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.
