Using a balloon-borne instrument chilled to nearly absolute zero, a team of U.S. scientists has serendipitously detected an unexplained hiss of relatively low-frequency radio waves that seems to pervade the universe.
The discovery represents yet another type of "cosmic background radiation" — distant emission coming from everywhere in the sky — to go along with the backgrounds previously discovered in microwaves, infrared light, X rays, and gamma rays.
The microwave background is from the hot gas that filled the whole universe as seen 380,000 years after the Big Bang. The other backgrounds are from later, but still extremely distant, astronomical sources that are too faint and numerous to resolve individually in most cases.
The new radio background "is exciting evidence for something new in the cosmos," said team leader Alan Kogut (NASA/Goddard Space Flight Center) at a press conference last Wednesday at the American Astronomical Society meeting in Long Beach, California.
Kogut and his colleagues found the radiation using an instrument named ARCADE (Absolute Radiometer for Cosmology, Astrophysics, and Diffuse Emission, seen above), which flew 120,000 feet over Texas on July 22, 2006. The ARCADE detector is housed in a dewar, with liquid helium circulating around the instrument to keep it cooled to 2.7° C above absolute zero (minus 454.8° F). By chilling the detector, the team could be assured that it was detecting weak emission from space, not from the detector itself. It listened at frequencies around 3, 8, 10, 30, and 90 gigahertz.
The group was hoping to detect redshifted heat emission from the very first stars: the long-sought "Population III" superstars that first lit up the universe near the end the Dark Age, several hundred million years after the cooling of the Big Bang.
But to the surprise of everyone, ARCADE found an excess hiss of lower-frequency radio waves, peaking in the 3 and 8 GHz detectors. Even after subtracting for Milky Way and other known sources, the hiss remained. It appeared throughout the big region of sky (7% of the entire celestial sphere) surveyed during the 2.5-hour part of the flight that ARCADE took data. Overall, the radio noise is six times louder than the combined radio emission from all known radio sources.
"We really don't know what it is, but it's not the combined emission of radio galaxies in the universe, there simply aren't enough of them," says team member Michael Seiffert (NASA/ Jet Propulsion Laboratory). "You start out on a path to measure something — in this case, the heat from the very first stars — but run into something else entirely, something unexplained."
Given that these results were just announced, theorists haven't had much time to digest the information. So for now, there are no convincing explanations for this excess radio hiss. But the team is confident that the detection is real, especially because the team has found confirmation of the signal in previous experiments.
"If the result holds up, it is quite interesting," says cosmologist Gary Hinshaw (NASA/Goddard), who is not a member of the ARCADE team. "It either means that radio galaxies, or some class of radio galaxies, have different properties than were previously thought, or perhaps there is some other, more exotic mechanism for producing radio emission in the universe."
Making this discovery more interesting is the fact that some of the flight hardware was built by high-school students in Greenbelt, Maryland, and by college undergraduates, helping to keep costs low.
"The payload is a good example of the kind of risk-taking you can take with a sub-orbital program," says Kogut. The total cost of ARCADE is less than $4 million, a tiny fraction of the cost of a typical astronomical satellite. And that cost was spread over a decade.
For more information: NASA press release.
The group has submitted four papers on the ARCADE results to the Astrophysical Journal: ARCADE 2 Measurement of the Extra-Galactic Sky Temperature at 3-90 GHz, Interpretation of the Extragalactic Radio Background, ARCADE 2 Observations of Galactic Radio Emission, and The ARCADE 2 Instrument.