3 mins read 12 Nov 2021

Detecting Dark Matter Using Warmed Up Neutron Stars

A team of scientists has been working to perfect the use of neutron stars as dark matter detectors. 

An artist’s impression of a neutron star. Credit: NASA.

If you thought that LIGO (the 4-kilometre-long gravitational wave detector) was a big detector, then think again. Scientists have been working on detecting dark matter using a detector that is out-of-this-world big. In a paper published recently in Physical Review Letters, a team has outlined several key improvements that can be made to calculations in order to use neutron stars as dark matter detectors. 

The team consisted of researchers from ARC (Australian Research Council) Centre of Excellence for Dark Matter Particle Physics, the Max Planck Institute for Nuclear Physics in Germany, and the University of Adelaide.

Looking for the Invisible

Scientists know that dark matter exists because of its interactions with normal matter. One of these instances is how it affects the mass of our galaxy, which is heavier than it should be if only the normal matter was taken into account. Artist illustration of the expected dark matter halo (blue) surrounding the Milky Way galaxy. Credit: ESO/L. Calcada.

To put it mildly, dark matter is hard to spot. A huge amount of matter in the universe is the dark kind (meaning it is invisible and does not interact with electromagnetic radiation), and yet it is exceedingly difficult to detect and measure. Scientists have inferred that dark matter exists through its gravitational influence, but have yet to directly observe it. The main issue with trying to find dark matter is that we cannot perceive it unless it interacts with normal matter, which rarely happens. 

Some scientists are trying to use Earth-based detectors in order to observe this elusive matter, like the soon-to-be-completed Stawell Underground Physics Lab, which is located one kilometre underground in Victoria. However, other scientists are looking to potentially use detectors which are on the cosmic scale. 

Neutron Stars as Dark Matter Detectors

Neutron stars are incredibly dense. While a neutron star may have roughly the mass of the Sun, it only has a 10 kilometre diametre. This image shows a neutron star compared with Manhattan Island, New York. Credit: NASA Goddard Space Flight Centre.

This new research paper has set out to try to perfect the calculations that could see the use of neutron stars as dark matter detectors. 

Neutron stars are the super dense left-overs of the core of a giant star which originally had a mass roughly between 10 and 25 solar masses. The neutron star is formed when the massive star undergoes a supernova, causing its core to be crushed down by gravity until a little more than a solar mass is compressed into a tiny 10 km radius. This type of star gets its name from the fact that the protons and electrons become so compressed that they combine to form neutrons.  

This immense density means - other than that one teaspoon of neutron star material has a mass of about a billion tons which is approximately the weight of all of humanity combined - that neutron stars can be used as ‘cosmic laboratories’ to detect dark matter. 

Theoretically, when dark matter collides with a neutron star, it becomes trapped and accumulates due to the star’s gravity. This is expected to heat up old, cold, neutron stars to a level that may be within the detectable reach of future observations, or even trigger the collapse of the star into a black hole. 

This latest research into neutron stars as dark matter detectors saw the team of scientists correct some imperfect calculations which otherwise didn’t fully take into account neutron star characteristics. This included accounting for nucleon structure and the effects of strong forces between nucleons. This then allowed the team, in turn, to better understand various factors such as how fast dark matter accumulates in neutron stars, and bring physics one step closer to detecting dark matter with these super-dense cosmic structures.