by Astronomers continue to struggle with their quest dark dark, the obscure and mysterious form of matter that dominates the mass of the universe. But one astronomer suggests that instead of building huge, expensive experiments on Earth, we should try another method of searching for dark matter: stars.
Multiple independent lines of evidence suggest that it is dark matter there. Something is keeping the stars within galaxies despite their enormous speeds. Something is keeping galaxies within clusters despite the excessive motion. Something is bending the path of light around massive objects. Something started building the large-scale structure of the universe long before ordinary matter had a chance.
All that “something” currently sits under the label of dark matter, and the vast majority of scientists believe that dark matter is some kind of new type of particle that is not currently included in the Standard Model of particle physics and avoids direct detection because it hardly, if ever, interacts with light or ordinary matter.
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In this picture, dark matter particles stream through every part of the galaxy, including the room you’re sitting in right now. Because these particles are invisible, however, they have gone unnoticed. Scientists have spent the last few years building huge laboratories on Earth to try to find the interaction of a ship with a dark matter particle, to no avail.
But nature has its own dark matter labs, and they’re far more powerful than ours, noted Thong Nguyen, an astronomer based at the Academia Sinica in Taiwan, in a paper published to the preprint database. onXiv.
In the standard picture of dark matter, dark matter begins to accumulate early in the universe well before regular matter arrives. Galaxies, which make up less than 20% of the total mass in the universe, are simply pockets of lit-up material that have fallen into these dark matter gravitational wells. So there should be much more dark matter sitting in the core of each galaxy, with a density thousands of times higher than the dark matter out in the solar neighborhood.
So, if we want to hunt for dark matter, we’ll have a much tougher time if we stick to Earth-based experiments, because the local dark matter density is so low. On the other hand, it’s not like we can launch an experiment flying 25,000 light years away into the galactic center. So how can we directly search for dark matter there?
The answer, according to Nguyen, is no neutron starsthe hearts of the giant stars left behind after their departure supernova. After black holes, neutron stars are the most dense objects in the universe; a typical neutron star packs two to three suns worth of material into a size smaller than Manhattan. Their densities are so high that they are essentially atomic nuclei a mile wide. If they were closer, they would simply fall into black holes.
The density of a neutron star is critical to its use as a dark matter detector. Although interactions between dark matter and regular matter must be rare (otherwise, we would have observed dark matter by now), they need not be completely absent. If there is any small, rare interaction between the two types of matter, it is more likely to occur within a neutron star, simply because there are many things to interact with crammed into one.
Neutron stars are found throughout the galaxy, but they are especially common in the core, which is also a hive of galactic activity. In the galactic core, there is more material to build stars and more interactions, which, when combined, result in higher rates of star formation. This leaves many dead remnants, such as neutron stars. In fact, there could be as many as a thousand neutron stars within a few light years of the center of the galaxy.
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If dark matter interacts with normal matter, then when it collides with a neutron star, it will have so many interactions that it will lose energy. Over millions of years, a significant amount of dark matter can accumulate inside neutron stars. The high density of dark matter is susceptible to annihilation, in which two dark matter particles interact and annihilate each other, decaying into other particles in the process. These other particles then go on to produce many other products, such as neutrinowhich can escape from the neutron star.
Ar Worldwe have many neutrino telescopes, like the IceCube Neutrino Observatory at the South Pole in Antarctica. Nguyen used publicly available data from these telescopes to search for a neutrino excess signal coming from the galactic center. Although he found no conclusive evidence of the existence of dark matter, he was able to limit the ability of dark matter to interact with normal matter through this process.
Although we still lack a complete understanding of dark matter, ideas like these, especially regarding the use of nature as a testing laboratory, will help narrow the search and hopefully reveal the identity of this elusive particle.
