ARCADE 2 was a balloon experiment flown in 2006 that measured the cosmic radio background. With particular emphasis on measuring radio waves between the 3GHz to 10GHz frequency range, its intention was to study the birth of the earliest stars in our universe. It’s an impressive instrument that flew up to 35 kilometers, or 21 miles, up in the sky and works near a temperature of absolute zero, cooled by almost 2000 liters, or 500 gallons, of liquid helium. Instead of finding radio emissions from the earliest stars, however, they found a mysterious radio background that drowned out such emissions.
The radio background is far in excess of what estimates based on known sources of radio waves from within and without our galaxy would suggest. Even a plausible increase in estimated excess under the assumption that we don’t quite understand the physical processes behind the known radio sources does not seem to be enough to account for the actual excess observed. The shape of the curve is also not consistent with what would be expected from star-forming galaxies, so it’s unlikely that the mysterious radio background is in fact the radio emissions from the birth of the earliest stars that ARCADE 2 had set out to observe. Simple instrument error is also unlikely, as the observations are consistent with those from the COBE satellite.
What could be the source of the mysterious radio background? The ARCADE researchers speculate that it might be from low-luminosity active galactic nuclei that do not emit much heat in the infrared. Whatever it is, it is yet another thing to be discovered about the early universe.