The Bullet Cluster, located about 3.8 billion light years from Earth, formed after a violent collision of two giant clusters of galaxies. This image combines an X-ray image from Chandra with optical data from the Hubble Space Telescope and the Magellan telescope in Chile.
Credit: X-ray: NASA/CXC/CfA/M.Markevitch et al.; Optical: NASA/STScI; Magellan/U.Arizona/D.Clowe et al.
Scientists are on the hunt for matter?s arch nemesis, antimatter, and new evidence suggests the search may have become even trickier.
The data, collected at a supercluster of galaxies called the Bullet Cluster, show no evidence of primordial antimatter. But the non-finding helps to set a limit on where the wacky particles could be hiding, the researchers say.
Antimatter is real. It is made of elementary particles, each with the same mass but opposite charge and magnetic properties as a corresponding counterpart of matter. A proton's antimatter counterpart is called an antiproton and that for an electron is called a positron.
While the stuff is either not around today or present only in miniscule amounts, scientists say that just seconds after the Big Bang, the universe was flooded with particles of both matter and antimatter. When the rival particles collided, they destroyed each other and produced energy (in the form of gamma rays). Most of this material annihilated early on.
But since slightly more matter than antimatter is thought to have existed initially, only matter was left behind, at least in the local universe, which includes the stars and galaxies located less than 500 million light-years away. (One light-year is the distance light will travel in a year, or about 6 trillion miles, or 10 trillion km.).
There's a chance antimatter could have survived in the more distant reaches of the universe.
Today, scientists are pretty sure of antimatter's existence. For instance, research has suggested stars getting torn apart by black holes and neutron stars can produce trace amounts of antimatter (though it wouldn't stay around long with normal matter nearby). And in the lab, high-energy particle accelerators such as the Large Hadron Collider (LHC) could churn out the anti-particles, once the instrument is fully up and running.
Even so, no evidence has yet been found for antimatter remaining from the infant universe.
The only way to detect antimatter is to look for a certain range of gamma rays produced when the anti- and normal matter particles collide. And while scientists have searched for such energy signatures in separate galaxies or clusters of galaxies, this new research is one of the first to probe a smash-up of groups of galaxies called a supercluster.
"If there's any antimatter still around in the universe today, we know it can't be fully mixed with ordinary matter in the solar system or the galaxy or probably in our local group of galaxies," said lead researcher Gary Steigman of Ohio State University. "But the question is, on some bigger scale, could there be separate regions of antimatter?"
Bubbles of antimatter
The thinking goes that perhaps regions of predominantly matter and regions of predominantly antimatter somehow got cordoned off from each other. These bubble worlds then could have remained hidden from one another as the universe ballooned up big time with inflation, or the theoretical exponential expansion of the universe after the Big Bang.
"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," Steigman said. "But they might be separated on smaller scales, such as those of superclusters or clusters, which is a much more interesting possibility."
When such clusters collide, the smash-up would bring the rivals into contact, resulting in destruction of both and a release of gamma rays.
The search continues
Steigman and his colleagues chose to study one of the most violent collisions between two large clusters of galaxies, which resulted in the Bullet Cluster. The researchers looked for gamma rays by analyzing data collected by NASA's Chandra X-ray Observatory and Compton Gamma Ray Observatory.
Officially known as 1E 0657-56, the Bullet Cluster is located about 3.8 billion light-years from Earth in the constellation Carina.
"This is the largest scale over which this test for antimatter has ever been done," said Steigman, whose paper was published in the October issue of the Journal of Cosmology and Astroparticle Physics. "I'm looking to see if there could be any clusters of galaxies which are made of large amounts of antimatter."
The researchers did not find any gamma rays, meaning the Bullet Cluster contains less than three parts per million of the mysterious stuff.
"This is the first example on the scale as big as the scale of the colliding clusters that says there's no evidence for antimatter," Steigman told SPACE.com. "So it pushes the scale on which antimatter could be hiding to even larger length scales or mass scales in the universe."
He added, "If somehow there was this separation in the early universe and not all the antimatter annihilated and disappeared, it must be separated in regions bigger than the size of those two original clusters." (The two colliding galaxies were originally separated by 65 million light-years.)
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