January 21, 2013
NASA’s Fermi Telescope Turns Its Lens To Search For Dark Matter
John P. Millis, Ph.D. for redOrbit.com — Your Universe Online
For much of the last century, scientists have been searching for a mysterious substance thought to make up about 85 percent of the total mass of the Universe. Without it, galactic rotation curves, galaxy interactions and the very structure of the Universe are inconsistent with our knowledge of physics. Yet, this so-called dark matter remains elusive.
One reason researchers have had such a difficult time finding and measuring dark matter is because, by its very definition, it neither emits nor absorbs light and does not interact with the electromagnetic field. Since light does not reflect off of its surface we can only measure it indirectly as it interacts with other matter.
But evidence of its existence abounds, and scientists can predict with some accuracy how it will lead galaxies and galaxy clusters to interact. But such observations have not yet yielded any clues into what particle or particles constitute this missing mass.
Detecting Dark Matter
Currently, experiments such as the XENON detector seek to measure dark matter interactions directly. As dark matter particles stream through Earth they could possible interact with heavy nuclei. The resulting excitation would create a signature flash of light that these detectors can isolate. However, these research efforts are still in relative infancy and, to date, no statistically significant detections have been noted.
There exists, however, another promising method to detect dark matter interactions. Leading models of dark matter suggest that they might be Majorana particles, which are characterized by their unique property of being self-annihilating — that is, they are their own anti-particle.
This property is actually common among bosons (such as photons of light), but no fermions (like electrons, protons, etc.) are known to be their own anti-particle. If this is true, and dark matter is the first such example, then a new method of detecting dark matter interactions exists.
During the process of annihilation, dark matter would convert to pure energy in the form of gamma-radiation. This dark matter signature could then be detected on Earth using space-based and ground based detectors.
In general, sources of gamma-rays are easy to identify, as only a few mechanisms — active galaxies driven by supermassive black holes, pulsars, supernovae, etc. — can produce such high energy photons. So by looking at regions of the Universe that lack these sources, researchers may have an opportunity to see dark matter annihilations.
One candidate region lies at the center of our Milky Way galaxy, where dark matter is expected to be present in significant quantities and the region is well understood in terms of traditional gamma-ray sources. Another option is nearby dwarf galaxies. These objects should be rich with dark matter, but are decidedly lacking other mechanisms that produce gamma-ray signatures.
Ground-based gamma-ray detectors, such as VERITAS and HESS, have searched these regions but have yet to announce any positive detections. But forthcoming upgrades to these instruments will increase their sensitivity to ensure they are able to find even the weakest signals, should they exist.
Dark Matter Hunting With Fermi
In the meantime, an opportunity awaits. NASA´s Fermi Gamma-ray Space Telescope, the most advanced space-based gamma-ray mission ever launched is turning its eye to the problem of dark matter.
Last year two measurements by the orbiting observatory indicated “bumps” in the data, suggesting dark matter annihilations may have been detected near our galactic center. But, as is the case with all such experiments, a large amount of data is needed to create statistics convincing enough to gain general acceptance by the research community.
The results from these studies lacked a high enough statistical significance to rule out the possibility that the “bump” was simply an anomaly instead of a measure of actual dark matter interactions. Nonetheless, the results were strong enough to pique the interest of other astronomers and physicists.
Now, Fermi scientists are thinking outside the box and imagining novel ways to approach the problem; and they are soliciting ideas from the broader scientific communities. In the near future, Fermi may execute a plan to search for these dark matter interactions in earnest.
The problem of dark matter is a complicated one but the advancement of observatories such as Fermi, HESS and VERITAS should hopefully begin to bring some answers in the coming years.