A year and a half ago, the team led by Alexander Kashlinsky of NASA proposed the controversial and ominously named “dark flow,” a massive gravitational force that is tugging at galaxy clusters, and that Kashlinsky says could be coming from beyond the limits of our own visible universe. Now the team is back with a follow-up study in The Astrophysical Journal Letters, and Kashlinsky says the team has tracked the dark flow out twice as far as before.
A quick note on dark flow: The reason Kashlinsky noticed it is thanks to the cosmic microwave background, a signature left over from 380,000 years after the Big Bang that permeates the universe. “The hot X-ray-emitting gas within a galaxy cluster scatters photons from the cosmic microwave background (CMB),” the NASA press release says. “Because galaxy clusters don’t precisely follow the expansion of space, the wavelengths of scattered photons change in a way that reflects each cluster’s individual motion.” Using data from the Wilkinson Microwave Anisotropy Probe (WMAP), which mapped the microwave background, the team managed to find this tiny effect when they looked at huge clusters of galaxies, and found something totally unexpected.
In a bizarre finding that has disrupted the current understanding of the universe, astronomers have detected evidence of a massive gravitational force beyond the horizon of the observable universe. What’s being called a dark flow appears to be pulling vast clusters of galaxies toward a 20-degree-wide patch of sky between the constellations of Centaurus and Vela. “It does fly in the face of everything we know,” said astronomer Dale Kocevski…. “I’m sure it’s going to be controversial” [Discovery News].
When scientists talk about the observable universe, they don’t just mean as far out as the eye, or even the most powerful telescope, can see. In fact there’s a fundamental limit to how much of the universe we could ever observe, no matter how advanced our visual instruments. The universe is thought to have formed about 13.7 billion years ago. So even if light started travelling toward us immediately after the Big Bang, the farthest it could ever get is 13.7 billion light-years in distance. There may be parts of the universe that are farther away (we can’t know how big the whole universe is), but we can’t see farther than light could travel over the entire age of the universe [SPACE.com].