Axion dark matter might convert into radio frequency electromagnetic radiation when it comes too close to the strong magnetic fields churning around neutron stars — and thus be detectable to high-precision astronomical instruments, according to a recent study published in the journal Physical Review Letters.
In other words, scientists created a new method for monitoring and confirming the existence of dark matter, and while they didn't find it right away, the search is far from over.
Scientists tested new method of dark matter detection
If this happens, the radio signal would be identifiable as an ultra-narrow spectral peak — at a frequency dependent on the mass of the axion dark matter particle behind it, and might be observable via high-precision astronomical instruments.
Researchers from the University of Illinois at Urbana-Champaign, the University of Michigan, and other institutes globally have completed a search for traces of this axion dark matter conversion — using data amassed with the help of two powerful telescopes: the Effelsberg Telescope, and the Green Bank Telescope (GBT).
The researchers' study came on the heels of earlier efforts along with theoretical predictions, the last one of which was cited in a 2018 paper.
'Some skepticism' for new dark matter detection method
"The idea proposed in our earlier work and fleshed out in many subsequent publications from throughout the community, is that axion dark matter may convert to narrow-band radio emission in the strong magnetic fields surrounding neutron stars," said one of the researchers involved in the study Benjamin R. Safdi, to Phys.org. "However, these older works are purely theoretical and involve speculations about how a signal might actually be found in the presence of noisy real-world telescope data."
"Understandably, there is some skepticism about the feasibility of such a search," explained Safdi, to Phys.org.
Detection expected as 'excess power in one frequency channel'
To begin their search, Safdi and his colleagues collected a large body of relevant data initially gathered via radio telescopes. This was done with the Effelsberg Radio Telescope and the GBT — two of the largest radio telescopes in the world based in Ahr Hills (Germany) and West Virginia, respectively.
The researchers directed the telescopes to a plenum of targets in the Milky Way, along with other, nearby galaxies. Some observed neutron stars aren't very far from the sun, while others persist in patches of space abundant with neutron stars (near the galaxy's center).
The team then recorded the power the telescope monitored throughout a spectrum of different frequencies. Signals linked to the conversion of axion dark matter would create excess power in a single frequency channel, according to their theory.
Desired 'signal' often drowned in Earth-bound radio noise
"We then developed and implemented novel and sophisticated data-taking and analysis techniques in order to separate a putative axion signal from confounding backgrounds," said Safdi. "Our search is very much like looking for a needle in a haystack, in the sense that we collect power across millions of different 'frequency channels,' but the axion is only expected to contribute excess power in one of these channels, and we do not currently know which one."
One of the challenges the team faced in hunting for axion dark matter conversion signatures via radio telescope data involves the possibility of misleading signals. Terrestrial background noise — from microwave ovens, radio communications, and other Earth-bound equipment — along with signals from other astrophysical phenomena might end up mischaracterized as signals linked to the conversion of axion dark matter in the magnetospheres of neutron stars.
'No evidence' of axion dark matter present in data, this time
To avoid this possible pitfall, Safdi and his colleagues used several strategies. For example, since real-world axion dark matter conversion signals should only be monitored in the region under the observation of the telescope, whenever the research team suspected Earth-bound signals might "show up" in the relevant area under observation, the team rapidly and continuously switched the telescope from "off source" to "on source" locations — without pulling it away from blank patches of sky.
"We also implemented sophisticated data analysis techniques to filter and 'learn' the properties of the background from the data itself," said Safdi. "Combining all of these techniques together, we were able to collect and analyze data and conclude, conclusively, that no evidence for axions is present in the data," he added.
Tapping into greater magnitudes, sensitivity could enhance dark matter search
"This was a non-trivial task, but this means that we now have developed and demonstrated an observation and analysis framework that can be used in future studies. This, to me, is the main significance of the paper," said Safdi.
So while the intrepid team found "no joy" in their hunt for indirect evidence of axion dark matter conversion — in pursuing that goal, they became creative enough to invent a new means of detecting one of the "holy grails" of physics, one they intend to use at greater levels of magnitude and sensitivity. With a little luck, they might even find it.