Another astronomer with experience making background observations, Judd Bowman of the California Institute of Technology, concurs: "The authors have made a careful analysis, but at this early stage there may still be room for unanticipated effects to conspire."
Even if the signal is real, astrophysicist Doug Finkbeiner of Harvard University warns that it may not be a truly cosmic signal—that is, one that originates in the distant universe as opposed to our own Milky Way Galaxy. He says the team subtracted the galaxy's contribution to the signal using a highly idealized model. A slight infidelity in this model might mimic an extragalactic signal. Both galactic and extragalactic processes produce light in the same way—namely, by electrons spiraling around magnetic field lines—so the galactic subtraction is especially error-prone. "You're subtracting two things that are almost exactly the same across frequencies," Finkbeiner says.
If the signal is real, where might it come from? Kogut's team speculates it may be a curtain of light emitted by the earliest stars to form in the universe—so-called Population III stars. That would explain why the radio background does not match up with the infrared background, as it would if the radio sources were, like most celestial bodies, surrounded by dust. Dust is produced in stars and litters space only upon their demise, so the first stars lived in dust-free environs. But the team acknowledges that evidence is thin for this hypothesis. Astronomers have yet to do the calculations to verify whether these stars could indeed account for the emission.
"There's a lot of invoking of Population III sources for various observations that people make," says astronomer Michael Hauser of the Space Telescope Science Institute. "I think that's jumping to conclusions."
Another culprit might be active galactic nuclei (AGN), which are blazingly strong light sources powered by black holes. Kogut and his colleagues argue that there are not enough AGN to produce the signal they see. Others, though, are not ready to rule out this possibility.
"They're extrapolating from the known AGN sources, and there could be more," Spergel says. The infrared, visible and x-ray backgrounds turned out to come mostly from hidden populations of AGN.
Finkbeiner speculates the source may be electrons given off by dark matter in our galaxy or extraneous emission that accompanied the release of the microwave background in the primordial universe. Whatever the answer, astronomers are clearly relishing a brand new phenomenon to explore.
"This is a fascinating observation that will take some time to decipher properly," says Michael Vogeley of Drexel University in Philadelphia. Stay tuned, even if you have digital TV.