Scientists speculate that some dead stars made of the densest matter in the known universe, so-called “neutron stars,” may act as traps for dark matter particles that collide and annihilate each other at high speeds. The annihilation process likely heated the dead star from the inside out, the crew said.
In general, dark matter is a problematic subject for researchers because, although dark matter makes up an estimated 85 percent of the matter in the universe, it is actually invisible because it does not interact with light. Dark matter also doesn’t appear to interact with “ordinary matter,” which is made up of protons, neutrons, and electrons—or, if it does exist, these interactions are rare and weak. We never see them. This raises an interesting question: Can dark matter interact with itself?
Nicole Bell is a theoretical physicist at the University of Melbourne who is interested in the dark matter self-interactions that occur when electrons in normal matter meet and annihilate their antiparticles, positrons, releasing energy. Dark matter is electrically neutral, meaning any particle that makes up the matter could theoretically also have its own antiparticle.
And, just like normal matter annihilation, dark matter annihilation should occur when dark matter particles meet – and neutron stars could be the ideal extreme environment for such interactions.
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“Annihilation is when particles and antiparticles collide and destroy each other. This would happen if dark matter was its own antiparticle, as is often assumed in the most widely studied dark matter models,” Bell told Space.com . “The capture and annihilation of dark matter in neutron stars would provide a source of heating that would prevent the star from becoming very cold.”
This means that if neutron stars can act as “dark matter traps,” they can emit a heat signature. If this could be detected, neutron stars could act as basic “dark matter detectors,” which would help scientists hunt down this virtually invisible form of matter.
What it takes to capture dark matter
Neutron stars are born when a star at least eight times the mass of the Sun exhausts the supply of fuel needed for nuclear fusion to occur in its core. This ends the outward force driven by radiation pressure that has supported the star’s inward force against its own gravity for millions or even billions of years.
As a result, the star’s core collapses, sending out a shock wave that triggers a supernova explosion. This shock wave blew away the dying star’s outer layers and most of its mass, leaving behind a stellar core with one to two times the mass of the Sun that had collapsed to about 12 miles (20 kilometers) across.
Crushing the equivalent of more than half a million Earths’ mass into an object within the city limits of Chicago would naturally have extreme effects on the material at the star’s core. It forces electrons and protons to clump together, creating a sea of neutrons, particles that normally only exist in the centers of atoms. The sea of neutrons that make up a neutron star is so dense that if a tablespoon of neutrons were brought to Earth, it would weigh over a billion tons. That’s about the same weight as Mount Everest.
Therefore, neutron stars are made of the densest matter in the known universe, which is why scientists believe their gravitational influence may be large enough to trap dark matter – which, despite its lack of interaction with light and matter, does interact with gravity effect.
Bell explained that dark matter annihilation was thought to have occurred frequently when the universe was less than a second old when it was 13.8 billion years old, but rarely occurs in today’s universe. The only exceptions are regions with large amounts of dark matter.
Bell and colleagues discovered that if dark matter can indeed accumulate inside neutron stars, this would provide precisely the environment rich in dark matter to annihilate in the aging universe.
“You may end up with a large amount of dark matter in a small area, enough to allow large amounts of dark matter annihilation to occur in these stars,” Bell said.
Bell added that in dark matter experiments in Earth laboratories, scientists look for signals of interactions between dark matter particles and ordinary matter, but that celestial bodies have a natural advantage in this regard.
“In the laboratory, we are looking for collisions between dark matter particles and atomic nuclei,” she said. “But if this can happen, then dark matter must also be able to collide with neutrons and protons in neutron stars. And neutron stars have a lot of neutron.
In addition, while studying neutron stars and dark matter, Bell was surprised to find that the huge gravity of neutron stars may create another condition that makes dark matter particles more likely to self-interact within these dead stars.
“When dark matter falls into a neutron star, it accelerates to nearly the speed of light,” Bell said. “This is useful because it increases the rate of interactions, potentially allowing us to detect certain types of dark matter interactions, while This is almost impossible to see in experiments on Earth.”
Dark matter annihilation releases thermal energy into these Death Star traps, so the team also studied how long it takes for neutron stars and the dark matter they capture to reach a state called “thermal equilibrium.” This is the point at which the two substances reach the same temperature and heat no longer flows between them.
This study shows that a dark matter-saturated neutron star can reach thermal equilibrium in no more than 10,000 years and as little as a year, depending on the model used. From a cosmic perspective, it’s just the blink of an eye.
To test this theory, researchers needed to measure the temperature of the neutron star. The discovery that these extreme dead stars are hotter than expected would suggest that dark matter particles are indeed annihilating within them. Such a discovery would be no easy feat, however, because only older, cooler neutron stars emit thermal radiation that is not drowned out by other light. This will require the most powerful observational instrument ever launched into space: the James Webb Space Telescope.
“The neutron stars that interest us most are very cool stars that are difficult to see,” Bell added. “The temperature of these stars results in near-infrared radiation that may be seen by the James Webb Space Telescope (JWST).”
A lack of understanding of neutron stars may mean that this dark annihilation model may be easier to test with the type of stellar remnant left behind when smaller stars like our sun die: white dwarfs.
“Because neutron stars are extremely dense, they are good for capturing dark matter. But they are also stars about which relatively little is known,” Bell concluded. “Similar ideas could be applied to other stars that we better understand, such as white dwarfs.”
If this theory proves to be correct, it will not only bring light to the study of dark matter, but will also help scientists better understand the evolution of neutron stars.
The study by Bell and colleagues has not yet been peer-reviewed and can be found on the paper repository website arXiv.
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