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A team of particle physicists from the University of Melbourne, the Australian National University, King’s College London and the Fermi National Accelerator Laboratory has calculated that the energy transferred when dark matter particles collide and annihilate inside Cold neutron stars can heat up the stars very quickly; It was previously thought that this energy transfer could take a long time, in some cases even longer than the age of the Universe, making this warming irrelevant.
Artist’s impression of a neutron star.
Recently there has been much work on capturing dark matter in neutron stars as a sensitive probe of dark matter interactions with ordinary matter.
This can potentially be used to probe dark matter interactions in a way that is highly complementary to experiments on Earth, especially since dark matter accelerates to relativistic speeds during the fall into a neutron star.
In some cases, neutron star techniques could potentially investigate interactions that would be difficult or impossible to observe in direct dark matter detection experiments. This includes dark matter that is too light to leave a detectable signal in nuclear recoil experiments, or interactions in which the momentum of the non-relativistic scattering cross section is suppressed.
It was recently noted that old, isolated neutron stars in the solar neighborhood could be warmed by capturing dark matter, leading to a temperature rise of 2000 K.
At ages greater than 10 million years, isolated neutron stars are expected to cool to temperatures below this, provided they are not reheated by the accumulation of standard matter or by internal heating mechanisms.
As a result, observing a local neutron star could impose tight constraints on dark matter interactions. Importantly, neutron star temperatures in this range would give rise to near-infrared emissions, potentially detectable by future telescopes.
“Our new calculations show for the first time that most of the energy would be deposited in just a few days,” said Professor Nicole Bell of the University of Melbourne, first 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 our Universe, but we can’t see it.”
“It does not interact with light: it does not absorb light, it does not reflect it, it does not emit light.”
“This means that our telescopes cannot observe it directly, even if we know it exists.”
“Instead, its gravitational pull on objects we can see tells us it must be there.”
“It’s one thing to predict dark matter theoretically, but another to observe it experimentally.”
“Experiments on Earth are limited by the technical challenges of making sufficiently large detectors.”
“However, neutron stars act as huge natural dark matter detectors, which have been collecting dark matter for astronomically long time scales, so they are a good place to focus our efforts.”
“Neutron stars form when a supermassive star runs out of fuel and collapses,” Professor Bell said.
“They have a mass similar to that of our Sun, forming a ball only 20 kilometers wide. “If they were denser, they would become black holes.”
“While dark matter is the dominant type of matter in the Universe, it is very difficult to detect because its interactions with ordinary matter are very weak.”
“In fact, it is so weak that dark matter can directly pass through the Earth or even the Sun.”
“But neutron stars are different: They are so dense that dark matter particles are much more likely to interact with the star.”
“If dark matter particles collide with the star’s neutrons, they will lose energy and become trapped.”
“Over time, this would lead to an accumulation of dark matter in the star.”
“This is expected to heat old, cold neutron stars to a level that could be within reach of future observations, or even trigger the star’s collapse into a black hole,” said the Ph.D. from the University of Melbourne. candidate Michael Virgato, co-author of the study.
“If the energy transfer happens fast enough, the neutron star would heat up.”
“For this to happen, the dark matter must undergo many collisions in the star, transferring more and more energy from the dark matter until, finally, all the energy has been deposited in the star.”
“Until now it was not known how long this process would take because, as the energy of the dark matter particles becomes smaller and smaller, they are less and less likely to interact again.”
“As a result, transferring all the energy was thought to take a long time, sometimes 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 that dark matter can heat neutron stars to a level that can potentially be detected,” Virgato said.
“As a result, observing a cold neutron star would provide vital information about the interactions between dark matter and regular matter, shedding light on the nature of this elusive substance.”
“If we want to understand dark matter, which is everywhere, it is essential that we use all the techniques at our disposal to discover what the hidden matter of our Universe really is.”
He study was published in the Journal of Cosmology and Astroparticle Physics.
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Nicole F. Bell et al. 2024. Thermalization and annihilation of dark matter in neutron stars. JCAP 04, 006; doi:10.1088/1475-7516/2024/04/006
