Scientists suggest that some dead stars made of the densest material in the known universe, called “neutron stars”, may serve as traps for dark matter particles that crash into each other at high speeds and annihilate each other. The crew says the destruction process will likely heat the dead stars from the inside out.
Dark matter in general is a problematic issue for researchers because although it makes up an estimated 85% of the matter in the universe, it is virtually invisible because it does not interact with light. Dark matter also does not appear to interact with “ordinary matter,” which consists of protons, neutrons, and electrons; or if they do interact, these interactions are rare and weak. We never saw 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 self-interaction of dark matter, which occurs in normal matter when an electron and its antiparticle, a positron, meet and annihilate, thus releasing energy. Dark matter is electrically neutral; This means that the particles that make up matter can theoretically also have their own antiparticles.
And as with normal matter annihilation, dark matter must annihilate when dark matter particles encounter it, and neutron stars may be the ideal extreme environment for such interactions.
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“Annihilation is when a particle and antiparticle collide and annihilate each other. This is what would happen if dark matter had its own antiparticle, as is often assumed in the most studied models of dark matter,” Bell told Space.com. “Trapping and destroying dark matter in neutron stars would provide a heating source that would prevent the star from actually cooling.”
This means that neutron stars could emit a thermal signature if they could act as “dark matter traps.” If this can be detected, neutron stars could act as primitive “dark matter detectors,” helping scientists find this virtually invisible form of matter.
What it takes to trap dark matter
A neutron star is born when the fuel source needed for nuclear fusion, which occurs in the core of a star at least eight times larger than the Sun, runs out. This puts an end to the external force driven by radiation pressure that has supported the star against the inward force of its own gravity for millions, sometimes even billions, of years.
As a result, the star’s core collapses, emitting shock waves that trigger a supernova. This shock wave blows away the dying star’s outer layers and most of its mass, leaving behind a stellar core with a mass between one and two times that of the Sun that collapses to a width of about 12 miles (20 kilometers).
Turning a mass equivalent to more than half a million Earths into an object that could be located within the city limits of Chicago naturally has an extreme effect on the material of the stellar core. It combines electrons and protons, creating a sea of neutrons, particles normally found only at the heart of atoms. This neutron sea that forms the neutron star is so dense that if a tablespoon of it were brought to Earth, it would weigh over 1 billion tons. This is about the same weight as Mount Everest.
Neutron stars are made of the densest matter in the known universe, and so scientists believe their gravitational pull could be large enough to trap dark matter, which does interact with gravity despite not interacting with light and matter.
Bell explains that dark matter extinctions are thought to occur frequently when the 13.8 billion-year-old universe was less than a second old, but are thought to occur rarely in the present-day universe. The only exception would be regions where there is a lot of dark matter.
And if dark matter could indeed accumulate in the interiors of neutron stars, Bell and his colleagues found that this would be exactly what would provide a dark matter-rich environment that would allow it to disappear in the old universe.
“In a small area, there could be enough dark matter in these stars to eliminate significant amounts of dark matter,” Bell said.
Bell added that in dark matter experiments conducted in laboratories on Earth, scientists look for signals of dark matter particles interacting with ordinary matter, but neutron stars have a natural advantage in this respect.
“In the laboratory, we look for collisions between dark matter particles and atomic nuclei,” he said. “But if this could happen, then dark matter should also be able to collide with neutrons and protons in neutron stars. share from neutrons.”
Additionally, while researching neutron stars and dark matter, Bell was surprised to learn that the immense gravity of neutron stars could create another condition inside these dead stars that makes the self-interaction of dark matter particles more likely.
“Dark matter accelerates to speeds close to the speed of light when it falls on a neutron star,” Bell said. “This is useful because it could increase the speed of interactions and potentially allow us to investigate some dark matter interactions that are almost impossible to see in experiments on Earth.”
The extinction of dark matter releases thermal energy into these dead star traps; so the team 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 no heat flows between them anymore.
This research revealed that a neutron star saturated with dark matter could reach thermal equilibrium in as little as a year, or no more than 10,000 years depending on the model used. In cosmic terms, this is just a blink of an eye.
To confirm this theory, researchers will need to measure the temperature of neutron stars. Finding that these extremely dead stars are hotter than expected would reveal that dark matter particles are indeed destructive inside them. But such a discovery would be no mean feat, since only older and cooler neutron stars can emit thermal radiation that is not suppressed by other light. This will require the most powerful observation tool humanity has ever launched into space: the James Webb Space Telescope.
“The neutron stars we are most interested in are very cool stars that are difficult to see,” Bell added. “The temperatures of these stars will lead to near-infrared emission, and it may be possible to see this with the James Webb Space Telescope (JWST).”
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The lack of understanding of neutron stars may mean that this dark extinction model may be easier to test with White dwarfs, the type of stellar remnants left behind when smaller stars like the Sun die.
“Neutron stars may 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 turns out to be correct, it will not only shed light on dark matter, but will also help scientists better understand the evolution of neutron stars.
Bell and colleagues’ research, which has not yet been peer-reviewed, is available in the arXiv article repository.