Two Americans won a Nobel prize yesterday for taking baby pictures of the universe. George Smoot of Lawrence Berkeley National Laboratory in California and John Mather of NASA’s Goddard Space Flight Center in Maryland were awarded science’s highest honor for depicting the universe as it was 380,000 years after its birth in the big bang. They will share the $1.4 million prize equally for their groundbreaking work.

Their feat, precisely measuring the faint light that showed the seeds of today’s galaxies and superclusters, affirmed the big-bang theory to even the most stubborn skeptics.

They were the chief architects of a NASA satellite observatory named COBE — short for cosmic background explorer. Launched in 1989, the spacecraft measured feeble remnants of light that originated early in the history of the universe, about 380,000 years after the big bang. Until then, the universe was opaque to light, making it impossible to directly observe anything older.

The findings of Messrs. Smoot and Mather showed the ancient seeds of stars, galaxies, and other celestial objects.

“It’s the farthest out we can see in the universe, and it’s the furthest back in time,” a spokesman for the American Institute of Physics, Phillip Schewe, said.

The big-bang theory predicts that this primordial light should display a classic “blackbody” spectrum, an indicator that the whole universe started out at a uniform temperature before expanding into the much less homogeneous state we now observe. That is exactly what COBE found.

“It’s just a magnificent verification of the big bang,” a professor of physics at Case Western Reserve University in Cleveland, Lawrence Krauss, said.

The measurements also showed tiny ripples in the light’s intensity, representing “lumps” no more than 0.001% richer in matter than the space around them. From those humble origins arose massive galaxies and galactic superclusters hundreds of millions of lightyears across.

In announcing the prize, the Royal Swedish Academy of Sciences noted that life itself depends on the existence of those tiny fluctuations because without them matter would be spread uniformly and thinly throughout space.

“It is one of the greatest discoveries of the century. I would call it the greatest,” the chairman of the Nobel physics committee, Per Carlson, said. “It increases our knowledge of our place in the universe.”

“We did not know how important this was at the time when it happened. We only knew it was important,” Mr. Mather said.

They weren’t the only ones. When Messrs. Mather and Smoot presented their observations at a 1992 physics meeting, “there was an audible gasp in the hall,” Mr. Schewe said.

University of Chicago physicist John Carlstrom said COBE’s observations provide strong, though circumstantial, evidence for inflation theory, which posits that space itself suddenly expanded faster than the speed of light a fraction of a second after the universe’s birth.

COBE also inspired even more detailed studies that made increasingly accurate estimates of the age, history, and composition of the universe, revolutionizing what had been a frustratingly vague and theoretical field.

“It really was a watershed event,” Mr. Carlstrom said.

The cosmic background radiation was a superhot 3,000 degrees Celsius when it was generated nearly 14 billion years ago. But as the universe has expanded since then, it has cooled to

barely 2.7 degrees above absolute zero, a vanishingly small signal compared to the stars, interstellar gas clouds, and other sources of energy in the universe.

“It’s just a really really difficult experimental measurement to make,” Mr. Schewe said.

Messrs. Smoot and Mather battled tight budgets, technical challenges, and launch delays caused by the 1986 explosion of the space shuttle Challenger, which was originally supposed to carry COBE into space. After more than a decade of development, the satellite was finally launched in 1989 on a Delta rocket.

“It’s vindication for an awful lot of people’s faith in us,” Mr. Mather told reporters who gathered at his suburban Maryland home yesterday morning.

Across the country in California, Mr. Smoot called the award “a great thrill. It’s not quite the same thrill as making the discovery.”

The significance of the two scientists’ achievement is reflected in the swiftness with which the Swedish academy chose to recognize it. Nobel prizes are typically awarded decades after the work that they commemorate, but both of the prizes announced so far this year were for research completed in the last 15 years.

The physics prize is also the second this year to go to a pair of Americans. On Monday, Andrew Fire of Stanford University and Craig Mello of the University of Massachusetts Medical School in Worcester won the Nobel prize in medicine for discovering a way to block the activity of genes that has proved useful to both researchers and clinicians.

Americans have so dominated the Nobels in recent times that at least one American scientist has been a recipient in one of the three science categories every year since 1992. In two recent years — 1998 and 2004 — seven American laureates have been named in the sciences and economics. Each prize can be shared by as many as three individuals.

The winner of the Nobel Prize in chemistry will be named today. The Bank of Sweden Prize in Economic Sciences in Memory of Alfred Nobel will be announce October 9. The winner of the peace prize will be announced October 13 in Oslo, Norway. A date for the literature prize has not yet been set.

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