The
identity of the mysterious dark matter thought to pervade the universe has
eluded astrophysicists for decades. Now, for the first time a team hopes to
look inside the sun for one of the prime candidates.
The invisible
stuff called dark matter is thought to make up as much as 90 percent of
the universe's matter. To date, astrophysicists have only inferred the existence of
some mysterious substance by identifying its gravitational effects on visible
matter such as stars and galaxies. (For instance, dark matter makes galaxies
spin faster than otherwise expected.)
Two
hypothetical particles have become the prime suspects to explain the
fundamental make-up of dark matter: so-called axions and WIMPs (Weakly Interacting Massive Particles). Tens of teams are
on the hunt for the heavyweight WIMPs, such as the GLAST team, which hopes to
detect the gamma rays produced when, hypothetically, WIMPs and their antimatter
selves annihilate each other.
Only
a handful of groups are searching for the lightweight particles called axions. For both sociological and technical
reasons, WIMP searches far outnumber axion ones, according to David Tanner, a
physicist at the University of Florida, and others. For instance, he said,
detectors for WIMPs build more on the expertise of many astrophysicists. In
addition, these massive particles are more fantastical.
"WIMPs
also imply things about supersymmetry
and extra dimensions," Tanner told SPACE.com. "And so if they
were detected, they would give theorists lots of new toys to play with, and new
ideas to follow."
A team led
by X-ray astronomer Hugh Hudson of UC Berkeley says, however, that they are
onto a promising and new way to search for the axion: Looking inside the sun.
Hudson presented his research at a recent
meeting of the American Astronomical Society (AAS) in St. Louis.
Solar
axions
The axion
is extremely lightweight with neither electric charge nor spin, so it hardly
interacts with the universe's surrounding matter — that's if the particle
even exists.
The sun is
thought to possibly be a factory for these axions. When photons at the sun's
core feel a magnetic field, they become axions, the thinking goes. Since the
teensy particles only weakly interact with ordinary matter, they are thought to
easily fly through the sun's core toward the surface unimpeded by other
particles. Once at the solar
corona, where the sun's magnetic field is strong, the axions would convert
back into photons.
It's these
photons that Hudson's team hopes to find using existing instruments on three
satellites capable of observing solar X-rays: Yohkoh, RHESSI and Hinode.
The X-ray
images to date, Hudson said, have turned up empty of axion signatures. He and
his team hope to increase the sensitivity of their searches by combining lots
of images to yield, potentially, a stronger, composite signal. The composite
image would help the astrophysicists to get rid of so-called background noise
produced by everything else but the axions.
The
search is on
Other axion
searches are ground-based.
The CERN
Axion Solar Telescope in Geneva aims to detect axions from the sun's super-hot
core. Hypothetically, the axions should hit the telescope's superconducting
magnet (and associated magnetic field). The axions would transform back into
photons due to the magnetic field. An X-ray detector would then pick up the
X-ray signal of these photons.
And in the
Axion Dark Matter Experiment at Lawrence Livermore National Laboratory in California, astrophysicist Karl van Bibber and his colleagues hope to create their own
axions. They are manufacturing intense magnetic fields in the hopes of
detecting microwave signals of an axion decaying into a single, real photon.
"The
axion is so light that it doesn't decay into two photons in free space.
However, you can play a very remarkable trick," van Bibber said. "If
I shoot a photon into a magnetic field (which you can think of as a sea of
virtual photons), a real photon and a virtual photon [interact] to make an
axion and vice versa."
Pending
award
Whether
it's made of axions or WIMPs, or something else, the invisible stuff seems
to be everywhere.
"Everyone
in the business agrees that there is an unknown particle that is the dark
matter of both the universe and of our galaxy," Tanner said.
"Galaxies have a halo of dark matter, so their mass is much greater than
the mass of the luminous stars in them."
If Hudson's
or another team were to reel in axions, the announcement that they are the dark
matter particle would not immediately follow. For one, evidence that axions
exist would not exclude the existence of WIMPs, van Bibber said.
"It
might be that we live in a universe that is kind of a cocktail, that it might
be 90 percent WIMPs and 10 percent axions, or fifty-fifty or something like
that," van Bibber said.
That's not
ideal, of course. "Nobody knows and nobody is so dogmatic as to say that
my type of dark matter is the only type," Tanner said. "One hopes
there aren't thousands of types of dark matter because that makes the problem
very messy."
Love
affair
Even with
the odds of detecting
dark matter not in their favor, astrophysicists maintain an unwavering
optimism.
"I
think if one were not optimistic, one would probably throw in the towel. I think
I'm an optimist about that. I think I expect someday the phone will ring and
somebody will say 'Did you hear that so and so found the axion?'" van
Bibber said during a telephone interview. "And they will do it in a clever
way that we hadn't thought about. I'm expecting to be surprised, because it
will be found in a pleasantly unsuspected way."
He
describes his search for the axion as one involving romance. "Personally,
it's like any love affair, you kind of get smitten," he said. "I've
been smitten for about 20 years now."
For Hudson, solar axions hold another prize, a window inside the sun.
"It
would be revolutionary for solar and stellar physics to be able to make use of
the axions, if real, to see inside the sun," Hudson said, "and also
to study the coronal magnetic field via the conversion process [of axions to
photons]." Axions could help astrophysicists to make more accurate
measurements of the temperature of the sun's core, for instance.
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