A balloon-borne instrument called ARCADE mapped a doughnut-shaped region that covered some 7% of the sky (coloured region), turning up an unexplained radio signal (Illustration: NASA/ARCADE)

A balloon-borne experiment has turned up a mysterious radio signal that seems to be coming from beyond the Milky Way. Astronomers do not yet have a clear explanation for the static, but say it could come from the universe’s first generation of stars.

The noise was found with a balloon-borne instrument called ARCADE, which flew for four hours at an altitude of 37 kilometres above Texas in July 2006. The instrument mapped a doughnut-shaped region that covered some 7% of the sky.

The team intended to look for slight deviations in the spectrum of the cosmic microwave background, the first radiation emitted after the big bang.

Instead, after subtracting known radio sources in the Milky Way and other galaxies, an unexplained radio static was left that seemed to pervade the sky and was some six times louder than all known astronomical sources combined at the same radio frequency.


“It is exciting evidence of something new and interesting going on in the universe,” ARCADE leader Alan Kogut of the Goddard Space Flight Center in Greenbelt, Maryland, said on Wednesday at a meeting of the American Astronomical Society in Long Beach, California.

First starlight?

The source of the signal is not clear. One possibility is that it is the last gasp of the universe’s first stars, stellar behemoths that were hundreds of times more massive than the Sun and died within the first billion years after the big bang. As their cores collapsed into black holes, the stars may have spewed out jets of charged particles that produced radio emission. That emission might account for the unexplained signal.

In 2005, another team of astronomers made a similar claim about detecting infrared light from the earliest stars. They said that infrared light that the Spitzer telescope could not trace to individual stars or galaxies might have been the diffuse glow of the universe’s first stars, but others countered that it came from nearby galaxies that were just too faint for Spitzer to see.

Another possibility is that the mysterious radio signal could be created in distant galaxies, whose supermassive black holes whip charged particles up to high speeds, generating radio emission.

High precision

This signal has not been seen until now because ground-based telescopes did not have the required precision to detect it, team member Michael Seiffert of NASA’s Jet Propulsion Laboratory in Pasadena, California told New Scientist.

To see the radio signal, ARCADE carried some 2000 litres of liquid helium to cool its detectors and instruments, bringing it to some -270 °C. Creating similar conditions on Earth would require sealing a telescope in a vacuum chamber. That would reduce the precision of the observations by placing a barrier between the telescope and the radiation it is meant to detect.

The team is confident they have found a true signal. “We don’t think there’s a lot of wiggle room in the result,” Seiffert said.

David Spergel of Princeton University, who is not on the team, is impressed with the research. “This is an exciting result,” he told New Scientist. “The excess does not seem to be coming from the Galaxy nor does it appear to be an instrumental artifact.”

“The most plausible explanations are either emission from young star-forming galaxies or from an early generation of black holes. A more exotic possibility would be the annihilation signal from dark matter,” he says. “We will need more data to understand the source of this new background.”