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.
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