Searching for Dark Matter With a Detector Larger Than We Can Build on Earth

Using neutron stars to detect dark matter

The search for dark matter may require a detector larger than the one we build on Earth, but neutron stars may be able to do the job.

Exploring the nature of dark matter is one of the biggest challenges in science today, but the key to understanding this mysterious matter is likely to lie in stars.

Or to be precise, it is a special type of star: a neutron star.

So far, scientists have been able to infer the existence of dark matter, but they cannot directly observe it. Actually detecting dark matter particles in experiments on Earth is a difficult task, because the interaction between dark matter particles and conventional matter is extremely rare.

Spinning Neutron Star in Space
Neutron stars are dense enough to capture dark matter. Picture: Animation of a spinning neutron star in space. Credit: NASA’s Goddard Space Flight Center Conceptual Image Lab

In order to search for these extremely rare signals, we need a very large detector, which may be so large that it is impractical to build a large enough detector on Earth. However, nature offers another option in the form of a neutron star: a complete neutron star can serve as the ultimate dark matter detector.

In research published in Physical Review Letters, we have determined how to use the information obtained from these unique natural dark matter detectors more accurately.

Neutron stars are the densest stars known to exist and are formed when giant stars die in supernova explosions. What is left is a collapsed nucleus in which gravity exerts so much pressure on the matter that protons and electrons combine to form neutrons. The mass is comparable to the sun, compressed into a radius of 10 kilometers, and the mass of a teaspoon of neutron star material is about 1 billion tons!

These stars are “cosmic laboratories” that allow us to study the behavior of dark matter under extreme conditions that cannot be replicated on Earth.

The interaction between dark matter and ordinary matter is very weak. For example, it can travel for a light-year (approximately 10 trillion kilometers) without stopping. However, it is unbelievable that the density of neutron stars is so high that they can capture all the dark matter particles that pass through them.

Outer Space
While the existence of dark matter has been inferred, it has yet to be directly observed. Credit: NASA

In theory, dark matter particles would collide with the neutrons of stars, lose energy, and be trapped by gravity. Over time, dark matter particles will accumulate in the core of the star. This is expected to heat up the old, cold neutron star to levels possible in future observations. In extreme cases, the accumulation of dark matter can cause the star to collapse into a black hole.

This means that neutron stars can allow us to detect certain types of dark matter that are difficult or impossible to observe in experiments on Earth.

On Earth, dark matter experiments look for tiny signals of nuclear recoil, which are caused by very rare collisions of slow-moving dark matter particles. In contrast, the strong gravitational field of a neutron star accelerates dark matter to a quasi-relativistic speed, leading to higher-energy collisions.

Another problem with Earth exploration is that nuclear recoil experiments are more sensitive to dark matter particles with a mass similar to the nucleus, so it is difficult to detect dark matter that may be lighter or heavier.

However, in theory, dark matter particles can be trapped in large numbers in stars and planets, no matter how light or heavy they are.

A key challenge in using neutron stars to detect dark matter is to ensure that the calculations used by scientists fully take into account the unique environment of the star. Although the capture of dark matter in neutron stars has been studied for decades, existing calculations ignore important physical effects.

Mathematics Formulas Calculations
The calculations used to detect dark matter in neutron stars need to fully account for the star’s unique environment.

Therefore, our team set out to make key improvements in calculating the dark matter capture rate (that is, the speed at which dark matter accumulates in a neutron star), which greatly changed the answer.

Our research correctly explained the structure of nucleons, instead of treating neutrons as point particles, and included the influence of strong forces between nucleons, instead of modeling neutrons as particle-free gas. This builds on our previous work. We combine star composition, relativistic effects, quantum statistics, and gravitational focus.

In short, we showed how to correctly think about dark matter collisions in the extreme environment of neutron stars, which is very different from dark matter detectors on Earth.

This new research has greatly improved the accuracy and robustness of our estimation of the dark matter capture rate. This paved the way for us to better determine the strength of the interaction between dark matter and ordinary matter.

In the end, the evidence (or lack of evidence) of the accumulation of dark matter in stars will provide valuable clues to the goals of experimental work on Earth and help unlock the mysteries of dark matter.

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