Scientific Method / Science & Exploration

Supernova research challenges cosmic “dark flow” mystery

New research contradicts the finding that everything in the visible universe …


Many scientists were baffled in 2008, when a team of researchers presented evidence suggesting that all galaxies in our visible universe were traveling like a school of fish on a single course. This mysterious collective motion was called dark flow.

The problem is that cosmological models are based on the idea that the universe is more or less the same everywhere and that galaxies, tugged by the gravity of their neighbors, move relatively randomly. A preferred direction in the velocity of matter would seem to challenge this assumption.

"The work has been very controversial, and there's been a lot of debate about it," said cosmologist Richard Watkins of Willamette University in Salem Oregon, who coauthored the new paper that goes against the dark flow finding, currently in press in the Monthly Notices of the Royal Astronomical Society.

The original dark flow results were based on analysis of photons from the cosmic microwave background, a remnant radiation left over from the Big Bang that permeates the entire universe. When these photons pass through galaxies, they are scattered off dust and particles, which alters them slightly.

By studying the change in the photons, researchers can extract information about how fast and in which direction the galaxies were moving. The original dark flow work analyzed this light and concluded that galaxies in the visible universe were moving in one direction at a speed of 1.3 million-2.2 million miles per hour relative to the cosmic microwave background.

Watkins and his team wanted to measure the relative movement of galaxies using a different dataset, a catalog of Type 1a supernova. Because all Type 1a supernovae undergo very similar explosions, astronomers can use their light to accurately measure the distance to, and speed of, the galaxy they occur in.

While Watkins' team found that galaxies were moving in the same direction as the original dark flow research, the velocities were considerably lower, around only 560,000mph. Furthermore, the collective movement was only seen out to relatively close distances, not the entire visible universe. This result wouldn't contradict cosmological models, which allow for small eddies of galaxies flowing in the same direction as long as the bulk of movement is random.

"This is one of the strongest recent papers that works against the dark flow claim," said physicist William Kinney of University at Buffalo in New York who has published previous work challenging the dark flow results.

Because the supernova data has much smaller uncertainties than that of the cosmic microwave background photons, they are more trustworthy, said theoretical cosmologist Dejan Stojkovic, also of the University at Buffalo, who has worked with Kinney. The dark flow team is likely underestimating their error bars, he added, and a better statistical analysis would give their findings as 2.2 million mph plus or minus 1.8 million mph, which brings the two discrepant results more or less in agreement.

But advocates of the dark flow effect are not entirely convinced.

"While our current estimation probably overestimated the dark flow velocity somewhat, [the overestimation] is likely to be around 30 percent," said cosmologist Alexander Kashlinsky at NASA's Goddard Space Flight Center in Greenbelt, Md., the lead author of the original work on dark flow. The new analysis is based on somewhat different assumptions than what his group used for their findings, which could also account for the discrepancy, he added.

The matter is likely to stay unsettled until the European Space Agency's Planck satellite releases new accurate cosmic microwave background radiation data in 2012.

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