Could we find dark matter in clouds around neutron stars?
Looking for dark matter
It’s thought that around 85% of all matter in the universe is dark matter. We can’t see this mysterious substance, or detect it with any currently known method … but we think it exists because we can measure its gravitational effects on normal matter. A leading theory says that dark matter could be composed of axions: hypothetical subatomic particles that have not yet been detected.
On October 17, 2024, a team of physicists from the universities of Amsterdam, Princeton and Oxford said that axions should form dense clouds around neutron stars. And if so, we might be able to observe these dark matter candidates through today’s telescopes.
In October 2023, the same researchers theorized that it’s possible to detect axions that have escaped from a neutron star. Now, their followup study focuses on the axions that wouldn’t be able to escape the star’s gravity. They published their peer-reviewed findings in the journal Physical Review X on October 17, 2024.
Dark matter: A missing piece
When faced with a gap in our theories about how the universe works, physicists sometimes come up with something entirely new to fill the hole. In the early 20th century, various astronomers found that the universe must contain more mass than we can see. And in the 1960s, astronomer Vera Rubin discovered that galaxies rotate so fast their mass shouldn’t prevent them from flying apart.
The only way to explain these discrepancies was to hypothesize the existence of a new, unseen form of matter: dark matter. This unknown and strange substance has not yet been found, but if it is discovered, it would solve a long list of problems.
In the 1970s, scientists came up with axions to explain an inconsistency in the way neutrons should function according to the Standard Model of particle physics. The Standard Model is our best guess at how the universe works at a fundamental level, but it’s not perfect. And the existence of axions would help clean up one of its mysteries, which is why they were named after a brand of soap!
Another intriguing thing about axions is that they might also solve the conundrum of what is dark matter. Dark matter is invisible because it doesn’t seem to interact with light or matter. But there’s a chance it does interact with other matter, just incredibly weakly. Axions also seem to be invisible and would interact incredibly weakly with other particles. Coincidence? Some scientists think not. So they believe axions could be an explanation for dark matter.
Observing axions
If axions do exist, how can scientists observe them? The solution, according to the researchers, lies with neutron stars.
Neutron stars are some of the most bizarre phenomena in the known universe. They’re the small, super-dense objects left over when massive stars explode as supernovae and their cores collapse. They typically have about 1.4 times our sun’s mass that’s squeezed into a sphere roughly 12-25 miles (20-40 kilometers) across. So they’re incredibly dense. In fact, a teaspoon of neutron star material weighs more than Mount Everest.
When a star’s core collapses down to form a neutron star, its magnetic field lines compress. That makes its magnetism stronger. A neutron star’s magnetic field is one of the strongest in the universe, billions of times stronger than any on Earth.
That’s important, because scientists believe axions should transform into light particles when exposed to a strong-enough magnetic field. The amount of light that a single axion could produce would barely register. But a huge amount of axions – in contact with a hugely powerful magnetic field – should produce enough light that today’s radio telescopes could see it.
Clouds of dark matter?
And it turns out neutron stars could produce a lot of axions. In their original October 2023 study, the researchers found that pulsars – rapidly spinning neutron stars – could produce a 50-digit number of axions every second. They went on to explore the possibility of detecting some of these axions as they escape the neutron star.
Their new study focuses on the axions that stay behind. This idea relies on another extreme property of neutron stars: their immense gravity. As you might expect given their density, neutron stars have an incredibly strong gravitational pull. And, since axions interact with gravity, that makes neutron stars excellent axion traps.
For the same reason, black holes are also thought to collect huge numbers of axions. But the gravity of black holes is so much that they would also absorb what they capture. Neutron stars are thought to have just the right gravitational force to capture and hold axions around them. And since axions interact very weakly with other particles, the researchers think they would simply accumulate around the neutron star. Over millions of years, they would theoretically form a dense cloud, providing the perfect opportunity for scientists to detect them.
Making the detection
There are two main ways that scientists could detect light from axion clouds. It could be visible as a continuous signal emitted during much of a neutron star’s lifetime. Or it could appear as a one-time burst of light at the end of the neutron star’s life.
Importantly, the researchers said that axion clouds would be generic and should theoretically occur around any neutron star.
So far, axion clouds have not been observed. But the researchers now know what they’re looking for. And if they find direct evidence of axions, it will be a major step in answering several of physics’ biggest problems.
Bottom line: Researchers say that we might be able to detect dark matter clouding around neutron stars in the form of axions, a hypothetical subatomic particle.
Source: Axion Clouds around Neutron Stars
Read more: Can we detect dark matter using light from pulsars?
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