Not a Whisper be Lost When astronomy aficionados hear the word “background,” they immediately think of the famous cosmic microwave background (CMB). This pervasive radio emission appears to have a truly diffuse origin—namely, a hot plasma that filled the universe when it was only 400,000 years old. Through the expansion of the universe, this radiation is today observed at a peak wavelength of about one millimeter, corresponding to a temperature of 2.7 kelvins. The study of the spectrum and distribution of the CMB has provided compelling evidence for the big bang theory.
Yet the CMB is only part of the story. The whole electromagnetic background is actually a mixture of components, each of which dominates a particular range of wavelengths. Besides the CMB are the lesser-known cosmic x-ray background (CXB), cosmic infrared background (CIB) and cosmic optical background (COB).
The precise measurement of these components is one of the most trying tasks in observational astronomy. Conceptually it seems so simple: look at the sky to measure the total signal and then subtract all the known sources between Earth and the deep universe (the foreground): the noise of the detectors, the signals from within our solar system, the emission from the rest of the galaxy, and so forth. In addition, one has to correct for any foreground attenuation of the background signal.
Performing all these subtractions with sufficient precision, though, is tricky; subtraction is an operation that amplifies error. In certain wavelength bands, observers are lucky that the background is the brightest emission in the sky, but in other bands they have to extract a cosmic whisper from a foreground roar. Most often the limiting factor is the accuracy with which astronomers know the foreground emission. They try to skirt this problem by concentrating on regions of the sky that are utterly devoid of stars and other known foregrounds—the more boring, the better. Despite the obstacles, observers have now determined the cosmic background spectrum with quite high precision over a broad range of the spectrum.
The x-ray component, discovered in 1962, has a characteristic hump at about 30 kiloelectron-volts—corresponding roughly to the wavelength used for medical x-rays—and a long tail toward higher energies, including gamma rays. Below about 1 keV, and superposed on this continuum, are a number of atomic emission lines that appear to be the fingerprint of a gas heated to several million kelvins and most likely located inside or around our galaxy.
In the 1970s the first x-ray satellites, such as UHURU, ARIEL V and HEAO-1, showed the higher-energy x-radiation to be spread uniformly over the sky. Thus, its origin has to be mainly extragalactic: if it came from our solar system or galaxy, the brightness would be strongly skewed in certain directions corresponding to the plane of the planets or galactic disk. Gamma-ray satellites such as SAS-3, COS-B and the Compton Gamma Ray Observatory have found a similar uniformity at still higher energies.
Whereas the CMB and CXB dominate the sky in their respective bands, the other cosmic background components account for only a small fraction of the radiation in their respective wavelength bands. A few years ago several groups independently detected the far-infrared background signal in the high-frequency tail of the CMB [see “Glow in the Dark,” by George Musser; Scientific American, March 1998]. In the near- to mid-infrared range, the bright zodiacal light obscures the background, so astronomers have generally resorted to interpolating measurements from other wavelength bands. They have also derived upper limits from observations of high-energy gamma rays: too thick a haze of infrared photons would interfere with the propagation of gamma rays. Only in the past two years have observers made direct measurements at infrared wavelengths.
In the optical and ultraviolet, the first direct background measurements were announced last December by Rebecca A. Bernstein of the University of Michigan and her colleagues. Before their work, astronomers had relied on constraints derived by summing up the light from the faintest galaxies seen by the Hubble Space Telescope. In the extreme ultraviolet, the background is obscured by the interstellar medium, so the background level can be estimated only by interpolating between the ultraviolet and x-ray measurements.