Image courtesy ESA/NASA
Published January 20, 2012
Mysterious, unseen structures on the outskirts of creation most likely aren't tugging on our universe, according to a new study. The paper reexamines "dark flow"—an unusual, one-way motion of matter—using measurements of supernovae and the existing laws of physics.
In 2008, a team of scientists took measurements of hundreds of galaxy clusters and calculated that everything in the visible universe—and likely beyond—is flowing at 2 million miles (3.2 million kilometers) an hour in the same direction.
The data couldn't be explained by the distribution of matter in the known universe, so the scientists suggested that chunks of matter had been pushed out shortly after the big bang, and their gravity is now pulling on everything around us.
In 2010 the same team released a second study with data on twice as many galaxy clusters as their 2008 work. That research found that dark flow extends even deeper into the universe than previously reported: out to at least 2.5 billion light-years from Earth.
If proven accurate, the original version of dark flow would revolutionize our understanding of our place in the universe, said Alexander Kashlinsky, the astrophysicist at NASA's Goddard Space Flight Center in Maryland who led the 2008 team.
That's because the presence of massive structures outside the known universe could be evidence that we are in fact just one universe in a multiverse.
(Also see "Other Universes Finally Detectable?")
No Place Like Home, After All
The new study—based on an analysis of supernovae instead of galaxy clusters—also detected a one-way flow.
But the supernovae data show that matter is moving at a mere 550,000 miles (900,000 kilometers) an hour out to about 240 million light-years from Earth.
This is a slightly faster than would be expected based on a standard model of the big bang theory, but "it is not a huge concern," said study leader Stephen Turnbull, of the University of Waterloo in Ontario, Canada.
"It is still perfectly consistent and perfectly well behaved" according to the standard set of equations scientists use to explain the universe, he added.
This slower flow could be caused not by forces beyond our universe but rather by the gravitational pull of a giant supercluster of galaxies, a collection of superclusters, or another as yet unseen but recognizable object within our universe.
If so, the flow may not be dark after all—"dark" being physics-speak for something unexplained by existing models.
Supernova "Candles" Light Up Dark Flow
In the original dark flow studies, Kashlinsky's team determined the motion of galaxy clusters by studying subtle temperature changes in the cosmic microwave background radiation, or CMB. This light is thought to have been released about 380,000 years after the big bang and now permeates the universe.
(Related: "Universe's Most Distant Object Spotted.")
The team could track the clusters' direction and speed by looking at how gas in the clusters has warmed the CMB as the radiation has passed through them.
For the new study, Turnbull's team looked at a special class of exploded stars called Type Ia supernovae.
The intensity of light from these supernovae is roughly the same in each case, so astronomers can use the explosions as "standard candles" for measuring how fast things are moving at different points across the universe.
Type Ia supernovae, for instance, have helped astronomers see that our universe is not only expanding but that its expansion is accelerating.
Turnbull and colleagues examined data from 245 of these supernovae and did some tricky math that lumps their motion together.
This allowed the team to derive the collective motion of matter in an imaginary sphere with a radius of about 240 million light-years.
"I found that that object was moving at 249, plus or minus 76, kilometers [155, plus or minus 47, miles] per second in a certain direction," Turnbull said.
The findings were posted to the website arXiv.org in December 2011 and have been accepted for future publication in the Monthly Notices of the Royal Astronomical Society.
NASA's Kashlinsky said of the new finding: "At face value, yes, there is a contradiction between what we are measuring and what they are measuring."
"Not globally, but in some important details, like the amplitude of the motion."
In fact, Kashlinsky's dark flow extends farther, is moving about four times faster, and runs in a different direction than the flow detected by Turnbull's team.
As things stand now, both teams doubt the two results can be reconciled.
But the scientists admit that they each have error bars in their results, which could possibly bring the data sets closer to agreement as the respective catalogs of supernovae and galaxy clusters grow and the measurements are refined.
More likely, though, one version of dark flow will emerge as the winner of this universal tug-of-war.
"Within a year or two, we hope to lock up this story one way or another," said Kashlinsky, who added that Turnbull's study is precisely the sort of science needed to test the dark flow theory.
There is one possibility that could allow both results to survive, Turnbull added.
"Consider a boat—a really big boat," he said. Kashlinsky had measured the motion of the boat—the collective bulk of galaxy clusters—which he found to be speeding across the ocean.
Meanwhile, Turnbull said, with the supernovae data, "I'm measuring the motion of a person. Now that person is on the boat, [and] the boat is moving, but ... that's going to give us some additional motion inside the boat."
If the person Turnbull measured is moving—i.e., not sitting down but running—inside the boat, then both motions could be happening simultaneously, and "the two results could be reconcilable," he said.
"It is unlikely, but it could be done."
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