A team of particle physicists from the University of Melbourne, Australian National University, King’s College London, and Fermi National Accelerator Laboratory has discovered that the energy transferred when dark matter particles collide and annihilate inside a cold neutron star. They calculated that the star could be heated rapidly. Previously, this heating was thought to be irrelevant because this energy transfer takes a very long time, in some cases longer than the age of the universe itself.
An artist’s impression of a neutron star.
A number of recent studies have focused on trapping dark matter in neutron stars as sensitive probes of the interaction of dark matter with ordinary matter.
This could potentially be used to test dark matter interactions in a way that is highly complementary to experiments on Earth, especially since dark matter is accelerated to relativistic speeds during a fall into a neutron star. there is.
In some cases, neutron star technology may be able to probe interactions that are difficult or impossible to observe with direct dark matter detection experiments. These include dark matter, which is too light to leave a detectable signal in nuclear recoil experiments, and interactions where non-relativistic scattering cross sections are momentum suppressed.
It was recently pointed out that an isolated old neutron star near the Sun could be heated by the capture of dark matter, increasing its temperature by 2000 K.
Once older than 10 million years, an isolated neutron star is expected to cool to temperatures below this unless reheated by standard matter accretion or internal heating mechanisms.
As a result, observations of local neutron stars may place severe constraints on dark matter interactions. Importantly, neutron stars with temperatures in this range produce near-infrared radiation that could be detected by future telescopes.
“Our new calculations show for the first time that most of the energy is stored in just a few days,” said Professor Nicole Bell from the University of Melbourne, lead author of the study.
“The search for dark matter is one of science’s greatest detective stories.”
“Dark matter makes up 85% of the matter in the universe, but we can’t see it.”
“It doesn’t interact with light. It doesn’t absorb, reflect, or emit light.”
“This means that even if we know it exists, we can’t directly observe it with our telescopes.”
“Rather, its attraction to an object that we can see tells us that it must be there.”
“Predicting dark matter theoretically and observing it experimentally are two different things.”
“Earth-based experiments are limited by the technical challenges of building a large enough detector.”
“But neutron stars act as huge natural dark matter detectors, collecting dark matter over astronomically long timescales, so they are a good place to focus our efforts.”
“Neutron stars form when supermassive stars run out of fuel and collapse,” Professor Bell said.
“They have a similar mass to our sun and are squeezed into a sphere just 20km wide. If they got any denser, they would become black holes.”
“Dark matter is the main type of matter in the universe, but it is very difficult to detect because it interacts very weakly with normal matter.”
“In fact, dark matter is so weak that it can pass straight through the Earth and even the Sun.”
“But neutron stars are different. Because neutron stars are so dense, dark matter particles are much more likely to interact with the star.”
“If dark matter particles collide with neutrons inside a star, they lose energy and become trapped.”
“Over time, this will lead to an accumulation of dark matter within the star.”
“We expect this to cause old, cold neutron stars to heat up to a point where they can be observed in the future, or even cause the star to collapse into a black hole,” said the University of Melbourne doctor. candidate Michael Vilgat, co-author of the study.
“If the energy transfer happens quickly enough, the neutron star will heat up.”
“For this to happen, the dark matter would have to collide within the star many times, transferring more and more of the dark matter’s energy until all the energy is stored in the star.”
“Until now it was unknown how long this process takes, because as dark matter particles become less and less energetic, they become less and less likely to interact again.”
“As a result, it was thought that it would take a very long time to transfer all the energy, in some cases longer than the age of the universe.”
Instead, the researchers calculated that 99% of the energy is transferred in just a few days.
“This is good news, because it means dark matter can potentially heat neutron stars to detectable levels,” Birgat said.
“As a result, observations of cold neutron stars will provide important information about the interactions between dark matter and ordinary matter and shed light on the nature of this elusive matter.”
“If we are to understand the ubiquity of dark matter, it is important to use every technology at our disposal to understand what the hidden matter in our universe actually is.” .”
of study Published in Journal of Cosmology and Astroparticle Physics.
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Nicole F. Bell other. 2024. Thermalization and extinction of dark matter in neutron stars. JCAP 04,006; doi: 10.1088/1475-7516/2024/04/006
