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.
Finding
it
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.)
advertisement
