Radio astronomers have found the biggest hole ever seen in the universe. The void, which is nearly a billion light years across, is empty of both normal matter and dark matter. The finding challenges theories of large-scale structure formation in the universe.
Lawrence Rudnick and colleagues of the University of Minnesota in Minneapolis, US, stumbled upon the void by accident. Rudnick’s team had been studying data from a survey carried out by the Very Large Array radio telescope in New Mexico, also in the US. “One morning I was a little bored, and said, ‘why don’t I look in the direction of the WMAP cold spot’,” says Rudnick.
The cold spot in question is an unexplained anomaly in the map of the cosmic microwave background (CMB) created by NASA’s WMAP satellite. The photons of the CMB coming from a region of the sky in the direction of the constellation Eridanus are colder than expected.
Rudnick’s team started looking for radio sources such as radio galaxies and quasars in the direction of the cold spot. “Radio sources track the distribution of mass in the universe,” says Rudnick. “They are the signposts for galaxies, clusters of galaxies and dark matter.”
The team was in for a surprise. They saw little or no radio sources in a volume that is about 280 megaparsecs or nearly a billion light years in diameter. The lack of radio sources means that there are no galaxies or clusters in that volume, and the fact that the CMB is cold there suggests the region lacks dark matter, too.
The void, which is about 6 billion to 10 billion light years away, is considerably larger than any found before. Until now, optical surveys have found no voids larger than 80 megaparsecs wide – making the new hole 40 times larger in volume than the previous record holder.
Rudnick says that these optical surveys could easily miss the void his team found simply because they don’t study large enough volumes.
He thinks that the void is a confirmation that dark energy is at work in the universe. Normally, when the CMB photons pass through a gravitational well, created say, by a supercluster of galaxies, they first gain energy as they fall into the well, then lose energy as they climb out.
If the expansion of the universe is accelerating due to dark energy, then by the time the photons climb out, the supercluster has expanded, and its gravity is a little less strong. So the photons exit relatively easily and with more energy than they had when they entered the gravitational well.
But photons going through a void actually lose energy, ending up colder than if they had been flying through a series of superclusters. Rudnick thinks that the discovery of the void ties in neatly with the WMAP cold spot and the existence of dark energy. “What the community says remains to be seen,” he told New Scientist. “People will take shots at it now.”
Because the CMB is leftover radiation from the big bang, some cosmologists have said that the cold spot is a problem for the theories of the early universe. But Rudnick says that the void could have been created billions of years after the big bang. “We have taken the problem away from the very early universe and put the problem in the time of structure formation,” he says.
Computer simulations that recreate the formation of clusters and super-clusters have never seen voids of this size. That could be because modellers have not simulated large enough volumes to see such a void, says Rudnick. If they did, maybe a void would emerge. “It is an open question whether this will create problems for structure formation,” he says.