A collision of two terribly dense, collapsed stars within the distant universe is offering potential clues to the axion, a darkish matter candidate first proposed half a century in the past.
The stellar remnants are neutron stars, the corpses that stay after huge stars collapse in on themselves. These lifeless stars are so dense that their electrons collapse onto their protons—therefore, “neutron star.” Their excessive density additionally makes them a venue for unique physics: particularly, they’ve been proposed as a source of axions, a hypothetical particle that would contribute to the universe’s darkish matter content material.
New analysis, published earlier this month in Bodily Overview Letters, places constraints on how axion-like particles may couple with photons, primarily based on spectral and temporal knowledge from a neutron star merger roughly 130 million light-years away.
Axion-like particles (or ALPs) are a extra basic class of hypothetical darkish matter candidates than axions, and scientists imagine their nature may very well be revealed by finding out photons and constraining the mass vary of the particles. The axion-like particles produced within the neutron star merger escape the remnant and decay again into two photons, the group wrote within the paper, producing an electromagnetic sign detectable to telescopes. The information was collected from 2017 observations of the collision taken by the Fermi Massive Space Telescope (Fermi-LAT).
“For a neutron star merger, there’s a novel alternative the place you would get the photon sign,” mentioned Bhupal Dev, a physicist at Washington College in St. Louis and lead writer of the examine, in a cellphone name with Gizmodo. “We might make the most of this multimessenger examine, this knowledge, to probe some new physics past the Commonplace Mannequin.”
Darkish matter appears to constitute 27% of the universe, but it surely interacts so weakly with odd matter that scientists can solely detect it through its gravitational effects on what we can see. Popular dark matter candidates (which is to say, theorized accountable events for darkish matter’s obvious existence) are Weakly Interacting Large Particles (WIMPs), hidden (or darkish) photons, huge compact halo objects (MACHOs), and, after all, axions.
Named for a model of laundry detergent, the axion is a hypothetical particle that was proposed within the Seventies as an answer to physics’ strong-CP problem, which describes the truth that quarks’ adherence to the legal guidelines of physics stays the identical, even when the particles are changed with their mirror photos.
Neutron stars are a few of the densest objects within the universe, overwhelmed solely by black holes. In contrast to black holes, mild can escape neutron stars, making them observable on the electromagnetic spectrum.
Dev explains that axions might come up from neutron star mergers in a few methods, if axions certainly couple to photons. Via photon coalescence, axions would emerge from photons coming collectively within the intensely scorching astrophysical atmosphere and fusing. The opposite means axions might come up is thru the Primakoff course of, through which a photon interacts with a shower of electrons, producing axions.
The axion, because it’s proposed, is so small that it will generally behave extra like a wave than a particle, which means it flees the scene of the crime with relative ease. However the proton is (comparatively) huge, so it takes a second for the particle to emerge from this hotbed of interplay. Particularly, it takes 1.7 seconds: the quantity of delay the researchers noticed between the gravitational wave sign from a neutron star merger and the electromagnetic sign from it.
“We get numerous photons from the sky. So how do we actually know that this photon sign is coming from the axion?” Dev mentioned. “That is coming from a decay of the particle, versus astrophysical processes the place the photons disappear from scattering. So there’s a distinction within the spectrum. We are able to analyze each the timing info and we will additionally analyze the spectral options. And that’s the place we will disentangle these sorts of recent physics indicators from the usual astrophysical processes.”
Earth-based experiments are additionally working to slender the potential mass ranges of the axion. LUX-Zeplin, XENON-1T, and the ALPS II experiment, which started operations in Might 2023, are all designed to hunt out axions deep underground. However there are additionally different initiatives, like ADMX and the Dark Matter Radio Pathfinder, working to constrain the mass vary on hidden (or darkish) photons, one other class of darkish matter candidates. Later generations of the Darkish Matter Radio will hunt axions.
The brand new analysis “offers some new constraints on the axion-like particles, as a result of to this point we didn’t see any sign of axions,” Dev mentioned. “It additionally offers us hope that sooner or later, utilizing these astrophysical observations, we might achieve extra perception into axion-like particles. And this will likely be complementary to the laboratory searches which are occurring.”
The hunt for axions is quite a bit like utilizing a steel detector on a really, very giant seaside. Most of the time, physicists and astronomers are detecting nothing. However looking the complete vary of potential lots for axions and axion-like particles is one of the best ways to ultimately monitor them down.
Extra: What Is Dark Matter and Why Hasn’t Anyone Found It Yet?
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