Could a Nearby Supernova Hold the Key to Solving the Dark Matter Mystery?
The Mystery of Dark Matter
The quest to uncover the universe's dark matter could reach its conclusion as early as tomorrow—if a nearby supernova offers the right conditions. For 90 years, astronomers have been puzzled by the elusive nature of dark matter, which constitutes 85% of the universe's mass yet remains invisible to telescopes. Current research focuses on the axion, a lightweight particle widely regarded as the leading candidate.
The Role of Axions in Dark Matter Research
Identifying Axions Through Supernova Gamma Rays
Researchers at the University of California, Berkeley, propose that axions could be identified almost instantly following the detection of gamma rays from a nearby supernova. If axions exist, they would be generated in vast numbers during the first 10 seconds of a massive star's collapse into a neutron star. These particles would then escape and convert into high-energy gamma rays within the star's powerful magnetic field.
The Challenge of Detecting Gamma Rays
At present, such a detection can occur only if the Fermi Gamma-ray Space Telescope, the only operational gamma-ray telescope in orbit, is oriented toward the supernova at the moment of its explosion. With the telescope's restricted field of view, the probability of this alignment is approximately 10%.
Implications of Gamma-Ray Detection
Determining the Mass of the QCD Axion
A single gamma-ray detection could precisely determine the mass of the axion, specifically the QCD axion, across a vast range of theoretical possibilities, including those under investigation in Earth-based experiments. Conversely, the absence of such a detection would rule out a significant portion of potential mass ranges, rendering many current dark matter searches obsolete.
The Need for a Nearby Supernova
The challenge lies in the fact that gamma rays must be sufficiently bright to detect, which requires the supernova to occur nearby—within the Milky Way or its satellite galaxies. However, such events are rare, with nearby stars exploding only every few decades. The most recent occurrence was in 1987, when a supernova erupted in the Large Magellanic Cloud, a satellite galaxy of the Milky Way. Although the Solar Maximum Mission gamma-ray telescope was directed toward this supernova, its sensitivity was insufficient to detect the predicted gamma-ray intensity, as analyzed by the UC Berkeley team.
"A modern gamma-ray telescope observing a supernova like 1987A could confirm or eliminate the existence of the QCD axion—a fascinating theoretical particle—over a vast range of parameter space," said Benjamin Safdi, UC Berkeley associate professor of physics and senior author of a paper published on November 19 in Physical Review Letters. "This includes nearly all parameter regions unreachable by laboratory methods and a considerable portion that laboratories are currently investigating, and it would happen within 10 seconds."
The Proposed GALAXIS Satellite Network
The Urgency of Detecting Axions
The researchers express concern that when the long-anticipated supernova occurs in the nearby universe, the opportunity to observe gamma rays produced by axions might be missed. To address this, they are collaborating with colleagues involved in gamma-ray telescope development to explore the possibility of deploying a fleet of satellites capable of monitoring the entire sky continuously. They have even proposed a name for this ambitious constellation of full-sky gamma-ray satellites: the GALactic Axion Instrument for Supernova, or GALAXIS.
The Risks of Missing the Opportunity
"We're all deeply concerned about the possibility of the next supernova occurring before the necessary instruments are in place," said Safdi. "Missing the chance to detect axions due to a supernova happening tomorrow would be a profound loss—such an opportunity may not arise again for another 50 years."
Safdi's co-researchers are graduate student Yujin Park and postdoctoral scholars Claudio Andrea Manzari and Inbar Savoray. Together, they are part of UC Berkeley's physics department and the Theoretical Physics Grou p at Lawrence Berkeley National Laboratory.
Understanding the QCD Axion
Dark Matter and the Search for Axions
Dark matter searches began by focusing on faint, massive compact halo object (MACHOs), which were expected to be spread throughout our galaxy and beyond. When MACHOs couldn't be detected, attention turned to theoretical elementary particles that should be ubiquitous and detectable in lab experiments on Earth. Yet, the hunt for weakly interacting massive particles (WIMPs) also yielded no results.
Axions and Their Role in Physics
The axion is currently the leading candidate for dark matter, a particle that seamlessly integrates into the standard model of physics while addressing several unresolved issues in particle physics.
Additionally, axions arise naturally from string theory, a theoretical framework for the universe's fundamental geometry, and could potentially reconcile gravity, which governs large-scale interactions, with quantum mechanics, which governs the microscopic.
According to Safdi, it appears nearly impossible to formulate a consistent theory that integrates gravity with quantum mechanics without including particles such as the axion.
The leading candidate for an axion, known as the QCD axion—so named after the prevailing theory of the strong face, quantum chromodynamics—hypothetically interacts with all forms of matter, albeit weakly, through the four fundamental forces: gravity, electromagnetism, the strong force (which binds atoms), and the weak force (which governs atomic decay).
One consequence is that in a powerful magnetic field, an axion should occasionally transform into an electromagnetic wave, or photon. This contrasts with the neutrino, a similarly lightweight, weakly-interacting particle, which only engages with gravity and the weak force, entirely disregarding the electromagnetic force.
Experimental Efforts and Simulations
Axion Detection in Laboratory Experiments
Experiments in laboratory settings, such as the ALPHA Consortium (Axion Longitudinal Plasma HAloscope), DMradion, and ABRACADABRA, all involving UC Berkeley researchers, utilize compact cavities that resonate like a tuning fork, enhancing the faint electromagnetic field or photon produced when a low-mass axion undergoes transformation in the presence of a strong magnetic field.
Supercomputer Simulations and Neutron Stars
Rather than focusing on distant galactic magnetic fields, the researchers investigated the generation of gamma rays by axions within the strong magnetic fields surrounding the star that produced them. Supercomputer simulations revealed that this process efficiently generates a burst of gamma rays, with intensity linked to the mass of the axion.
Neutron Stars as Axion Laboratories
The Role of Neutron Stars and Magnetars
Gamma ray burst should coincide with a burst of neutrinos from within the neutron star. However, this axion burst only lasts for 10 seconds following the formation of the neutron star, after which the production rate rapidly decreases, hours before the outer layers of the star begin to explode.
"This has prompted us to consider neutron stars as ideal locations for axion searches, effectively turning them into 'axion laboratories,'" Safdi explained. "Neutron stars offer many advantages. Not only are they incredibly hot, but they also possess some of the strongest magnetic fields in the universe. Magnetars, a type of neutron star, for instance, generate magnetic fields that are tens of billions of times stronger than anything we can produce in a laboratory. These intense magnetic fields help convert axions into detectable signals."
Setting Limits on Axion Mass
Two years ago, Safdi and his team established the most precise upper limit on the mass of the QCD axion, pegging it at roughly 16 million electron volts, which is about 32 times smaller than the electron's mass. This conclusion was drawn from analyzing the cooling rate of neutron stars, which would accelerate if axions were produced alongside neutrinos within these dense celestial objects.
The Future of Axion Research
Predictions and Opportunities for Gamma-Ray Detection
In their latest publication, the UC Berkeley team details the generation of gamma rays following a core collapse into a neutron star. Additionally, they utilize the lack of gamma ray detection from the 1987A supernova to set the most stringent constraints on the mass of axion-like particles, which differ from QCD axions in their lack of interaction with the strong force.
The researchers predict that detecting gamma rays enable them to determine the mass of the QCD axion, provided it exceeds 50 microelectron volts (μeV), roughly one 10-billionth of the electron's mass. A single observation could redirect ongoing experiments to verify the axion's mass, according to Safdi. Although a network of dedicated gamma-ray telescopes is the optimal solution for detecting gamma rays from a nearby supernova, a fortunate event with Fermi would be an even more advantageous scenario.
The Role of Fermi in Axion Research
The optimal scenario for detecting axions would be for Fermi to capture a supernova. However, Safdi notes that the probability of this hap pening is low. "But if Fermi were to observe it, we would be able to measure the axion's mass and interaction strength, providing us with the critical information about the axion. The signal would be highly reliable, as no ordinary matter could produce such an event," he said.
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