Scientists propose that some dead stars made of the densest material in the known universe, so-called ‘neutron stars’, could act as traps for dark matter particles to collide at high speed and destroy each other. The crew says the destruction process likely heats the dead stars from within.
In general, dark matter is a problematic topic for researchers because, despite making up an estimated 85% of the matter in the universe, it is effectively invisible because it does not interact with light. Dark matter also does not appear to interact with ‘ordinary matter’ made up of protons, neutrons and electrons – or, if it does, these interactions are rare and weak. We’ve never seen them. This raises an interesting question: does dark matter interact with itself?
Nicole Bell, a theoretical physicist at the University of Melbourne, is interested in the kind of self-interaction with dark matter that occurs in normal matter when an electron and its antiparticle, a positron, meet and annihilate, releasing energy. Dark matter is electrically neutral, meaning that the particles that make it up can theoretically also have their own antiparticles.
And just like the destruction of normal matter, when dark matter particles meet, dark matter destruction should occur – and neutron stars could be the ideal extreme environment for such interactions.
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“Annihilation is when a particle and an antiparticle collide and destroy each other. This is what would happen if dark matter were its own antiparticle, as is often assumed in the most studied models of dark matter,” Bell told Space.com. ‘The capture and destruction of dark matter in neutron stars would provide a heating source that prevents the star from getting really cold.’
This means that if neutron stars can act as ‘dark matter traps’, they can emit a thermal signature. If this can be detected, neutron stars could act as rudimentary ‘dark matter detectors’, which could help scientists track down this effectively invisible form of matter.
What it takes to ensnare dark matter
A neutron star is born when a star at least eight times as massive as the Sun runs out of fuel for nuclear fusion to occur in its core. This ends the radiation pressure-driven force that has been supporting the star against the inward force of its own gravity for millions, or sometimes billions, of years.
As a result, the star’s core collapses, sending out shock waves that cause a supernova. This shock wave blows away the dying star’s outer layers, as well as most of its mass, leaving behind a stellar core with a mass between one and twice that of the Sun, which has collapsed to a width of about 20 kilometers.
Crushing a mass equivalent to more than half a million Earths into a body that could sit within the city limits of Chicago obviously has an extreme effect on the matter of that stellar core. It forces electrons and protons together, creating a sea of neutrons, which are particles normally only found in the hearts of atoms. This sea of neutrons that makes up a neutron star is so dense that if just a tablespoon of it were brought to Earth, it would weigh more than 1 billion tons. That’s about the same weight as Mount Everest.
So neutron stars are made of the densest matter in the known universe, which is why scientists believe their gravitational influence could be great enough to trap dark matter – which, despite its lack of interaction with light and matter, does interact with gravity.
Bell explains that the destruction of dark matter likely occurred frequently when the 13.8 billion-year-old universe was less than a second old, but that it rarely occurs in the current universe. The only exception to this are areas where a lot of dark matter is present.
And if dark matter can indeed accumulate in the interiors of neutron stars, Bell and colleagues found that this would create just the dark matter-rich environment that could destroy it in the aging universe.
“You could end up with a lot of dark matter in a small region, enough that there could be a significant amount of dark matter destruction in these stars,” Bell said.
Bell added that in dark matter experiments in laboratories on Earth, scientists look for signals from dark matter particles interacting with ordinary matter, but that neutron stars have a natural advantage in this regard.
“In the laboratory we look for collisions between dark matter particles and atomic nuclei,” she said. ‘But if that is possible, then dark matter should also be able to collide with the neutrons and protons in neutron stars. And neutron stars have one lot of neutrons.”
Furthermore, Bell was surprised when, during her research on neutron stars and dark matter, she discovered that the immense gravity of neutron stars could create another condition that makes the self-interaction of dark matter particles within these dead stars even more likely.
“Dark matter is accelerated to speeds close to the speed of light when it enters a neutron star,” Bell said. “This is useful because it can increase the speed of interactions, potentially allowing us to investigate certain types of interactions with dark matter that are almost impossible to see in experiments on Earth.”
The destruction of dark matter releases thermal energy in these traps of dead stars. The team therefore also looked at how long it would take for neutron stars and the dark matter they capture to reach a state called ‘thermal equilibrium’. This is the point at which two substances reach the same temperature and heat no longer flows between them.
This research found that a dark matter-saturated neutron star could reach thermal equilibrium in a period as short as 10,000 years, and even within a year, depending on the model used. In cosmic terms, this is just the blink of an eye.
To validate this theory, researchers would need to measure the temperatures of neutron stars. If these extremely dead stars turn out to be hotter than expected, it would reveal that dark matter particles are indeed being destroyed within their interiors. However, such a discovery would be no small feat, as only older and colder neutron stars would emit thermal radiation that is not overwhelmed by other light. This would require the most powerful observation instrument humanity has ever launched into space: the James Webb Space Telescope.
“The neutron stars we are most interested in are very cold stars, which are difficult to see,” Bell added. “The temperatures of these stars would lead to emission in the near-infrared, which could potentially be visible with the James Webb Space Telescope (JWST).”
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The lack of knowledge about neutron stars could mean that this model of dark destruction is easier to test with the type of stellar remnant left behind when smaller stars like the Sun die: white dwarfs.
‘Neutron stars would be good for capturing dark matter because of their extreme density. But they are also relatively poorly understood stars,” Bell concluded. “Similar ideas can be applied to other stars we understand better, such as white dwarf stars.”
If the theory proves correct, it would not only shed light on dark matter but also help scientists better understand the evolution of neutron stars.
Bell and colleagues’ research, which has yet to be peer-reviewed, is available on the paper repository site arXiv.