The collision of entire clusters of galaxies has helped set the strongest limit yet on the amount of antimatter in the universe. The research suggests that if antimatter exists in large amounts, it may have been pushed to the far reaches of the universe in the moments after the big bang.
In the early universe, the theory goes, matter and antimatter 鈥� which has the same mass as matter, but the opposite charge 鈥� should have been created in equal amounts. But as far as we can tell, our universe is made of matter.
In our galaxy, for instance, no primordial anti-protons or anti-helium atoms have been found by satellite or balloon-based experiments. 鈥淚t鈥檚 clear to a very high level of precision that our galaxy is made of [what] we by convention call 鈥榦rdinary matter鈥�,鈥� says Gary Steigman of Ohio State University in Columbus.
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Recently, astronomers have looked for signs of antimatter on an even larger scale 鈥� galaxy clusters, which each typically contain hundreds of galaxies.
If any fraction of a cluster鈥檚 gas is a mixture of matter and antimatter, then the two types of matter should annihilate on contact, producing gamma rays along with the X-rays expected from collisions between matter particles.
Now, Steigman has X-rays and gamma rays from two galaxy clusters that collided, producing the Bullet Cluster, which was discovered in 2006.
Matter bubble
The fraction of mixed matter observed is less than a few parts per million. Steigman says this excludes any significant amount of antimatter on scales up to about 65 million light years.
The research has implications for the theory of inflation, a period in which space expanded exponentially moments after the big bang. Inflation could have separated neighbouring regions of matter and antimatter before they annihilated each other.
Figuring out if there is any antimatter within the observable universe could tell cosmologists about how long inflation lasted.
For instance, some models of inflation suggest that antimatter that was once adjacent to matter would now be outside the observable universe. Other models show that it might be separated on the scale of superclusters of galaxies.
鈥淭hat鈥檚 why there is interest in whether we have entirely excluded antimatter [from the universe] or simply forced it to a very large scale,鈥� says Steigman.
Diego Casadei, a particle physicist at near Geneva, Switzerland, says Steigman has pushed the limits of antimatter鈥檚 domain. 鈥淗e goes in the direction of saying that we live in a big 鈥榖ubble鈥� of matter,鈥� Casadei told New 杏吧原创.
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