Along with dark matter and dark energy, astronomers can now add dark flow to the lexicon of cosmic mysteries. Researchers have discovered that 700 distant clusters of galaxies, gas, and dust are all being pulled in the same direction, apparently toward something invisible and possibly very large, confounding current cosmological models. So far, what that "something" is remains speculative, but it could turn out to be a vestige of the universe's earliest days.
Shortly after the big bang occurred some 13.7 billion years ago, cosmologists think, the universe underwent a brief period that defied current physical laws. The theory goes that during this time, called inflation, space itself expanded at a rate much, much faster than the speed of light. As a result, some of the matter formed with the big bang was pulled more than 13.7 billion light-years away--so far that its light hasn't reached us yet. As a result, that matter can't be observed--or at least, so cosmologists thought.
But now a team led by astrophysicist Alexander Kashlinsky of NASA's Goddard Space Flight Center in Greenbelt, Maryland, has unearthed what could be indirect evidence of inflation. The researchers had been surveying the motion of 700 clusters of galaxies to test an unrelated astronomical phenomenon when they made a startling discovery: All 700 clusters are flowing basically in the same direction and at speeds of as much as 1000 kilometers per second--or more than 30 times faster than Earth revolves around the sun. As the team reports in this week's online edition of Astrophysical Journal Letters, the clusters, which appear headed toward a region of the sky where the constellation Centaurus resides, are moving faster than they should be if their acceleration were due only to dark energy, the mysterious force discovered a decade ago that is slowly ripping the cosmos apart.
It was "a huge surprise," says astrophysicist and co-author Harald Ebeling of the University of Hawaii, Honolulu. The big question, he says, is what is generating the flow. The volume of galaxy clusters affected is much too big to be reacting to some single, massive object, Ebeling says. Instead, the team speculates that the flow is moving the clusters toward an irregularity in the mass distribution of the universe that was already in place before inflation began. If so, Ebeling says, the flow should extend even farther across the universe, and the researchers could have the answer when they examine the motion of an even larger and deeper catalog of close to 1500 galaxy clusters. "We should know more in a few years," he adds.
It's an "intriguing" finding, says astrophysicist Mike Hudson of the University of Waterloo in Ontario, Canada. Hudson, who with colleagues has submitted a related paper of his own observations to the Monthly Notices of the Royal Astronomical Society, says the direction of the flow agrees "very well" with his own findings, though those of Kashlinsky's team cover a greater distance and have found a much greater velocity. Hudson recommends awaiting confirmation of the results by other groups using other surveys, but "if this result holds up, then theorists will certainly have to make some major revisions to the standard cosmological model."