Dark matter is a mysterious substance that has evaded all of our attempts at directly detecting it in our experiments on Earth. Dark matter is an important ingredient of the Universe’s evolution as it is the foundation upon which galaxies form – without it we would not exist in the solar system we find ourselves in today. While we can clearly observe the bulk gravitational effects of dark matter, we have yet to identify its particle nature. In their recent paper, physicists at UCI partnering with NASA, have developed a technique which uses precision measurements on asteroid trajectories to improve our understanding of dark matter. To understand how they accomplish this, we must first ask: where is dark matter?
Our home galaxy, the Milky Way, is shaped like a large disk and Earth is located near the edge. Surrounding this disk is an even more massive dark matter bubble – or “halo” – which holds the Milky Way inside. Like riding a carousel and feeling a breeze, as our disk spins, the Earth sweeps through the dark matter bubble and encounters a dark matter “wind”. This dark matter wind is composed of dark matter particles, which experiments on Earth aim to detect; but so far the dark matter particle is so dark that it hasn’t interacted with any of our detectors here on Earth. But just as an airplane can encounter pockets of denser air and experience turbulence, it is possible that the Earth can encounter denser pockets of dark matter and these experiments might finally feel the subtle bump of a dark matter particle.
If the Earth encountered one of these dense pockets of dark matter where there is a more than average amount – in technical terms, an “overdensity” or “subhalo” of dark matter – our experiments would have a better chance of finally detecting it. Or, if we knew that we were currently in an overdense clump and we still couldn’t detect it, then we know that this mysterious stuff is even darker than we thought. So we’ve reached a puzzling problem: we need to know how much dark matter is around the Earth to understand it better, but we can’t measure how much is around us in the first place. Are we out of luck, or is there some way that we can map out the overdense clumps of dark matter? UCI physicists Jason Arakawa and Yu-Dai Tsai, have proposed a technique which uses the orbital trajectory of asteroids to look for these clumps of matter in our solar system.
There are about one million asteroids in orbit between Mars and Jupiter. Some of these, so-called “near-Earth objects”, pose potential threats to Earth and thus need to be monitored carefully. These objects have been studied by NASA in thorough detail, which tracked their motion through the solar system over dozens of years. One asteroid in particular, 10995 Bennu, has accumulated unprecedented levels of tracking due to NASA’s OSIRIS-REx mission, which aims to return a piece of Bennu to Earth by September 2023. Bennu is tracked in such detail that we can measure its orbit down to the meter when it is a staggering half a billion meters away; this precision is equivalent to knowing how many grains of sand you’d have to stack to reach the cruising altitude of a commercial jet. This incredible level of precision raises the possibility that dark matter might give a gravitational tug on Bennu, and that we might be able to detect this unexpected change in Bennu’s trajectory. This is the core idea behind the recent UCI paper.
Using Newton’s laws we can predict the motion of the asteroid. If we see that there is some anomalous motion to the orbit, then we know there is something else tugging on the asteroid. Furthermore, any mass that is near the asteroid will have a stronger pull. Using these facts we can constrain the overdensities of dark matter throughout our solar system using asteroids. This measurement provides a way to determine how much dark matter is around us without knowing anything about what the dark matter particle actually is.
While this is the first time asteroids have been used with this method, planets have been used for a similar purpose in the past. Saturn’s orbit, for example, was used to constrain overdensities of dark matter near Saturn to be less than 10,000 times the average density of dark matter. A critical difference between planets and asteroids, however, is that asteroids span much larger orbits and that there are many more asteroids than planets in our solar system. With enough tracking of enough asteroids, we might one day be able to map out the distribution of dark matter around our solar system!
If we could accurately measure this overdensity in the solar system, what could we do with it? With enough data, we might be able to determine when the Earth will encounter a dense blob of dark matter. During this encounter, experiments which look for the interaction of a single dark matter particle will become much more sensitive and we can scrutinize the haystack of data from this encounter to look for a dark matter signal. There are other exciting possibilities that we might see with our own eyes on Earth. For example, physicists have proposed that certain types of dark matter could lead to unusually straight lightning bolts in the sky! With the tracking data of asteroids we might one day know when to look out for these strange dark matter storms!
Whatever dark matter is, it is crucially important that we find it. The particle has constantly evaded every one of our efforts of identifying it, and it is one of the dominant guiding hands of our Universe. Without it, life as we know it would surely not exist. By tracking asteroids we might one day be able to better pinpoint where the dark matter is. The better we know where it is, the better chance we have to figure out what it is.
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https://kids.frontiersin.org/articles/10.3389/frym.2017.00029
Post by Max Fieg, a graduate student at UCI
Edited by Dylan Green