Background Radiation: Glow in the Dark

A second cosmic background radiation permeates the sky

By George Musser

Modern theories of the universe begin with the simplest of observations: the night sky looks dark. The darkness implies that the universe is not infinitely old, as scientists once thought. If it were, starlight would already have seeped into all corners of space, and we would see a hot, uniform glow across the sky. This insight is known as Olbers’s paradox, after the 19th-century German astronomer Wilhelm Olbers.

Some kinds of light, however, have had enough time to suffuse space. The famous cosmic microwave background radiation, considered to be the definitive proof of the big bang, fills the sky. Now astronomers say they have found a second, younger background. It is thought to be the first look at a previously unseen period of the universe—between the release of the microwave background and the formation of the earliest known galaxies, about a billion years later. “We’re really completing the resolution of Olbers’s paradox,” said Princeton University astronomer Michael S. Vogeley, one of the researchers who announced their findings at the American Astronomical Society meeting in January.

The greatest hoopla at the meeting concerned the far-infrared part of the background, first hypothesized in 1967 by R. Bruce Partridge of Haverford College and P. James E. Peebles of Princeton. Two effects turn primordial starlight into an infrared glow: the expansion of the universe, which stretches visible wavelengths of light into the infrared; and the presence of dust, which absorbs starlight, heats up and reradiates.

The background proved too dim to be seen by the Infrared Astronomical Satellite (IRAS) and other detectors previously. The decisive measurements were made by the Cosmic Background Explorer (COBE) satellite during 1989 and 1990, although it was not until 1996 that a group led by Jean-Loup Puget of the Institute of Spatial Astrophysics in Paris tentatively detected the background.

Now three teams have confirmed and extended Puget’s findings. One, led by Dale J. Fixsen and Richard A. Shafer of the National Aeronautics and Space Administration Goddard Space Flight Center, used the same instrument on COBE—the Far Infrared Absolute Spectrometer (FIRAS)—that the French team did. Another, headed by Michael Hauser of the Space Telescope Science Institute and Eliahu Dwek of NASA Goddard, relied on COBE’s Diffuse Infrared Background Experiment (DIRBE). A third team, led by David J. Schlegel of the University of Durham and Douglas P. Finkbeiner and Marc Davis of the University of California at Berkeley, combined DIRBE and FIRAS data.

No other COBE result demanded such arduous analysis. Starting with the total amount of observed infrared light, the researchers had to subtract the so-called zodiacal light produced by dust within our solar system and infrared light from stars and dust in the rest of our galaxy. They were left with a faint, nearly uniform glow that exceeded the inherent instrumental error.

Although the teams took different approaches, all arrived at nearly the same background intensity: 2.3 times as bright as the visible light in the universe, according to Hauser. The first implication is that the universe is filled with dust—much more dust than in the Milky Way and nearby galaxies. The second is that some unidentified source generates two thirds of the light in the cosmos.

“I don’t think we know where this radiation is coming from,” said Princeton astrophysicist David N. Spergel. “This emission could be coming from big galaxies; it could be coming from a class of small galaxies in relatively recent times.”

To locate the source, a group directed by Puget and David L. Clements in Paris has started the first far-infrared search for distant galaxies, using the European Space Agency’s Infrared Space Observatory (ISO). Through the Marano hole, a dust-free patch in the southern sky, they discovered 30 galaxies—10 times more than IRAS surveys had implied and exactly the number required to explain the infrared background. Unfortunately, ISO couldn’t get a fix on the galaxies’ positions. Analogous efforts by Vogeley and others have already explained a similar remnant glow in visible- light images by the Hubble Space Telescope.

How do these background measurements affect theories of how and when stars and galaxies formed? The current thinking is that once star formation began, it slowly accelerated, peaked when the universe was about 40 percent of its current age and has since declined 30-fold. But the unexpectedly bright background may indicate that star formation got going faster and remained frenetic for longer. If so, theorists might need to revisit the prevailing theory of galaxy formation, which posits clumps of so-called cold dark matter and agglomerations of small protogalaxies into progressively larger units. “It would cause real trouble for the cold-dark-matter model,” Partridge said. “I think it’s safe to say that we’re seeing more energy than in all current models.”

Besides identifying the source of the background, observers want to measure the glow at shorter wavelengths, determine how it has varied with the age of the universe and look for fluctuations. Upcoming missions such as the Far Infrared Space Telescope may prove crucial. Meanwhile the light-subtracting techniques may improve measurements of other phenomena, such as large-scale galaxy motions and the expansion of the universe. In short, scientists are encountering a new kind of Olbers’s paradox. The night sky isn’t dark; it’s too bright.