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Now you see it, now you don't. Douglas Clowe of Ohio University in Athens, Ohio, is reporting on new Hubble observations that do not find an unusually dense
clump of dark matter in the universe that a different Hubble team reported on
earlier this year. The region of interest lies at the center of a collision
among massive galaxy clusters in Abell 520, located 2.4 billion light-years away.
"The earlier result presented a mystery. But in our observations we didn't see
anything surprising in the core," said Clowe. "Our measurements are in complete
agreement with how we would expect dark matter to behave." Because dark matter is not
visible, its presence and distribution is found indirectly through its gravitational
effects. The gravity from both dark and luminous matter warps space, bending and
distorting light from galaxies and clusters behind it like a giant magnifying glass.
Astronomers can use this effect, called gravitational lensing, to infer the presence
of dark matter in massive galaxy clusters. Both teams used this technique to map the
dark matter in the merging cluster. Clowe is encouraging other scientists to study
the Hubble data and conduct their own analysis on the cluster.