The search for the universe’s dark matter could end tomorrow—given a nearby supernova and a little luck.
Astrophysicists at the University of California, Berkeley, now argue that the axion could be discovered within seconds of the detection of gamma rays from a nearby supernova explosion.
The researchers are anxious, however, that when the long-overdue supernova pops off in the nearby universe, we won’t be ready to see the gamma rays produced by axions.
Safdi and his colleagues realized that that process is not very efficient at producing gamma rays, or at least not enough to detect from Earth.
While a fleet of dedicated gamma-ray telescopes is the best option for detecting gamma rays from a nearby supernova, a lucky break with Fermi would be even better.
With luck and a nearby supernova, the hunt for the universe’s dark matter may come to an end tomorrow. For ninety years, astronomers have been trying to figure out what dark matter is since they discovered that eighty-five percent of the universe’s matter is invisible to the naked eye. A lightweight particle that scientists worldwide are frantically searching for, the axion is currently the most likely dark matter candidate.
Today, astronomers at the University of California, Berkeley, claim that the axion might be found seconds after gamma rays from a nearby supernova explosion were detected. If axions are present, they would be created in large amounts in the first ten seconds following a massive star’s core collapse into a neutron star. In the star’s strong magnetic field, the axions would escape and change into high-energy gamma rays.
Only if the Fermi Gamma-ray Space Telescope, the only gamma-ray telescope in orbit, is pointing in the supernova’s direction at the moment of explosion is such a detection feasible today. About one in ten, considering the telescope’s field of view.
The mass of the axion, specifically the so-called QCD axion, could be determined with a single gamma-ray detection, however, across a vast range of theoretical masses, including those currently being investigated in Earthly experiments. However, the absence of a detection would rule out a wide range of possible axion masses and render the majority of ongoing dark matter searches pointless.
However, the supernova must be close by—in our Milky Way galaxy or one of its satellite galaxies—for the gamma rays to be bright enough to detect, and nearby stars only explode on average every few decades. The Large Magellanic Cloud, a satellite of the Milky Way, saw its most recent close-by supernova in 1987. Although the Solar Maximum Mission, a now-defunct gamma-ray telescope, was pointing in the direction of the supernova at the time, the UC Berkeley team’s analysis revealed that it was not sensitive enough to detect the expected intensity of gamma rays.
Benjamin Safdi, an associate professor of physics at UC Berkeley and the senior author of a paper published online in November, stated that “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.”. published in Physical Review Letters on page 19. And everything would take place in ten seconds. “..”.
Researchers fear, however, that we will not be prepared to witness the gamma rays from axions when the long-awaited supernova explodes in the nearby universe. The researchers are currently discussing the viability of launching one or a fleet of gamma-ray telescopes to cover the entire sky around-the-clock and ensure that they will detect any gamma-ray burst with colleagues who construct such telescopes. Even the GALactic AXion Instrument for Supernova, or GALAXIS, is the name they have suggested for their constellation of full-sky gamma-ray satellites.
As Safdi put it, “I think everyone on this paper is worried that a supernova will occur before we have the proper equipment.”. We might not be able to detect the axion for another 50 years, so it would be a great shame if a supernova went off tomorrow and we missed the chance. “.”.
Co-authors of Safdi’s work include postdoctoral fellows Claudio Andrea Manzari and Inbar Savoray, as well as graduate student Yujin Park. All four belong to the Theoretical Physics Group at Lawrence Berkeley National Laboratory and the physics department at UC Berkeley.
Axions of QCD.
When faint, massive compact halo objects (MACHOs), which are theoretically scattered throughout our galaxy and the universe, failed to appear, physicists turned their attention to searching for elementary particles, which are theoretically ubiquitous and should be detectable in labs located on Earth. Moreover, these weakly interacting massive particles (WIMPs) did not appear.
The axion, a particle that neatly fits into the standard model of physics and resolves a number of other unresolved issues in particle physics, is currently the best candidate for dark matter. Axions are also a good fit for string theory, which postulates the fundamental geometry of the universe. They may also be able to bring together the theories of quantum mechanics, which describes the infinitesimal, and gravity, which describes interactions on cosmic stages.
Safdi stated, “It appears nearly impossible to have a coherent theory of gravity combined with quantum mechanics that does not have particles like the axion.”.
The four forces of nature—gravity, electromagnetism, the strong force that holds atoms together, and the weak force that explains atoms breaking up—theoretically interact with all matter, albeit weakly, through the strongest candidate for an axion, known as a QCD axion, which is named after the dominant theory of the strong force, quantum chromodynamics.
An axion should occasionally transform into an electromagnetic wave, or photon, in a strong magnetic field, as a result. The neutrino, a lightweight, weakly interacting particle that completely ignores the electromagnetic force and only interacts through gravity and the weak force, is very different from the axion.
Compact cavities, which resemble tuning forks, resonate with and amplify the faint electromagnetic field or photon created when a low-mass axion transforms in the presence of a strong magnetic field. This is used in lab bench experiments like ABRACADABRA, DMradio, and the ALPHA Consortium (Axion Longitudinal Plasma HAloscope), all of which involve UC Berkeley researchers.
As an alternative, astrophysicists have suggested searching for axions created inside neutron stars such as 1987A right after a core-collapse supernova. However, they have so far mostly concentrated on detecting gamma rays from these axions’ gradual conversion into photons in galaxies’ magnetic fields. It was discovered by Safdi and his associates that the process was inefficient in generating enough gamma rays for detection from Earth.
Instead, they investigated how axions in the powerful magnetic fields surrounding the star that produced the axions could produce gamma rays. Supercomputer simulations demonstrated how effectively that process produces a gamma-ray burst that is reliant on the axion’s mass. The burst should coincide with a neutrino burst from within the hot neutron star. However, that axion burst occurs just 10 seconds after the neutron star forms, and the production rate significantly decreases thereafter, even though the outer layers of the star explode hours later.
Safdi stated, “This has brought us to consider neutron stars as the best targets for looking for axions as axion laboratories.”. “Neutron stars have many advantages. They are incredibly hot items. They also contain extremely powerful magnetic fields. Magnetars, which are neutron stars, have magnetic fields tens of billions of times stronger than anything we can create in a lab. These stars have the strongest magnetic fields in the universe. As a result, these axions are transformed into detectable signals. “.
Safdi and his associates established the best upper bound on the mass of the QCD axion two years ago at roughly 16 million electron volts, which is roughly 32 times less than the mass of an electron. This was based on the idea that if axions were created inside these hot, compact bodies in addition to neutrinos, the cooling rate of neutron stars would accelerate.
This paper presents the best constraints to date on the mass of axion-like particles, which are different from QCD axions in that they do not interact via the strong force. The UC Berkeley team uses the non-detection of gamma rays from the 1987A supernova to describe the production of gamma rays after core collapse to a neutron star.
They anticipate that if the QCD axion mass is greater than 50 microelectron volts (micro-eV, or μeV), or roughly one 10-billionth the mass of the electron, they could detect it using a gamma ray detector. According to Safdi, a single discovery could refocus current research to validate the axion’s mass. The best way to find gamma rays from a nearby supernova is with a fleet of specialized gamma-ray telescopes, but a lucky break with Fermi would be even better.
