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.
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."
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."
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.