HomeScienceOuter SpaceThe meteor detection technique can be used to find dark matter particles...

The meteor detection technique can be used to find dark matter particles entering the atmosphere

Researchers at Ohio State University have come up with a new method to detect dark matter, based on existing technology for detecting meteors. By using ground radar to search for ionization trails, similar to those produced by meteors as they streak across the sky, they hope to use Earth’s atmosphere as a supersized particle detector. The results of experiments using this technique would help researchers narrow down the range of possible characteristics of dark matter particles.

The existence of dark matter is fairly well accepted by mainstream physicists. Since Lord Kelvin calculated that the mass of all the stars in the Milky Way galaxy was much less than the mass of the galaxy itself, we know that much of the matter in the universe is not visible to us. As technology has improved, we’ve learned how to detect things that were hidden from visible-light telescopes, but we still can’t explain all of the missing matter. We call this missing material “dark matter,” and according to current estimates, 85% of the mass of the universe is made up of dark matter. Most physicists now believe that dark matter is made of an undiscovered particle.

Dr. John Beacom of Ohio State University has proposed an experiment to determine the properties of this particle. He wants to adapt the radar technology used to detect and measure meteors as they streak through the atmosphere, and use it to look for similar streaks that could indicate a collision of a dark matter particle with air molecules. This technique uses ground-based radar stations to detect and measure traces of ionization through the upper layers of the atmosphere.

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When a meteoroid enters Earth’s atmosphere, it rams through the air faster than the air itself can get out of the way. This compresses the air in front of the meteor and makes it so hot that it ionizes – individual air molecules collide so hard that they lose electrons. Ionized air not only glows, but it is also opaque to radio waves. This bounces radar signals back to Earth, allowing meteors to be detected even during the day.

Theoretical physicists have calculated the physical properties that dark matter particles could possibly have. Unfortunately, since most of what we know about these particles is that they interact weakly with normal matter (we’ve only detected it through the influence of gravity so far), this leaves a wide range of possibilities. Dr. Beacom points out that if dark matter particles fall within the larger, heavier end of the range of possibilities, they would more easily interact with “normal” matter, although these interactions would still be rare.

“One of the reasons dark matter is so hard to detect could be because its particles are so massive,” Beacom said. “If the mass of dark matter is small, the particles are normal, but if the mass is large, the particles are rare.”

If these particles are large, traditional ground-based detectors may never see them, because the particles are absorbed by the Earth’s atmosphere. But if this happens, they should have enough energy to produce an ionization track, similar to what we see in meteors. Meteor detection radar installations could therefore be adapted to search for dark matter particles as well – essentially turning the entire Earth’s atmosphere into one giant particle detector.

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The existence of dark matter was first predicted in 1884 by Lord Kelvin. He had calculated the mass of the Milky Way galaxy from the speed at which it rotates, and found that it must be significantly more massive than the combined visible stars. He theorized that most of the galaxy’s mass must be made of “dark” material — things that couldn’t be seen with the telescopes of the day. However, most scientists assumed that meant there would be a lot of cold gas, dust, exoplanets and other objects that don’t shine with their own light. The phrase “dark matter” was first used to describe these things in a French newspaper in 1906.

abell 611 and its galaxies and dark matter
Hubble Space Telescope offers a cosmic web of galaxies and invisible dark matter in the cluster Abell 611. Credits: ESA/Hubble, NASA, P. Kelly, M. Postman, J. Richard, S. Allen

Many other pieces of evidence have emerged since then: Fritz Zwicky observed in the 1930s that galaxies in the Coma cluster move as if the entire cluster were 400 times more massive than the total mass of all its visible members. Early radio astronomers in the 1960s saw spiral galaxies spinning around their edges far too quickly — they should just fly apart unless there was an additional gravitational source to hold them together. Vera Rubin, Kent Ford and Ken Freeman made the same discovery soon after, using recently improved spectrographs to measure the rotational curve of galaxies in visible light. And a series of deep cosmological observations in the 1980s discovered gravitational lensing and anisotropies in the cosmic microwave background radiation (CMBR), adding to evidence for the existence of dark matter.

It’s worth noting that no one yet knows whether dark matter particles will actually produce these ionization trails. Detectors built using this technique may not see anything at all. But any result, detections or no detections, would be a good thing. One way or another, an experiment using this detection technique will answer the question: “Are dark matter particles big, heavy and rare? Or are they small, light and numerous?

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Read the original research paper to learn more about this technique



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