A new study tracing the path of many galaxy clusters scattered over a wide span of sky shows that they are all moving toward one point in the universe.
The finding was unexpected and contradicts most theories about the state of the universe, said study author Dale Kocevski, a postdoctoral researcher at UC Davis.
Astronomers have long held to the notion that the universe is relatively uniform in nature, which Alexander Kashlinksy at NASA’s Goddard Space Flight Center compares to the viewpoint from the middle of a quiet ocean.
“As far as you can see to the horizon, the ocean seems smooth … the same in every direction and you may conclude that the entire cosmos is like that you see inside your horizon,” Kashlinsky said in an e-mail interview. “But then you find a small flow in some direction extending across the entire field of view. The flow would then indicate the existence of other very different structures (say ravines to sink to or mountains to flow from) from your local part of space-time (ocean).”
Although local flows of galaxies and galaxy clusters have been identified – the Milky Way and Andromeda galaxies, for example, are moving toward the Virgo cluster – this flow is much larger in scale, extending over billions of light-years, Kocevski said. The pull of gravity by dense matter – such as nearby galaxy clusters – is responsible for local flows and decreases with further distance. However, there isn’t any matter or dark matter dense enough to account for the extensive bulk flow, leading researchers to conclude that the pull is coming from outside the observable universe.
The discovery came when researchers identified a method to measure bulk flow using a large sample of galaxy clusters, each consisting of thousands of galaxies, Kocevski said.
Photons from residual cosmic microwaves left by the Big Bang (CMB) shift in temperature when passing through hot gas in galaxy clusters. Cluster movement accounts for a tiny fraction of this temperature shift. Kashlinsky and colleagues determined that this effect can be amplified and measured by stacking signals from many galaxy clusters, allowing their velocity to be calculated.
“Galaxy clusters were identified by their X-ray emission and their distances measured using optical telescopes,” said Harald Ebeling, an astronomer at the University of Hawaii Institute for Astronomy, in an e-mail interview. “This approach ensured that all clusters included in the study were real systems held together by gravity and helped greatly to find clusters even behind the Milky Way.”
With a catalog of nearly 1,000 galaxy clusters and the CMB data, researchers determined that they are all moving at roughly 600 kilometers per second toward a point in the sky between the Vela and Centaurus constellations.
Study authors posit that inflation, the notion that the universe expanded very rapidly after the Big Bang, may explain the flow and provide context for probing the original structure of space-time.
“What our finding may indicate is the tilt across the observable universe generated perhaps by the pull from a far-away inhomogeneity (dense region) … blown by inflationary hyper-expansion to well beyond our observable universe,” Kashlinksy said.
“Our explanation seems the most straightforward, but we will be interested to see what others can come up with,” he said.
Future research will verify the finding using more clusters over larger distances and more CMB data collected over the next two years, Kocevski said.
What the galaxy clusters are converging on will remain a mystery.
“We’ll never in fact reach the object(s) that is pulling us,” Kocevski said. “This is in part because of the great distance to that object(s), but also because the universe itself is expanding and essentially pushing that region of space away from us.”
ELAINE HSIA can be reached at firstname.lastname@example.org