found by Smoot's team.

These tiny deviations (anisotropy) have been attributed to the first structures to form in the Universe - the so-called seeds of galaxies appearing in the vast clouds of hot gas that was all the Universe consisted of in its earliest phase.

Astronomers believe that the CMB contains a great deal of information about the origin of the Universe, and why it has assumed the structure we see around us today. They also believe the CMB holds clues to the Universe's eventual fate - that it is likely to go on expanding forever.

Nasa launched a follow-up mission, WMap, in 2001 to look at the CMB in even finer detail.

'Fantastic tool'

Mather said Tuesday's prize could not have been won without the efforts of many people on the Cobe project.

"In total there were 1,500 people, so it's a huge team effort that we're recognising today," he told Reuters.

"I didn't expect this, it was a wonderful surprise."

Smoot said: "The discovery was sort of fabulous, it was an incredible milestone. Now this is a great honour and recognition. It's amazing," he told Associated Press.

A member of the Nobel committee for physics, Professor Lars Bergstrom, said Mather's and Smoot's work represented an important breakthrough for our understanding of the infant Universe.

"These two laureates of this year have really helped us to find the missing link in cosmology," he said. "And also they have found a fantastic tool to make detailed measurements of the universe when it was very young and much simpler than today."

CMB repeat

Professor Michael Rowan-Robinson, president of the Royal Astronomical Society, UK, commented: "The demonstration of the perfect blackbody form of the cosmic microwave background spectrum by John Mather and his team, and the detection of fluctuations in the cosmic background radiation by George Smoot and his team, are among the most significant discoveries in astronomy of the past century.

"The blackbody form demonstrates the correctness of the Hot Big Bang model, in which matter and radiation were locked together in thermal equilibrium for the first 150,000 years after the initial singularity.

"The fluctuations show that galaxies and clusters of galaxies grew from very small seed fluctuations in the early universe through gravitational aggregation.

Arno Penzias and Robert Wilson, who first detected the CMB in 1965, shared the 1978 Nobel Prize in physics for their achievement.

The Nobel Prizes - which also cover chemistry, medicine, literature, peace and economics (more properly called the Sveriges Riksbank Prize) - are valued at 10m Swedish Kronor (£0.7m; US$1.4m).

Laureates also receive a medal and a diploma.