Just as unseen dark energy is accelerating the universe’s expansion rate,
there is something else out there causing an unexpected motion in distant galaxy clusters.
Scientists believe that the cause of this is the gravitational attraction of matter
beyond the observable universe. They refer to it as “Dark Flow”
to distinguish it from two other cosmic mysteries: dark matter and dark energy.
The clusters have a small but measurable velocity that is unaffected by the
expansion of the universe and does not change as distance increases.
The distribution of matter in the observable universe cannot explain
this motion of galaxy clusters. The clusters appear to be traveling
along a path extending from our solar system to Centaurus/Hydra, although the
direction of this motion is uncertain. Evidence suggests that the clusters are
moving away from Earth along this path, but scientists cannot rule out the possibility
of the clusters moving towards Earth. They can see movement along this axis,
but their data can’t state as clearly as they’d like whether the clusters are coming or going.
The dark flow is contentious since the distribution of matter in the observable universe
cannot account for it. Its existence implies that something beyond the observable universe,
beyond our horizon, is pulling on the matter in our vicinity.
Cosmologists regard the microwave background, a flash of light emitted 380,000 years after the
universe formed, as the ultimate cosmic reference frame. In relation to it, all large-scale motion
should show no preferred direction. A galaxy cluster contains hot X-ray
emitting gas that scatters photons from the cosmic microwave background (CMB).
Because galaxy clusters do not accurately follow the expansion of space, the wavelengths of
scattered photons shift in a way that reflects the motion of each individual cluster.
This results in a minute shift in the microwave background’s temperature in the direction of the
cluster. The change, which is known to astronomers as the Kinematic Sunyaev-Zel’dovich (KSZ) effect,
is so minuscule that it has never been observed in a single galaxy cluster.
But a team of researchers led by Alexander Kashlinsky, an astrophysicist at NASA’s
Goddard Space Flight Center in Greenbelt, Md, discovered it when they studied a huge catalog
of 700 clusters, spanning up to 6 billion light-years, or half the universe away.
They matched this catalog to the map of the CMB taken by NASA’s Wilkinson Microwave
Anisotropy Probe (WMAP) satellite. They discovered that the clusters
were speeding toward a region in the sky between the constellations Centaurus and
Vela at almost 3.2 million km/hr. This motion is different from the
outward expansion of the universe, which is accelerated by a force called dark energy.
They found a substantial velocity, which does not decrease with distance
as far as they can measure. The scientists deduced that
whatever is causing the clusters’ movements must exist outside of the known universe.
According to inflation theory, the universe we see today is nothing more than a tiny bubble
of space-time that rapidly expanded after the Big Bang. There could be sections of the cosmos
beyond this bubble that we cannot see. Space-time may be radically different in
these regions of the universe, and stars and galaxies are unlikely to exist.
It could include gigantic, massive structures that are much
larger than anything in the observable universe. Researchers suspect these structures are pulling
the galaxy clusters, causing the dark flow. The suspected structures have been pushed so far
away by inflation, perhaps hundreds of billions of light-years away, that we cannot see them
even with the most powerful telescopes because the light emitted there might not have reached us.
The structures would probably be very strange to create such a coherent flow,
perhaps some warped space-time. However, this is just pure speculation.
Even though inflation theory predicts many strange aspects of the distant cosmos,
few scientists predicted the dark flow. It came as a great surprise to most scientists.
Scientists may be able to learn more about what happened to the cosmos before inflation,
as well as what’s going on in those distant realms we cannot see.