COSMOLOGISTS are about to undertake the most comprehensive study yet made of what John Mather calls ``the most important `fossil' available.'' Dr. Mather is project scientist for the National Aeronautics and Space Administration's (NASA) Cosmic Background Explorer satellite program. After 15 years of planning and construction, 5,000-pound COBE is ready to head for orbit later this month to measure what literally is the afterglow of our universe's creation.
This is the microwave radiation that permeates the cosmos. It's all that's left of the intense radiant energy of the Big Bang explosion from which most cosmologists think our universe has been expanding and evolving over the past 10 to 20 billion years. ``It's 99 percent of all the radiant energy in the universe - still the dominant factor,'' Mather says.
COBE's launch - now set for no earlier than Nov. 17 from Vandenburg Air Force Base in California - also marks a watershed for NASA. The $150 million satellite is to ride the last NASA-owned and NASA-launched Delta rocket. Future unmanned launches will be carried out by commercial contractors on commercial rockets.
The cosmic background radiation fascinates scientists because it justifies the Big Bang theory. As Prof. David Wilkinson of Princeton University noted in reviewing the subject in Nature, ``Why should one believe anything based on such a fantastic extrapolation of current physics and astronomy?'' The reason, he added, is that ``no other cosmological model has explained [the cosmic radiation] spectrum, and no known astrophysical radio source [other than the hot Big Bang] has such a spectrum.''
This means that the intensity of the cosmic radiation at different frequencies - that is, its spectrum - almost exactly matches that of radiation from what physicists call a blackbody, meaning a perfect absorber and radiator of energy. The frequencies at which blackbodies radiate most strongly depend on the body's temperature.
The cosmic background radiation is highly uniform in all directions. That's what physicists would expect of blackbody radiation that reflects conditions in the early universe when things were so hot that particles of matter and photons (particles of radiation) freely exchanged energy.
By the time the universe was 300,000 to 500,000 years old, it would have cooled to 3,000 Kelvin (3,000 K or 4,940 degrees F). At that temperature, matter and radiation would cease to interact freely and the radiation would evolve on its own. The cosmic ``fossil'' now looks like blackbody radiation at a temperature around 2.75 K (-455 degrees F). This is exactly what physicists would expect for Big Bang radiation that evolved in this manner.
With so much theory hanging on the exact nature of the cosmic radiation, scientists want to study it in great detail. Also, while the radiation's uniformity reflects early conditions, any tiny variations should reveal what happened to it after the first few hundred thousand years. For example, the early clumping of matter before galaxies formed could have subtly distorted its character.
COBE is equipped to map the radiation during a year on orbit 559 miles out from Earth. Mather explains that variations that hint at individual galaxy formation would be smaller than the 7 degree resolution of COBE instruments. But COBE could detect variations that reflect the coalescence of large-scale structures such as super clusters of galaxies.
Mather says that, three years after launch, he expects COBE data will be available to the world's scientists in an electronic database. It will be a complete set of radiation maps at 100 different wavelengths.
The Soviets expect to launch a cosmic background satellite of their own - Relict 2 - within a few years. Mather says his team is not working closely with the Soviets. But he adds: ``I expect they will have very good data and these will overlap our data somewhat. It's such a hard experiment to do, we welcome confirmation.''