by Scientists argue that some dead stars made of the densest matter in the known universe, so-called “neutron stars”, could act as traps for dark matter particles that smash together at high speeds and annihilate each other. After that, the team says, the extinction process is likely to heat the dead stars from the inside out.
In general, dark matter is problematic for researchers because, despite making up an estimated 85% of the stuff in the universe, it is effectively invisible because it does not interact with light. Dark matter doesn’t seem to interact with “normal matter” made up of protons, neutrons and electrons, either — or, if it does, these interactions are rare and weak. We didn’t see 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 type of dark matter self-interaction that occurs in ordinary matter when an electron and its antiparticle, a positron, meet and annihilate, releasing energy. Dark matter is electrically neutral, meaning that whatever particles are in the substance could theoretically also have their own antiparticles.
And, like normal matter annihilation, when dark matter particles come together, dark matter annihilation should occur — and neutron stars could be an incredibly ideal environment for such interactions.
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“Annihilation is when a particle and an antiparticle collide and annihilate each other. This is what would happen if dark matter were its own antiparticle, as is often assumed in the most studied dark matter models, ” Bell told Space.com. “The capture and annihilation of dark matter in neutron stars would provide a source of heat that stops the star from becoming really cold.”
That means, if neutron stars can act as “dark matter traps”, they could emit a thermal signature. If this can be detected, neutron stars could act as key “detectors of dark matter”, helping scientists look for this effectively invisible form of matter.
What it takes to contain dark matter
A neutron star is born when a star eight times the size of the sun runs out of the fuel supply it needs for nuclear fusion to take place in its core. This ends the outward force driven by radiation pressure that has supported the star against the inward force of its own gravity for millions, or sometimes billions, of years.
As a result, the core of the star collapses, sending out shock waves that trigger a supernova. This shock wave blows away the outer layers of the dying star along with most of its mass, leaving behind a stellar core with a mass of one to two times that of the sun that has collapsed to a width of about 12 miles ( 20 kilometers).
Squeezing a mass equal to over half a million Earths into a body that could sit within the city limits of Chicago has a profound effect on the contents of that stellar core. It forces electrons and protons together, creating a sea of neutrons, which are particles normally only found in the core of atoms. This neutron sea that makes up a neutron star is so dense that if only 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.
Neutron stars are thus made of the densest matter in the known universe, which is why scientists believe their gravitational effects could be large enough to contain dark matter – which it does, despite its lack of interaction with light and matter, interacting with gravity.
Bell explains that dark matter annihilation is thought to have occurred frequently when the 13.8 billion-year-old universe was less than two years old, but is rare in today’s universe. The only exception to this is in regions with a lot of dark matter.
And if dark matter can indeed accumulate in the interior of neutron stars, Bell and his colleagues found that this would provide precisely the dark matter-rich environment, allowing it to annihilate in the old universe.
“You could have a lot of dark matter in a small area, enough that a significant amount of dark matter annihilation could be happening in these stars,” Bell said.
Bell also said that in dark matter experiments in laboratories on Earth, scientists search for signs of dark matter particles interacting with normal matter, but neutron stars 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 that can happen, then dark matter must be able to collide with the neutrons and protons in neutron stars. full neutron.”
In addition, Bell was surprised to discover, while investigating neutron stars and dark matter, that the massive center of gravity of neutron stars may create another condition that makes the self-interaction of dark matter particles within stars more likely. this dead.
“Dark matter is accelerated to speeds close to the speed of light when it collapses into a neutron star,” Bell said. “This is useful because it can improve the rate of interactions, allowing us to explore some types of dark matter interactions that would be nearly impossible to see in experiments on Earth.”
The annihilation of dark matter releases thermal energy into these dead star traps, so the team also looked at how long it would take neutron stars and the dark matter they absorb to reach a state known as “thermal equilibrium .” This is the point at which two substances reach the same temperature, and heat no longer flows between them.
This investigation revealed that a dark-matter-saturated neutron star could reach thermal equilibrium in a period of no longer than 10,000 years, and as short as one year, depending on the model used. In cosmic terms, this is just a blink of an eye.
To validate this theory, researchers would need to measure the temperature of neutron stars. Finding these giant dead stars to be hotter than expected would indicate that dark matter particles are annihilating within their interiors. Such a discovery would be no mean feat, however, because only older and cooler neutron stars would emit thermal radiation that is not overwhelmed by other light. This would require the most powerful observational instrument that humanity has ever sent into space: the James Webb Space Telescope.
“The neutron stars we’re most interested in are very cold stars, which are hard to see,” Bell said. “The temperature of these stars would result in near-infrared emissions, which could be seen by the James Webb Space Telescope (JWST).”
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The lack of understanding of neutron stars may mean that it may be easier to test this dark extinction model with the type of stellar remnant left behind when stars smaller than the sun die: dwarfs white
“Neutron stars would be good for trapping dark matter because of their extreme density. But they are also relatively poorly understood stars,” Bell said. “Similar ideas can be applied to other stars that we understand better, such as white dwarf stars.”
If the theory turns out to be correct, it would not only shine a light on dark matter but help scientists better understand the evolution of neutron stars.
The research by Bell and his colleagues, which has not yet been peer-reviewed, is available on the paper repository site arXiv.
