The cold spot was surprising, because background radiation is remarkably uniform across the whole sky, interrupted only by small bumps and dips.
So Rudnick looked across the sky at galaxies that emit radio waves. He also tracked what the radio signals were like in the region of the WMAP cold spot to find a reason for the radiation dip.
Possible explanations included that the dip is a holdover from the beginnings of the universe or that a cosmic cloud is soaking up the radiation before it could reach Earth. (Related: "Proof of Big Bang Seen by Space Probe, Scientists Say" [March 17, 2006].)
But Rudnick found that in this region of the sky, there are also far fewer sources of radio waves.
The research team interpreted this as a huge void empty of both regular and dark matter that's nearly a billion light-years across.
"Although our surprising results need independent confirmation, the slightly lower temperature of the [radiation] in this region appears to be caused by a huge hole devoid of nearly all matter," Rudnick said. This hole is estimated to be about six to ten billion light-years away from Earth.
Open to Interpretation?
But some other researchers aren't convinced by this interpretation.
"The claims ... are interesting and important if correct," said Margaret Geller of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts. "But the argument for such a large completely empty void in the universe is not thoroughly convincing.
"Even the smaller voids detected in a wide variety of surveys are not completely empty," Geller added. "It is also odd that there are no other solid indications of structures approaching this scale."
"For this surprising finding to be taken seriously, more objects of comparable size should be found, something that is certainly not to be expected according to the standard model [of cosmology]," said Pablo Fosalba of the Autonomous University of Barcelona in Spain.
Yet the debate could be resolved soon.
The Planck satellite, due to launch in 2008, will produce "very clean extragalactic maps that will greatly help in resolving this puzzle," Fosalba said.
Free Email News Updates
Sign up for our Inside National Geographic newsletter. Every two weeks we'll send you our top stories and pictures (see sample).
SOURCES AND RELATED WEB SITES