Gaping "Hole" in the Sky Found, Experts Say

Mason Inman
for National Geographic News
August 24, 2007

There is a yawning gap of sky nearly a billion light-years across that contains no matter, a new study suggests.

But some researchers aren't buying it, in part because it would be a monumental surprise to find a void that large.

When seen on the scale of tens of millions of light-years, the universe has a foamy structure, with galaxies arranged as if on strings or sheets, with little matter in between them.

This arrangement applies to both visible matter that pumps out light, such as stars, and the mysterious dark matter, whose existence can be inferred only indirectly from how it holds galaxies together.

(Related: "'Cosmic Train Wreck' May Derail Theories of Dark Matter" [August 22, 2007].)

But at much larger scales, about 150 million light-years and beyond, researchers had expected the universe would be more uniform—so finding a void nearly a billion light-years across was a shock.

"Not only has no one ever found a void this big, but we never even expected to find one this size," said study lead author Lawrence Rudnick of the University of Minnesota in Minneapolis.

Rudnick and colleagues Shea Brown and Liliya Williams report their findings in a paper accepted for publication in the Astrophysical Journal.

Unusual Spot of Sky

The researchers began looking at this particular spot in the sky because it already showed a strange feature.

There the cosmic microwave background radiation—low-level light left over from the birth of our universe that bathes all of space—is especially dim.

This dark patch—where the sky appears "cooler"—is known as the "WMAP cold spot," named after the Wilkinson Microwave Anisotropy Probe satellite that mapped the radiation in 2003.

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.