For scientists, the discovery of this "fossil radiation" is akin to turning out all the lights in a bedroom only to find the walls, floor and ceiling aglow with an eerie luminescence.
The telltale infrared radiation puts a limit on the total amount of energy released by all the stars in the universe. Astronomers say this will greatly improve development of models explaining how stars and galaxies were born and evolved after the Big Bang.
The discovery reveals a surprisingly large amount of starlight in the universe cannot be seen directly by today's optical telescopes, perhaps due to stars being hidden in dust, or being too faint or far away to be seen.
The discovery culminates several years of meticulous data analysis from the Diffuse Infrared Background Experiment aboard NASA's Cosmic Background Explorer (COBE), which was launched in 1989. The difficulty in making the discovery is analogous to listening for a faint background hum in a shopping mall full of people talking, music playing, and other noises.
"This is another big step in bringing cosmology to a science based on observation as well as theory," said Michael Hauser of the Space Telescope Science Institute, Baltimore, MD, principal investigator on the Diffuse Infrared Background Experiment.
Reporting the results at the meeting of the American Astronomical Society in Washington, DC, Hauser added: "We set out to do this 23 years ago, and these results show it was worth it. Our discovery fulfills the third and final cosmology objective of the Cosmic Background Explorer mission."
One possible explanation is that the universe is very dusty, with many stars hidden in blankets of dust. Alternatively, many stars may have been born in a flurry of activity in the very early universe but faded away at earlier times than yet reached with large telescopes.
In either scenario, the existence of hidden stars is revealed by telltale dust which absorbs and re-radiates their light at infrared wavelengths, and so a permanent record of their existence is encoded in the infrared background.
But finding the infrared background was not easy. Unlike the cosmic microwave background, which at millimeter wavelengths outshines everything else in the universe, the infrared background is masked by infrared light from nearby dust in our solar system, stars and interstellar dust in the Milky Way Galaxy, and, for ground-based instruments, emission from the Earth's atmosphere and from the instrument itself. The COBE mission overcame the last two problems by observing from space using a small telescope and instrument cooled to within a few degrees of absolute zero.
The COBE science team began by using the Diffuse Infrared Background Experiment to scan half the entire sky once a week, over a 10-month period from Dec. 1989 to Sept. 1990. Astronomers then modeled and subtracted the infrared glow from foreground objects in our solar system, our galaxy's stars, and vast clouds of cold dust between the stars of our Milky Way.
Solar system dust was fairly easy to identify in the data because its brightness changes from week to week as Earth orbits the Sun. The interstellar dust of our Galaxy was identified in the data because it has structure, and so looks different across the sky. Light from stars was removed using a detailed model based on counts of the many types of stars in the various parts of the Galaxy.
When infrared light from these sources was subtracted from the all-sky maps, the astronomers found a smooth background of residual infrared light in the 240 and 140 micrometer wavelength bands in "windows" near the north and south poles of the Milky Way, which provided a relatively clear view across billions of light years.
Astronomers next plan to probe the early formation of stars and galaxies using infrared telescopes on new space missions such as the Space Telescope Infrared Facility, Wide Field Infrared Explorer, Next Generation Space Telescope and the Far Infrared Space Telescope, and hope some day to make more infrared background measurements using instruments launched deep into the solar system to escape the interplanetary dust.
Images accompanying this release can be obtained from the Internet
The images were compiled from data taken between December 1989 and September 1990 by the Diffuse Infrared Background Experiment (DIRBE) on board NASA's Cosmic Background Explorer (COBE). They illustrate the steps scientists used to find the cosmic infrared background, which is a radiative fossil containing cumulative starlight which now appears in the infrared due to the cosmic redshift and by absorption and re-emission by dust in the universe since the Big Bang.
The top picture represents the brightness of the full sky as seen in infrared light. The bright yellow-orange line across the center of the image arises from interstellar dust in the plane of our Milky Way Galaxy, with the center of the Galaxy at the center of the image. The red color above and below this line shows additional wispy clouds of interstellar dust. The blue S-shaped color arises from interplanetary dust in the solar system.
The middle picture represents a view of the sky after the foreground glow of the solar system dust has been extracted. This image is dominated by emission from interstellar dust in the Milky Way Galaxy. The two bright objects in the center of the lower right quadrant are nearby galaxies, the Large and Small Magellanic Clouds.
After the infrared light from our solar system and galaxy has been removed, what remains is a uniform cosmic infrared background. The line across the center is an artifact from removal of galactic light. The DIRBE team reports detection of this cosmic background light also at 140 micrometers, and has set limits to its brightness at eight other infrared wavelengths from 1.25 to 100 micrometers.
Credits: Michael Hauser (Space Telescope Science Institute), the COBE/DIRBE Science Team, and NASA's Office of Space Science