Astronomers may be on the verge of solving the mystery of dark matter by detecting the elusive axion particle, potentially with the help of a nearby supernova. Dark matter constitutes 85% of the universe’s mass and has remained undetected for almost 90 years.
Researchers at UC Berkeley proposed that axions could be spotted shortly after a supernova’s gamma rays, with the Fermi Gamma-ray Space Telescope having a 1 in 10 chance of observing the event. Axons would be created in the initial moments of a star’s collapse, where they would then transform into high-energy gamma rays in the star’s magnetic field.
A single detection of gamma rays from a nearby supernova could provide crucial information about the mass of the QCD axion, offering insights into a wide range of possible axion masses. This would significantly inform ongoing dark matter research.
However, if no gamma rays are detected, many potential axion masses would be ruled out, rendering some dark matter searches obsolete. The difficulty lies in the rarity of nearby supernovae, as they must occur within our Milky Way or its satellite galaxies to be detectable.
Such events only happen every few decades, and the last nearby supernova in 1987 was too distant. At the time, the gamma-ray telescope lacked the sensitivity needed to detect the expected gamma-ray intensity.
Benjamin Safdi, a UC Berkeley associate professor of physics and senior author of a paper, said, “If we were to see a supernova, like supernova 1987A, with a modern gamma-ray telescope, we would be able to detect or rule out this QCD axion, this most interesting axion, across much of its parameter space — essentially the entire parameter space that cannot be probed in the laboratory, and much of the parameter space that can be probed in the laboratory, too. And it would all happen within 10 seconds.”
Researchers are concerned that they may not have the proper instruments to detect the gamma rays associated with axions when the next supernova occurs nearby. To address this, they are collaborating with colleagues who design gamma-ray telescopes to explore the possibility of launching a fleet of telescopes capable of continuously covering 100% of the sky, ensuring they can detect any gamma-ray bursts.
They have proposed a satellite constellation named GALAXIS (GALactic AXion Instrument for Supernova) for this purpose. Safdi, one of the researchers, expressed anxiety that without the right equipment, the chance to detect axions could be missed if a supernova occurs soon, possibly delaying the opportunity for decades.
Safdi works on this project alongside graduate student Yujin Park and postdocs Claudio Andrea Manzari and Inbar Savoray, all from UC Berkeley and the Lawrence Berkeley National Laboratory.
Searches for dark matter initially focused on MACHOs (massive compact halo objects). Still, when they were not found, attention shifted to weakly interacting massive particles (WIMPs), which also failed to materialize. The leading dark matter candidate is the axion, a particle that aligns with the standard physics model and resolves several unresolved issues in particle physics.
Axions also emerge from string theory, which posits a fundamental structure of the universe and may offer a way to unify gravity (cosmic interactions) with quantum mechanics (micro-level interactions).
Safdi said, “It seems almost impossible to have a consistent theory of gravity combined with quantum mechanics that does not have particles like the axion.”
QCD axion is the strongest candidate for an axion that interacts with all matter, though weakly, through the four forces of nature: gravity, electromagnetism, the strong force, which holds atoms together, and the weak force, which explains the breakup of atoms.
In a strong magnetic field, an axion can occasionally transform into a photon (electromagnetic wave), unlike neutrinos, which only interact through gravity and the weak force and don’t respond to the electromagnetic force. Laboratory experiments, such as the ALPHA Consortium, DMradio, and ABRACADABRA—led by UC Berkeley researchers—use compact cavities that resonate like a tuning fork.
These cavities help amplify the faint electromagnetic signals produced when a low-mass axion transforms in a strong magnetic field.
Astrophysicists have previously focused on detecting gamma rays from axons transforming into photons in the magnetic fields of galaxies. However, Safdi and his colleagues found that this process needs to be more efficient to detect from Earth. Instead, they investigated axion production in the strong magnetic fields around the neutron star that generates them.
Supercomputer simulations revealed that this process produces a burst of gamma rays, highly dependent on the axion’s mass. This burst occurs simultaneously with a burst of neutrinos from the newly-formed neutron star.
However, the axion gamma-ray burst lasts only about 10 seconds after the neutron star forms before production drops off significantly, though it happens hours before the star’s outer layers explode.
Safdi said, “This has led us to think about neutron stars as optimal targets for searching for axions as axion laboratories. Neutron stars have a lot of things going for them. They are extremely hot objects. They also host very strong magnetic fields.”
“The strongest magnetic fields in our universe are found around neutron stars, such as magnetars, which have magnetic fields tens of billions of times stronger than anything we can build in the laboratory. That helps convert these axions into observable signals.”
Two years ago, Safdi and his team set a new upper limit for the mass of the QCD axion at around 16 million electron volts, based on the cooling rate of neutron stars. In their latest work, the UC Berkeley team extends this research by analyzing gamma rays produced during a star’s core collapse into a neutron star.
Using the absence of gamma rays from the 1987A supernova, they provide the best constraints yet on the mass of axion-like particles, which do not interact through the strong force. They predict that detecting gamma rays could reveal the QCD axion mass if it exceeds 50 microelectron volts (μeV), about one ten-billionth the mass of the electron.
Such a discovery could shift the focus of existing experiments, with a potential breakthrough from a nearby supernova or a lucky detection by the Fermi telescope.
Safdi said, “The best-case scenario for axions is Fermi catches a supernova. It’s just that the chance of that is small. But if Fermi saw it, we could measure its mass. We’d be able to measure its interaction strength. We’d be able to determine everything we need to know about the axion and incredibly confident in the signal because no ordinary matter could create such an event.”
Journal Reference:
- Claudio Andrea Manzari, Yujin Park, Benjamin R. Safdi, and Inbar Savoray. Supernova Axions Convert to Gamma Rays in Magnetic Fields of Progenitor Stars. Physical Review Letters. DOI: 10.1103/PhysRevLett.133.211002