The true nature of dark matter may be revealed in a supernova

The nature of dark matter has puzzled astronomers for 90 years since discovering that 85% of the universe’s matter is invisible through our telescopes. 

Currently, the most promising candidate for dark matter is the axion, a lightweight particle that scientists worldwide are fervently trying to detect.

Astrophysicists at the University of California, Berkeley, now suggest that axions could be discovered within seconds of detecting gamma rays from a nearby supernova explosion. 

If axions exist, they would be produced in large quantities during the first 10 seconds after a massive star collapses into a neutron star. These axions would escape and transform into high-energy gamma rays within the star’s intense magnetic field.

Detecting gamma rays from supernova explosions

Detecting these gamma rays today is possible only if the Fermi Gamma-ray Space Telescope – the sole gamma-ray telescope currently in orbit – is pointing in the direction of the supernova at the time it explodes. Given the telescope’s field of view, there’s about a one-in-ten chance of this happening.

Yet, a single detection of gamma rays would pinpoint the mass of the axion, particularly the so-called QCD axion, across a vast range of theoretical masses, including those currently being investigated in Earth-based experiments. 

Conversely, the lack of a detection would eliminate a large range of potential masses for the axion, rendering many current dark matter searches irrelevant.

Searching in our cosmic neighborhood

The issue is that, for the gamma rays to be bright enough to detect, the supernova must be nearby – within our Milky Way galaxy or one of its satellite galaxies. 

Nearby stars explode, on average, only every few decades. The last nearby supernova was in 1987 in the Large Magellanic Cloud, a satellite of the Milky Way. 

At that time, a now-defunct gamma-ray telescope, the Solar Maximum Mission, was pointing in the supernova’s direction but wasn’t sensitive enough to detect the predicted intensity of gamma rays, according to the UC Berkeley team’s analysis.

Seeking the true nature of dark matter

Benjamin Safdi is an associate professor of physics at UC Berkeley and senior author of the study.

“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,” explained Professor Safdi.

The researchers are concerned that when the long-overdue supernova occurs in the nearby universe, we won’t have the necessary instruments to observe the gamma rays produced by axions.

A fleet of gamma-ray telescopes

In discussions with colleagues who build gamma-ray telescopes, the experts are exploring the feasibility of launching one – or a fleet – of such telescopes to cover 100% of the sky 24/7, ensuring any gamma-ray burst would be detected.

The researchers have even proposed a name for their full-sky gamma-ray satellite constellation: the GALactic AXion Instrument for Supernova, or GALAXIS.

“I think all of us on this paper are stressed about there being a next supernova before we have the right instrumentation,” noted Safdi. “It would be a real shame if a supernova went off tomorrow and we missed an opportunity to detect the axion – it might not come back for another 50 years.”

The search for dark matter

Initial searches for dark matter focused on faint, massive compact halo objects (MACHOs) theorized to be sprinkled throughout our galaxy and the cosmos. 

When these didn’t materialize, physicists shifted their attention to elementary particles that should be detectable in Earth-bound labs. Weakly interacting massive particles (WIMPs) also failed to show up. 

In the quest to find the true nature of dark matter, the best candidate is the axion – a particle that fits well within the standard model of physics and solves several outstanding puzzles in particle physics.

“It seems almost impossible to have a consistent theory of gravity combined with quantum mechanics that does not have particles like the axion,” Safdi said.

Strongest candidate for an axion

Axions also naturally emerge from string theory – a hypothesis about the universe’s underlying geometry – and might help unify gravity, which explains interactions on cosmic scales, with quantum mechanics, which describes the infinitesimal. 

The strongest candidate for an axion is the QCD axion, named after quantum chromodynamics, the prevailing theory of the strong force. 

Theoretically, QCD axions interact weakly with all matter through the four fundamental forces: gravity, electromagnetism, the strong force, and the weak force. 

One consequence is that, in a strong magnetic field, an axion should occasionally transform into an electromagnetic wave, or photon. This distinguishes axions from neutrinos, which only interact through gravity and the weak force and ignore electromagnetism.

Neutron stars and axion detection

Laboratory experiments – such as the ALPHA Consortium (Axion Longitudinal Plasma Haloscope), DMradio, and ABRACADABRA, all involving UC Berkeley researchers – use compact cavities that resonate with and amplify the faint electromagnetic field or photon produced when a low-mass axion transforms in the presence of a strong magnetic field.

Astrophysicists have also proposed looking for axions produced inside neutron stars immediately after a core-collapse supernova, like 1987A. Until now, they’ve primarily focused on detecting gamma rays from these axions’ slow transformation into photons in the magnetic fields of galaxies.

Safdi and his colleagues realized that this process is not very efficient at producing gamma rays detectable from Earth.

Instead, they explored the production of gamma rays by axions in the strong magnetic fields around the very star that generated the axions. Supercomputer simulations showed that this process very efficiently creates a burst of gamma rays dependent on the axion’s mass. 

Burst of neutrinos from a hot star

This burst should occur simultaneously with a burst of neutrinos from inside the hot neutron star. However, the burst of axions lasts a mere 10 seconds after the neutron star forms – after that, the production rate drops dramatically – though it happens hours before the outer layers of the star explode.

“This has really led us to thinking about neutron stars as optimal targets for searching for axions as axion laboratories,” Safdi said. “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.”

Advancing the search for axions

Two years ago, Safdi and his colleagues established the best upper limit on the mass of the QCD axion at about 16 million electron volts, or about 32 times less than the mass of the electron. This was based on the cooling rate of neutron stars, which would cool faster if axions were produced along with neutrinos inside these hot, dense objects.

In the current paper, the UC Berkeley team not only describes the production of gamma rays following a core collapse to a neutron star but also uses the non-detection of gamma rays from the 1987A supernova to set the best constraints yet on the mass of axion-like particles, which differ from QCD axions in that they do not interact via the strong force.

They predict that a gamma-ray detection would allow them to identify the QCD axion mass if it is above 50 microelectron volts (μeV), or about one ten-billionth the mass of the electron. A single detection could refocus existing experiments to confirm the mass of the axion, Safdi said. 

While a fleet of dedicated gamma-ray telescopes is the best option for detecting gamma rays from a nearby supernova, a fortunate observation with Fermi would be even better.

“The best-case scenario for axions is Fermi catches a supernova. It’s just that the chance of that is small,” Safdi said. “But if Fermi saw it, we’d be able to 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 we’d be incredibly confident in the signal because there’s no ordinary matter which could create such an event.”

Uncovering the true nature of dark matter

The researchers are anxious about missing a rare opportunity to detect axions due to inadequate instrumentation. They will continue to discuss the feasibility of launching new gamma-ray telescopes to ensure continuous sky coverage.

Safdi’s team believes that capturing gamma rays from a nearby supernova could not only confirm the existence of axions but also provide detailed information about their properties. This could revolutionize our understanding of dark matter and the fundamental forces of nature.

The quest to uncover the true nature of dark matter continues to drive innovation in astrophysics. By focusing on gamma rays emitted during supernova explosions, scientists hope to finally detect axions and solve one of the most enduring mysteries in physics. 

The stakes are high, and time is of the essence. With the potential for a nearby supernova to occur at any moment, researchers are eager to have the right tools in place to seize this fleeting opportunity.

The study is published in the journal Physical Review Letters.

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