Spaceflight Now Home

Spaceflight Now +

Subscribe to Spaceflight Now Plus for access to our extensive video collections!
How do I sign up?
Video archive

Two shuttles sighted

Stunning aerial views of shuttles Atlantis and Endeavour perched atop launch pads 39A and 39B on Sept. 20.

 Play | Hi-Def

Endeavour to pad 39B

Space shuttle Endeavour made the journey from Kennedy Space Center to pad 39B in the predawn hours of Sept. 19.

 Play | Hi-Def

MAVEN to Mars

NASA has selected the Mars Atmosphere and Volatile Evolution spacecraft, or MAVEN, for launch to the Red Planet.


Endeavour to the VAB

For its role as a rescue craft during the Hubble servicing mission and the scheduled November logistics run to the space station, Endeavour is moved to the Vehicle Assembly Building.


STS-125: The mission

A detailed step-by-step preview of space shuttle Atlantis' STS-125 mission to extend the life and vision of the Hubble Space Telescope.


STS-125: The EVAs

The lead spacewalk officer provides indepth explanations of the five EVAs to service Hubble during Atlantis' flight.


STS-125: The crew

The seven shuttle Atlantis astronauts hold a press conference one month before their planned launch to Hubble.


STS-125: NASA leaders

The leaders of NASA's Space Operations and Science directorates give their insights into the upcoming shuttle mission to the Hubble Space Telescope.


STS-125: Shuttle boss

The head of NASA's space shuttle program discusses the risks and plans for Atlantis' trek to Hubble.


The Hubble program

An overview of the Hubble Space Telescope program and the planning that has gone into the final servicing mission.


Hubble's future science

The new instruments to be installed into Hubble and the future science objectives for the observatory are previewed.


Atlantis on the pad

Shuttle Atlantis makes the slow journey from the Vehicle Assembly Building to launch pad 39A for the STS-125 mission to service Hubble.


Meet the Hubble crew

Meet the crew launching on Atlantis' STS-125 mission to service the Hubble Space Telescope and learn how each became an astronaut in this special biography movie.


Become a subscriber
More video


Sign up for our NewsAlert service and have the latest news in astronomy and space e-mailed direct to your desktop.

Enter your e-mail address:

Privacy note: your e-mail address will not be used for any other purpose.

Cosmic 'dark flow' detected across billions of light years
Posted: September 23, 2008

WASHINGTON -- Using data from NASA's Wilkinson Microwave Anisotropy Probe (WMAP), scientists have identified an unexpected motion in distant galaxy clusters. The cause, they suggest, is the gravitational attraction of matter that lies beyond the observable universe.

Galaxy clusters like 1E 0657-56 (inset) seem to be drifting toward a 20-degree-wide patch of sky (ellipse) between the constellations of Centaurus and Vela. Credit: NASA/WMAP/A. Kashlinsky et al.
"The clusters show a small but measurable velocity that is independent of the universe's expansion and does not change as distances increase," says lead researcher Alexander Kashlinsky at NASA's Goddard Space Flight Center in Greenbelt, Md. "We never expected to find anything like this."

Kashlinsky calls this collective motion a "dark flow" in the vein of more familiar cosmological mysteries: dark energy and dark matter. "The distribution of matter in the observed universe cannot account for this motion," he says.

Hot X-ray-emitting gas in a galaxy cluster scatters photons from the cosmic microwave background. Clusters don't precisely follow the expansion of space, so the wavelengths of scattered photons change in a way that reflects each cluster's individual motion.

This results in a minute shift of the microwave background's temperature in the cluster's direction. Astronomers refer to this change as the kinematic Sunyaev-Zel'dovich (SZ) effect.

A related distortion, known as the thermal SZ effect, has been observed in galaxy clusters since the 1980s. But the kinematic version is less than one-tenth as strong and has not been detected in any cluster.

In 2000, Kashlinsky and Fernando Atrio-Barandela from the University of Salamanca, Spain, showed that astronomers could, in essence, amplify the effect isolating the kinematic SZ term. The trick, they found, is to study large numbers of clusters.

The astronomers teamed up with Dale Kocevski at the University of California, Davis, and Harald Ebeling from the University of Hawaii to identify some 700 X-ray clusters that could be used to find the subtle spectral shift. This sample includes objects up to 6 billion light-years - or nearly half of the observable universe - away.

Using the cluster catalog and WMAP's three-year view of the microwave background, the astronomers detected bulk cluster motions of nearly 2 million miles per hour. The clusters are heading toward a 20-degree patch of sky between the constellations of Centaurus and Vela.

What's more, this motion is constant out to at least a billion light-years. "Because the dark flow already extends so far, it likely extends across the visible universe," Kashlinsky says.

The finding flies in the face of predictions from standard cosmological models, which describe such motions as decreasing at ever greater distances.

Cosmologists view the microwave background - a flash of light emitted 380,000 years after the big bang - as the universe's ultimate reference frame. Relative to it, all large-scale motion should show no preferred direction.

Big-bang models that include a feature called inflation offer a possible explanation for the flow. Inflation is a brief hyper-expansion early in the universe's history. If inflation did occur, then the universe we can see is only a small portion of the whole cosmos.

WMAP data released in 2006 support the idea that our universe experienced inflation. Kashlinsky and his team suggest that their clusters are responding to the gravitational attraction of matter that was pushed far beyond the observable universe by inflation. "This measurement may give us a way to explore the state of the cosmos before inflation occurred," he says.

The next step is to narrow down uncertainties in the measurements. "We need a more accurate accounting of how the million-degree gas in these galaxy clusters is distributed," says Atrio-Barandela.

"We're assembling an even larger and deeper catalog of X-ray clusters to better measure the flow," Ebeling adds. The researchers also plan to extend their analysis by using the latest WMAP results, released in March.

The results will appear this week in the electronic edition of Astrophysical Journal Letters.