A Supernova Could Unlock the Secrets of Dark Matter

The long and winding quest to understand dark matter, which constitutes about 85% of the universe yet remains invisible to our telescopes, may be on the verge of a breakthrough—thanks to a nearby supernova and a stroke of luck. For over 90 years, astronomers have grappled with the enigmatic nature of dark matter, and current research is zeroing in on an exciting lead: the axion. This lightweight particle is viewed as today's prime candidate, and researchers are eagerly hoping for its discovery.

Astrophysicists at the University of California, Berkeley, propose that the detection of gamma rays emitted from a nearby supernova explosion could facilitate the immediate identification of axions. According to their research, if axions exist, they would be produced in large quantities during the first ten seconds following the collapse of a massive star into a neutron star. In the intense magnetic fields surrounding the star, these axions could convert into high-energy gamma rays—an opportunity no scientist wants to miss.

However, there lies a catch. The only gamma-ray telescope currently capable of making such crucial observations is the Fermi Gamma-ray Space Telescope, which must fortuitously be pointed at the supernova at the time of the explosion. The odds are slender, roughly one in ten, given the telescope’s limited field of view.

A successful detection could profoundly alter our understanding of dark matter. A single observation of gamma rays could substantially narrow down the mass of the axion, specifically the so-called Quantum Chromodynamics (QCD) axion, across an extensive range of theoretical masses that are actively being tested in experiments on Earth. Conversely, failure to detect gamma rays would cast doubt on a vast array of potential axion masses, sidelining many current dark matter investigations.

Unfortunately, supernova explosions in our Milky Way or its satellite galaxies are rare events, occurring only once every few decades. The last close encounter was over three decades ago with supernova 1987A in the Large Magellanic Cloud. At the time, the Solar Maximum Mission telescope—now defunct—was observing, but it lacked the sensitivity needed to detect the anticipated gamma rays.

"If we were to witness another supernova like 1987A, we would have the means to either confirm or refute the existence of this fascinating QCD axion across much of its theoretical range—literally within seconds," explained Benjamin Safdi, a UC Berkeley associate professor of physics, in a recent paper published in the journal *Physical Review Letters*.

Concerned about missed opportunities during these celestial events, Safdi and his colleagues are exploring the feasibility of deploying a constellation of gamma-ray telescopes—dubbed the GALactic AXion Instrument for Supernova, or GALAXIS—that could continuously monitor the entire sky for gamma-ray bursts.

The urgency is palpable: "It would be a devastating loss if we missed detecting the axion during the next supernova event. It might take another 50 years for a comparable opportunity to arise," Safdi lamented.

The Quest for the QCD Axion

Initially, searches for dark matter focused on hypothesized faint, massive compact halo objects (MACHOs) littering our galaxy, but these searches yielded no results. Physicists shifted their focus to weakly interacting massive particles (WIMPs), yet these, too, eluded detection. The spotlight now shines on axions, which not only mesh seamlessly with the current framework of physics but also provide solutions to several outstanding questions in particle physics.

Axions emerge naturally from string theory, which aims to unify the fundamental forces of nature, including gravity and quantum mechanics. The leading candidate, the QCD axion, interacts weakly with all matter through various forces—gravitational, electromagnetic, strong, and weak. This unique characteristic makes axions prime candidates for application in several ongoing experimental endeavors.

With existing laboratory experiments, such as the ALPHA Consortium, DMradio, and ABRACADABRA, researchers are implementing innovative techniques to detect axions—often using compact cavities that amplify signals produced when axions transform in the presence of strong magnetic fields. However, Safdi and his team realized that gamma rays generated in the vicinity of the very neutron stars that produce axions provide a more promising approach.

Neutron stars, particularly magnetars with their ultra-strong magnetic fields, act as stellar laboratories for axion research. They have been shown to create a burst of gamma rays, which correlates with a simultaneous burst of neutrinos emanating from the formation of the neutron star. Remarkably, this axion-induced gamma ray flux is brief, lasting only around ten seconds post-neutron star formation.

In their extensive examinations, the UC Berkeley team recently established new upper limit estimates for the mass of QCD axions, reinforcing their pursuit. They predict that any future gamma ray detection could yield valuable insights into the axion's properties.

The continued search for a nearby supernova could not only answer long-standing questions about dark matter but potentially reshape our entire understanding of the universe. The scientific community eagerly awaits its moment under the cosmic spotlight. Stay tuned—the universe might be about to reveal its best-kept secrets!