Loud sounds tend to startle us. But imagine being surprised by a sound six times louder than you expect. A balloon-borne instrument called ARCADE, (Absolute Radiometer for Cosmology, Astrophysics, and Diffuse Emission) was supposed to be used to search for heat signature from the first stars to form after the Big Bang. Instead it found an unexplained “booming” radio static that fills the sky.
In July 2006, the instrument launched from NASA’s Columbia Scientific Balloon Facility in Palestine, Texas, and flew to an altitude of 36,000 meters (120,000 feet) where the atmosphere thins into the vacuum of space. Its mission lasted four hours.
The team, led by Alan Kogut of NASA’s Goddard Space Flight Center said they found the radio noise almost immediately. “We were calibrating the instrument, and we saw this big point in the graph. I said, ‘What the heck is this — this shouldn’t be here.’ We spent the next year trying to make that point go away, but it didn’t.”
Detailed analysis has ruled out an origin from primordial stars, user error or a mis-identified galactic emission, and the scientists are sure there aren’t more radio sources than we expect. “Radio source counts are well known and they don’t even come close to making up the detected background,” said Kogut. “New sources, too faint to observe directly would have to vastly outnumber the number everything else in the sky.”
Dale Fixsen of the University of Maryland at College Park, added that to get the signal they detected, radio galaxies would have to be packed “into the universe like sardines,” he said. “There wouldn’t be any space left between one galaxy and the next.”
The sought-for signal from the earliest stars remains hidden behind the newly detected cosmic radio background. This noise complicates efforts to detect the very first stars, which are thought to have formed about 13 billion years ago — not long, in cosmic terms, after the Big Bang. Nevertheless, this cosmic static may provide important clues to the development of galaxies when the universe was less than half its present age. Unlocking its origins should provide new insight into the development of radio sources in the early universe.
“This is what makes science so exciting,” says Michael Seiffert, a team member at NASA’s Jet Propulsion Laboratory in Pasadena, Calif. “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.”
ARCADE’s revolutionary design makes it super-sensitive to cosmic noise. Chilled to 2.7 degrees above absolute zero by immersion into more than 500 gallons of liquid helium, each of ARCADE’s seven radiometers alternately views the sky and a calibration target. The project allows for significant high school and undergraduate student participation. ARCADE is the first instrument to measure the radio sky with enough precision to detect this mysterious signal.
This is the same temperature as the cosmic microwave background (CMB) radiation, the remnant heat of the Big Bang that was itself discovered as cosmic radio noise in 1965. “If ARCADE is the same temperature as the microwave background, then the instrument’s heat cannot contaminate the cosmic signal,” Kogut explains.
“We don’t really know what this signal is,” said Seiffert. “We’re relying on our colleagues to to study the data and put forth some new theories.”
Source: NASA, AAS Press Conference