[tt] dark flow dragged from the outside of visible universe

[Eugen Leitl]

http://www.space.com/scienceastronomy/080923-dark-flows.html

Mysterious New 'Dark Flow' Discovered in Space

By Clara Moskowitz

Staff Writer

posted: 23 September 2008

12:46 pm ET

As if the mysteries of dark matter and dark energy weren't vexing enough,
another baffling cosmic puzzle has been discovered.

Patches of matter in the universe seem to be moving at very high speeds and
in a uniform direction that can't be explained by any of the known
gravitational forces in the observable universe. Astronomers are calling the
phenomenon "dark flow."

The stuff that's pulling this matter must be outside the observable universe,
researchers conclude.

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.

Mysterious motions

Scientists discovered the flow by studying some of the largest structures in
the cosmos: giant clusters of galaxies. These clusters are conglomerations of
about a thousand galaxies, as well as very hot gas which emits X-rays. By
observing the interaction of the X-rays with the cosmic microwave background
(CMB), which is leftover radiation from the Big Bang, scientists can study
the movement of clusters.

The X-rays scatter photons in the CMB, shifting its temperature in an effect
known as the kinematic Sunyaev-Zel'dovich (SZ) effect. This effect had not
been observed as a result of galaxy clusters before, but a team of
researchers led by Alexander Kashlinsky, an astrophysicist at NASA's Goddard
Space Flight Center in Greenbelt, Md., found it when they studied a huge
catalogue of 700 clusters, reaching out up to 6 billion light-years, or half
the universe away. They compared this catalogue to the map of the CMB taken
by NASA's Wilkinson Microwave Anisotropy Probe (WMAP) satellite.

They discovered that the clusters were moving nearly 2 million mph (3.2
million kph) toward a region in the sky between the constellations of
Centaurus and Vela. This motion is different from the outward expansion of
the universe (which is accelerated by the force called dark energy).

"We found a very significant velocity, and furthermore, this velocity does
not decrease with distance, as far as we can measure," Kashlinsky told
SPACE.com. "The matter in the observable universe just cannot produce the
flow we measure."

Inflationary bubble

The scientists deduced that whatever is driving the movements of the clusters
must lie beyond the known universe.

A theory called inflation posits that the universe we see is just a small
bubble of space-time that got rapidly expanded after the Big Bang. There
could be other parts of the cosmos beyond this bubble that we cannot see.

In these regions, space-time might be very different, and likely doesn't
contain stars and galaxies (which only formed because of the particular
density pattern of mass in our bubble). It could include giant, massive
structures much larger than anything in our own observable universe. These
structures are what researchers suspect are tugging on the galaxy clusters,
causing the dark flow.

"The structures responsible for this motion have been pushed so far away by
inflation, I would guesstimate they may be hundreds of billions of light
years away, that we cannot see even with the deepest telescopes because the
light emitted there could not have reached us in the age of the universe,"
Kashlinsky said in a telephone interview. "Most likely to create such a
coherent flow they would have to be some very strange structures, maybe some
warped space time. But this is just pure speculation."

Surprising find

Though inflation theory forecasts many odd facets of the distant universe,
not many scientists predicted the dark flow.

"It was greatly surprising to us and I suspect to everyone else," Kashlinsky
said. "For some particular models of inflation you would expect these kinds
of structures, and there were some suggestions in the literature that were
not taken seriously I think until now."

The discovery could help scientists probe what happened to the universe
before inflation, and what's going on in those inaccessible realms we cannot
see.

The researchers detail their findings in the Oct. 20 issue of the journal
Astrophysical Journal Letters.