An intriguing hint of a certain type of gamma-ray light at the center of the Milky Way might be a product of elusive dark matter — or it might not be. For the past several years, scientists have debated whether the light is really there, and what it means. Now, researchers are petitioning the management team of NASA's Fermi Gamma-Ray Space Telescope, the observatory that saw the light, to change its observing strategy to determine once and for all whether the signal really exists.
However, even if there are extra gamma-ray photons coming from the center of the galaxy, scientists are a ways from knowing whether the photons were made by dark matter.
Theories suggest some mysterious form of matter that can't be seen or touched is rife throughout the universe, making its presence known only through its gravitational pull. The leading theory behind this dark matter posits that it's made of a new kind of fundamental particle called a WIMP (weakly interacting massive particle). [Graphic: Dark Matter Explained]
Because WIMPs are thought to be their own antiparticle (antimatter is a mirror version of normal matter that annihilates ordinary particles when it meets them), if two WIMPs were to collide, they would destroy each other on the spot. These explosions, which should be more common toward the center of the galaxy where dark matter would be densest, would likely create new particles that would give rise to gamma-ray photons of a precise energy. That light is what Fermi might have seen.
"It's pretty ambiguous — it could be a statistical fluke, it could be a systematic effect or it could be a true signal," said Christoph Weniger, an astrophysicist at the University of Amsterdam in the Netherlands. "Right now, there are signs of all three."
Weniger is lead author of a recent white paper suggesting the Fermi telescope spend more time looking toward the center of the Milky Way in search of this feature. The paper was submitted in response to a call for alternative Fermi observing strategies by the telescope's project scientist Julie McEnery, an astrophysicist at NASA's Goddard Space Flight Center in Greenbelt, Md.
A new strategy
Fermi was launched in June 2008, and has been surveying the entire sky evenly since then. Although one of its goals is to learn more about dark matter, the observatory is used for many areas of research, including spinning stars called pulsars and glowing supermassive black holes in other galaxies, both of which emit gamma-ray light.
Weniger's proposal recommends that Fermi observe the center of the galaxy whenever it is visible, which would more than double the rate at which it collects data from this part of the sky. However, the intent is not to divert too much time away from other projects.
"We are very concerned about having a negative impact on other people's science projects," said Harvard University astrophysicist Doug Finkbeiner, a co-author of the white paper. "We're just really trying to do what's right for the project."
Fermi is funded to continue operating through at least 2016, potentially offering plenty of time to settle the question of the galactic center light.
"We think if we started a new observing strategy immediately, we could have the answer by 2015," Finkbeiner told SPACE.com.
Armed with more data from the center of the galaxy, the scientists hope to determine if there really is an excess of gamma-ray light in the particular energy range — 130 gigaelectron volts (GeV) — that Fermi has seen hints of so far.