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 useless 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 might contribute to the universe’s darkish matter content material.
New analysis, published earlier this month in Bodily Evaluate Letters, places constraints on how axion-like particles would possibly 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 common class of hypothetical darkish matter candidates than axions, and scientists consider their nature could possibly 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 workforce 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 singular alternative the place you might get the photon sign,” mentioned Bhupal Dev, a physicist at Washington College in St. Louis and lead creator of the examine, in a cellphone name with Gizmodo. “We may make the most of this multimessenger examine, this knowledge, to probe some new physics past the Customary Mannequin.”
Darkish matter appears to constitute 27% of the universe, but it surely interacts so weakly with atypical 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 Huge Particles (WIMPs), hidden (or darkish) photons, huge compact halo objects (MACHOs), and, in fact, 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 pictures.
Neutron stars are a number of the densest objects within the universe, crushed solely by black holes. Not like black holes, gentle can escape neutron stars, making them observable on the electromagnetic spectrum.
Dev explains that axions may come up from neutron star mergers in a few methods, if axions certainly couple to photons. By means of photon coalescence, axions would emerge from photons coming collectively within the intensely scorching astrophysical atmosphere and fusing. The opposite approach axions may come up is thru the Primakoff course of, during which a photon interacts with a shower of electrons, producing axions.
The axion, because it’s proposed, is so small that it could typically behave extra like a wave than a particle, that 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 quite a lot of 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 will analyze each the timing info and we are able to additionally analyze the spectral options. And that’s the place we are able to disentangle these sorts of recent physics indicators from the usual astrophysical processes.”
Earth-based experiments are additionally working to slim the potential mass ranges of the axion. LUX-Zeplin, XENON-1T, and the ALPS II experiment, which started operations in Could 2023, are all designed to hunt out axions deep underground. However there are additionally different tasks, 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 “provides some new constraints on the axion-like particles, as a result of thus far we didn’t see any sign of axions,” Dev mentioned. “It additionally provides us hope that sooner or later, utilizing these astrophysical observations, we may achieve extra perception into axion-like particles. And this might be complementary to the laboratory searches which might be happening.”
The hunt for axions is rather a lot like utilizing a metallic detector on a really, very massive seashore. As a rule, physicists and astronomers are detecting nothing. However looking out the total vary of potential lots for axions and axion-like particles is one of the best ways to finally observe them down.
Extra: What Is Dark Matter and Why Hasn’t Anyone Found It Yet?
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