Astronomers have succeeded in detecting an infrared background glow across the sky produced by dust warmed by all the stars that have existed since the beginning of time. For scientists, the discovery of this "fossil radiation" is akin to turning out all the lights in a room 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, which should greatly improve models to explain how stars and galaxies were born and evolved after the Big Bang. It also reveals that a surprisingly large amount of starlight in the universe cannot be seen directly by today's optical telescopes, perhaps because stars are hidden by dust, or are too faint or far away to be seen.

The discovery is the result of several years of meticulous data analysis from the Diffuse Infrared Background Experiment onboard 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, principal investigator on the Diffuse Infrared Background Experiment. "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."


The unexpected preponderance of far infrared light implies that many stars have been missed in ultrasensitive visible light probes of distant reaches of the universe, such as the Hubble Deep Field survey. Some stars may be hidden in blankets of dust and others may have been born in a flurry of activity in the very early universe but faded away before large telescopes began to survey the sky. In either scenario, the existence of hidden stars is revealed by telltale dust that absorbs and reradiates their light at infrared wavelengths, and so apermanent record of their existence is encoded in the infrared background.


Data from two other instruments on COBE have already famously given the precise spectrum and a detailed map of temperature variations in the microwave background radiation from the Big Bang. Finding the infrared background was also an impressive feat, because it is masked by infrared light from nearby dust in our solar system, stars and interstellar dust in the Milky Way galaxy, and, for groundbased instruments, emission from the Earth's atmosphere and from the instrument itself. COBE mission overcame the last two problems by observing from space using a small telescope and instruments 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 between December 1989 and September 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 the Milky Way. Solar system dust was relatively easy to identify because its brightness changes from week to week as Earth orbits the Sun. Interstellar dust in 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.

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