Although dark matter's exact nature is still unknown, what we do know is that the amount of gravity in the universe is greater than the amount of visible matter that it corresponds to. This anomaly could be explained by some unseen source of extra mass, which provides the additional gravity that helps hold galaxies together. And the missing matter behind this extra mass has been dubbed dark matter.
Just as we surmised the existence of dark matter by detecting an abnormally large amount of gravity, an astrophysical instrument has detected an unusually large amount of radio waves coming from beyond this galaxy – and a new study is attributing those to dark matter as well, opening the door to new methods of dark matter detection.
Top image: Hubble Space Telescope/NASA.
NASA's Absolute Radiometer for Cosmology, Astrophysics and Diffusion Emission (ARCADE 2) is an airborne instrument that records, among other things, radio waves that originated outside this galaxy. Most of the signals can be linked to known extragalactic sources. But the ARCADE 2 team also discovered that known sources fail to account for an excess of radio waves in the frequency range between 3 and 90 gigahertz.
"The simplest explanation of such excess involves a ‘new' population of unresolved sources which become the most numerous at very low (observationally unreached) brightness," write the authors of a paper in the journal Physical Review Letters. They propose that the "unresolved sources" of these additional emissions could be a hypothetical candidate for dark matter called weakly interacting massive particles, or WIMPs.
How are WIMPs related to dark matter? As mentioned previously, the true nature of dark matter is unknown. Even astrophysicist Neil DeGrasse Tyson, in his recent profile, is quick to point out how little we know about it:
"But really we have no idea what's causing it. We so don't know what's causing it that we shouldn't even call it dark matter because that implies we have some understanding that it's matter. We don't know what it is. I could call it Fred. Eighty five percent all the gravity in the universe comes from something about which we know nothing…It's been with us since 1936 and it's one of the longest-standing unsolved problems in astrophysics."
Of the possible candidates for
dark matter, WIMPs are one of the most popular. We would expect a WIMP to be a relatively heavy subatomic particle, perhaps 40 times as massive as a proton, which rarely interacts with normal matter and then only weakly. (For example, once in a blue moon a WIMP might hit an atomic nucleus and make it vibrate slightly, and there are detectors looking for just that sort of interaction.) But we still aren't completely sure, since we have never actually detected a WIMP.
Nonetheless, based on what a WIMP should look like, the research team created a model for the radiation emitted by WIMPs – or rather, the radiation emitted by the secondary particles that are created every time that WIMPs decay or annihilate. They found that the expected WIMP emission closely matched the extra radio waves from the ARCADE 2 measurements.
Although the WIMPs could account for the unusual extragalactic radio signal, that does not necessarily mean that they are the true source. More research is required to even prove that the massive particles exist. But if they do produce these radio waves, it could help astrophysicists detect WIMPs without relying on gravity measurements, helping to reveal more about the true nature of dark matter.
Via the American Physical Society