Among Strongest Evidence Yet Substance Exists
The most common substance in the
universe is called dark matter. It doesn’t shine or reflect
light; in fact, humans can’t even see it. It is an invisible
substance composed of atoms that are far different from those that
make up the universe’s normal matter, such as stars and
galaxies.
Astronomers using NASA’s Hubble Space Telescope recently
got a first-hand view of how dark matter behaves during a titanic
collision between two galaxy clusters. The wreck created a ripple
of dark matter, which is somewhat similar to a ripple formed in a
pond when a rock hits the water.
The ring's discovery is among the strongest evidence yet that
dark matter exists. Astronomers have long suspected the existence
of the invisible substance as the source of additional gravity that
holds together galaxy clusters. Such clusters would fly apart if
they relied only on the gravity from their visible stars. Although
astronomers don't know what dark matter is made of, they
hypothesize that it is a type of elementary particle that pervades
the universe.
The ring-like structure is evident in a composite image of the
cluster made from Hubble observations. The ring can be seen in the
blue map of the cluster’s dark matter distribution, which is
superimposed on an image of the cluster.
The Hubble astronomers say it is the first time they have
detected dark matter as having a unique structure that is different
from the gas and galaxies in the cluster. The researchers spotted
the ring unexpectedly while they were mapping the distribution of
dark matter within the galaxy cluster Cl 0024+17 (ZwCl 0024+1652),
located 5 billion light-years from Earth. The ring measures 2.6
million light-years across.
Although astronomers cannot see dark matter, they can infer its
existence in galaxy clusters by observing how its gravity bends the
light of more distant background galaxies, a powerful effect called
gravitational lensing. The blue streaks near the center of another
Hubble image of the cluster are the distorted shapes of more
distant galaxies, whose light was bent and magnified by the
powerful gravity of Cl 0024+17.
The collision between the two galaxy clusters, the astronomers
explained, created a ripple of dark matter that left distinct
footprints in the shapes of the background galaxies. It's like
looking at the pebbles on the bottom of a pond with ripples on the
surface. The pebbles' shapes appear to change as the ripples pass
over them. So, too, the background galaxies behind the ring show
coherent changes in their shapes due to the presence of the dense
ring. Although the invisible matter has been found before in other
galaxy clusters, astronomers say it has never been detected to be
so largely separated from the hot gas and the galaxies that make up
galaxy clusters.
The astronomers found previous research that suggested the
cluster had collided with another cluster 1 to 2 billion years ago.
They then created computer simulations of galaxy cluster
collisions. The simulations show that when the two clusters smash
together, the dark matter falls to the center of the combined
cluster and sloshes back out.
As the dark matter moves outward, it begins to slow down under
the pull of gravity and pile up... like cars bunched up on a
freeway, according to NASA.