— Our universe is sliding steadily in a specific direction, in what researchers are calling "the dark flow."
— Some suspect the flow is caused by the pull of gravity from another universe.
— One way to detect the flow is seeing how galaxy clusters scatter radiation left over from the Big Bang.
The universe is not only expanding — it's being swept along in the direction of constellations Centaurus and Hydra at a steady clip of one million miles per hour, pulled, perhaps, by the gravity of another universe.
Scientists have no idea what's tugging at the known world, except to say that whatever it is likely dates back to the fraction of the second between the universe's explosive birth 13.7 billion years ago and its inflation a split second later.
"At this point we don't have enough information to see what it is, or to constrain it. We can only say with certainty that somewhere very far away the world is very different than what we see locally. Whether it's 'another universe' or a different fabric of space-time we don't know," Alexander Kashlinsky at NASA's Goddard Space Flight Center in Greenbelt, Maryland, told Discovery News.
Kashlinsky and colleagues have spent years building up evidence for what they call "the dark flow." They look at how the relic radiation from the Big Bang explosion scatters as it passes through gases in galaxy clusters, a process that is something akin to looking at stars through the bubble of Earth's atmosphere.
With data on more than 1,000 galaxy clusters, including some as distant as 3 billion light-years from Earth, the measurements show the universe's steady flow is clearly not a statistical fluke, Kashlinsky said.
"It was greatly surprising. When we first found it, we didn't know what to do with it. We knew how extraordinarily unexpected it was," he said.
The force and direction of the flow holds steady across space and through time.
"It's the same flow at a distance of a hundred million light-years as it is at 2.5 billion light-years and it points in the same direction and the same amplitude. It looks like the entire matter of the universe is moving from one direction to the next," Kashlinsky said.
The observation fits theoretical models of how our universe might be impacted by sibling universes, predicted by string theory, that we cannot directly detect.
It's like our universe is a box and everything that it contains is inside it like milk in a carton, physicist Laura Mersini-Houghton with the University of North Carolina at Chapel Hill, told Discovery News.
"If our universe is all that's there, then the liquid in the box shouldn't be sliding. Whatever is pulling it has to be bigger than the size of the box," she said. "There is a structure beyond the horizon of our universe and that structure is exerting a force on our universe and creating this flow."
The research expands on previously reported results based on fewer and closer galaxy clusters. Another team of scientists has turned up independent evidence for the flow with an alternative method that spatially correlates nearby galaxies.
Kashlinsky and colleagues' new findings appear in the March 20 issue of The Astrophysical Journal Letters.