A potential portal to other universes seems to have closed. The sharpest map yet made of light from the infant universe shows no evidence of "dark flow" – a stream of galaxy clusters rushing in the same direction that hinted at the existence of a multiverse.
That is the conclusion of 175 scientists working with data from the
European Space Agency's Planck spacecraft. But champions of dark flow are not ready to give up yet, including one Planck scientist who says his team's analysis is flawed.
The
first suggestion that the flow existed came in 2008, when a group led by
Alexander Kashlinsky of NASA's Goddard Space Flight Center in Greenbelt, Maryland, scrutinised what was then the best map of the cosmic microwave background radiation, the big bang's afterglow.
NASA's WMAP satellite measured the temperature of this ancient light, revealing fluctuations in the density of matter in the very early universe.
The light's wavelength can also change noticeably when photons are scattered off ionised gas moving through space, providing a way to probe the velocity of such gas. Kashlinsky's WMAP analysis found that hundreds of gas-rich galaxy clusters appeared to be streaming towards a region in the sky between the constellations Vela and Centaurus.
Tilted table
This flow suggested that the universe had somehow become lopsided, as if space-time itself was behaving like a tilted table and matter was sliding off, says Kashlinsky. That goes against the
standard model of cosmology, which says that the universe is increasingly uniform on larger scales, making it unlikely that structures big enough to produce such a tilt would form. Some researchers suggested that, instead,
other universes could be pulling on matter in ours, creating the flow. But
other groups looking at WMAP data
did not detect the controversial motion.
The latest search is based on a new, higher-resolution map of the cosmic microwave background from Planck. The Planck team says their multi-pronged analysis also found no evidence of galaxy clusters gushing along in a coherent stream.
"The Planck team's paper appears to rule out the claims of Kashlinsky and collaborators," says
David Spergel of Princeton University, who was not involved in the work. If there is no dark flow, there is no need for exotic explanations for it, such as other universes, says Planck team member
Elena Pierpaoli at the University of Southern California, Los Angeles. "You don't have to think of alternatives."
But it is too soon to rule out dark flow entirely, argues
Fernando Atrio-Barandela at the University of Salamanca in Spain. A member of the Planck team, he withheld his name from his colleagues' paper because he says they overestimated the uncertainty in their measurements, making what might be a subtle signal of dark flow look like mere noise.
"One has to be very careful not to wash the baby out with the bathwater," agrees Kashlinsky. He and Atrio-Barandela are running their own analysis with the new Planck data and expect to have results in just a few months.
