Astronomers search for antimatter in colliding galaxies

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Washington, Nov 1 : Astronomers have probed the collision of two large clusters of galaxies, using NASA's Chandra X-ray Observatory and Compton Gamma Ray Observatory, to search for evidence of primordial antimatter.

Antimatter is made up of elementary particles, each of which has the same mass as their corresponding matter counterparts - protons, neutrons and electrons - but the opposite charges and magnetic properties.

When matter and antimatter particles collide, they annihilate each other and produce energy according to Einstein's famous equation, E=mc2.

According to the Big Bang model, the Universe was awash in particles of both matter and antimatter shortly after the Big Bang.

Most of this material annihilated, but because there was slightly more matter than antimatter - less than one part per billion - only matter was left behind, at least in the local Universe.

Trace amounts of antimatter are believed to be produced by powerful phenomena such as relativistic jets powered by black holes and pulsars, but no evidence has yet been found for antimatter remaining from the infant Universe.

"If clumps of matter and antimatter existed next to each other before inflation, they may now be separated by more than the scale of the observable Universe, so we would never see them meet," said Gary Steigman of The Ohio State University, who conducted the study.

"But, they might be separated on smaller scales, such as those of superclusters or clusters, which is a much more interesting possibility," he added.

In that case, collisions between two galaxy clusters, the largest gravitationally-bound structures in the Universe, might show evidence for antimatter.

X-ray emission shows how much hot gas is involved in such a collision. If some of the gas from either cluster has particles of antimatter, then there will be annihilation and the X-rays will be accompanied by gamma rays.

Steigman used data obtained by Chandra and Compton to study the so-called Bullet Cluster, where two large clusters of galaxies have crashed into one another at extremely high velocities.

At a relatively close distance and with a favorable side-on orientation as viewed from Earth, the Bullet Cluster provides an excellent test site to search for the signal for antimatter.

"This is the largest scale over which this test for antimatter has ever been done," said Steigman. "I'm looking to see if there could be any clusters of galaxies which are made of large amounts of antimatter," he added.

ANI

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