2 months ago
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Scientists think it's possible that shrouds of hypothetical particles called "axions" could surround extreme, dead stars called "neutron stars." If true, this could bring us one step closer to solving the mystery of dark matter.
That's the conclusion reached by a team of researchers led by Dion Noordhuis of the University of Amsterdam, who previously investigated what would happen to particular dark matter candidates called "axions" that escape neutron stars. These incredibly lightweight particles, which are hypothetical because they have never actually been detected, are a good match for dark matter. That's because, if they exist, they would interact very weakly with "ordinary" matter and light. That's the aspect of dark matter that makes it effectively invisible.
Noordhuis and colleagues have now turned their attention to axions that would have been left behind after this daring escape, finding that these particles could congregate in dense clouds around neutron stars as a result of the dead stars' exotic properties.
"If axion clouds, and thereby axions, are discovered, this would be a massive step toward solving the dark matter problem," Noordhuis told Space.com. "The question is: Why are neutron stars the right celestial bodies to produce a cloud of axions?"But how could the gathering of axion clouds happen, and why are neutron stars and not, say, black holes the ideal bodies to gather these axion clouds? It begins with the fact that dark matter, and thus axions, interact with gravity — and neutron stars are the "Goldilocks" of this situation, having just enough gravitational influence but not too much.
Seeing dark matter more clearly through axion clouds
To understand why dark matter is such a challenge for scientists, consider that all the matter wrapped up in everything we see around us — from stars, planets, and moons down to humans, coffee tables and even cats — accounts for no more than 15% of the matter in the universe. The other 85%, dark matter, is effectively invisible because it either doesn't interact with light (or, at least, does so incredibly weakly).
Axions are the current prime suspects for dark matter particles, and the team theorizes that a crafty technique could allow current telescopes to observe these shrouds of dark matter around what are known as neutron star "traps," or wells in the fabric of space and time that these stars create.
The team reasons that if axions have masses within a certain range that current theoretical models support, then neutron stars would be surrounded by clouds of these dark matter particles.
According to the researchers, one surprising finding was that this cloud gathering would be "generic," meaning that it wouldn't only be restricted to neutron stars with certain qualities. That means axion clouds could gather around young neutron stars with a rapid rate of spin that blasts out the light from their poles, known as "pulsars," as well as older neutron stars that have slowed and thus don't act like spinning cosmic lighthouses. Additionally, the clouding action should occur for all types of axions.
Noordhuis and his colleagues found that axion clouds would be very dense, able to exceed local dark matter densities by more than 20 orders of magnitude over large periods of a neutron star's life. This density matters because, though axion interactions with light and matter would be weak and thus rare, a region where enough of these particles are clustered together could result in a "boosted signal" that is indeed detectable.
He added that there would be very small interaction strengths between the hypothesized axions and particles of light, or photons, which would be much weaker than we can probe with current telescope technology. However, when axions cluster in dense clouds around neutron stars built over the course of millions of years, that situation could change. Telescopes should be able to pick those clouds up.
"Once an axion cloud has built up sufficiently, there can be large amounts of axions converting into light particles, creating powerful observational signatures in the form of radio waves," Noordhuis said.
To discover why neutron stars are the ideal bodies to gather dense axion clouds, it is worth exploring where their extreme characteristics come from.
Why neutron stars?
Neutron stars are born when stars at least eight times more massive than the sun rea...
