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How do scientists hunt for dark matter? A physicist explains why the mysterious substance is so hard to find

  • Written by David Joffe, Associate Professor of Physics, Kennesaw State University
How do scientists hunt for dark matter? A physicist explains why the mysterious substance is so hard to find
Curious Kids[1] is a series for children of all ages. If you have a question you’d like an expert to answer, send it to CuriousKidsUS@theconversation.com[2]. Can we generate a way to interact with dark matter with current technology? – Leonardo S., age 13, Guanajuato, Mexico That’s a great question. It’s one of the most difficult and fascinating problems right now in both astronomy and physics, because while scientists know that the elusive substance called dark matter makes up the majority of all matter in the universe, we’ve never actually observed it directly. Dark matter[3] is so difficult to interact with because it’s “dark,” which means it doesn’t interact directly with light in any way. I’m a physicist[4], and scientists like me observe the world around us mainly by looking for signals[5] from different wavelengths of light. So no matter what type of technology scientists use, they run into the same issue in the hunt for dark matter. It’s not completely impossible to interact with dark matter, though, because it can interact with ordinary matter in other ways that don’t involve light. But those interactions are generally very weak. What we call dark matter[6] is really anything that we can see only through these weaker interactions, especially gravity. How we know dark matter exists One way that dark matter can interact with ordinary matter is through gravity. In fact, gravity is the main reason scientists even think dark matter exists at all. For decades, scientists have been observing how galaxies spin and move throughout the universe. Gravity acts on stars and galaxies, in the same way it keeps you from floating off into space. Heavier objects have a stronger gravitational pull. At these huge scales, researchers have spotted some unexpected quirks that gravity alone can’t explain. For example, almost 100 years ago, a Swiss astronomer named Fritz Zwicky[7] studied a cluster of galaxies called the Coma Cluster[8]. He noticed the galaxies inside it were moving very fast, so much so that they should have flown apart many millions of years ago. The only way the cluster could have stayed together for so long is if there was much more matter holding it together with gravity than the telescope could see. This extra matter necessary to hold the galaxies together became known as dark matter. About 40 years after Zwicky, an American astronomer named Vera Rubin[9] looked at the individual stars moving around the centers of spiral galaxies[10] as they rotated. She saw that the stars at the outside edges of the spiral were moving much faster than you’d expect if only the gravity from the stars you could see was keeping them from flying off into intergalactic space. Just as with the galaxies moving around the cluster, the motion of the stars around the edges of the galaxies could be best explained if there was much more matter in the galaxies than what we could see. A spiral-shaped galaxy with a bright spot in the center
A rotating spiral galaxy in the Coma Cluster. NASA, ESA, and the Hubble Heritage Team (STScl/AURA); Acknowledgement: K. Cook (Lawrence Livermore National Laboratory)[11]

More recently, scientists have combined optical telescopes[12] that observe visible light with X-ray telescopes. Optical telescopes can take pictures of galaxies as they move and rotate. Sometimes, galaxies in these images are distorted or magnified by gravity coming from large masses in front of them. This phenomenon is called gravitational lensing[13], which is when the gravity around a very heavy object is so strong that it bends the light passing by it, acting like a lens.

X-ray telescopes[14], on the other hand, can see the clusters of hot gases[15] that surround galaxies. By combining these two telescopes, astronomers can see galaxies as well as the gases surrounding them – all the observable matter. Then, they can compare these images with the optical results. If there’s more gravitational lensing seen than what could be caused by the gas, there must be more mass hiding somewhere and causing the lensing.

Clouds of blue and pink shown, with lots of bright spots representing galaxies shown in the background.
The picture combines optical images of the galaxies with X-ray images. The region in the pink shows the area where the X-ray telescope sees the distribution of gas around the galaxies, and the blue area shows the region where gravitational lensing can be observed. There is blue in places where there isn’t pink, so lensing is showing that there’s something else heavy there. Dark matter is again the best explanation. NASA, ESA, CXC, M. Bradac (University of California, Santa Barbara), and S. Allen (Stanford University)[16]

How we might be able to see dark matter

Unfortunately, all this tells astronomers only that dark matter must be there, not what it really is. The evidence for dark matter is all based on how it interacts with gravity at very large scales. It’s still “dark” to scientists in the sense that it hasn’t interacted directly with any measurement devices.

The good news is that light and gravity aren’t the only forces in the universe. A force called the weak force[17] might be able to interact directly with dark matter and give scientists a direct signal to observe. Most of the ideas about what the dark matter might be include the possibility of it interacting through the weak force, converting energy into signals that are visible.

The weak force is not observable at normal scales of distance. But for objects the size of an atom’s nucleus[18] or smaller, it can change[19] one type of subatomic particle into another. The weak force can also transfer energy and momentum at very short distances – this is the main effect scientists hope to observe with dark matter. These processes might be extremely rare, but in theory they should be possible to see.

Most experiments looking to see dark matter directly are searching for signals of rare weak interactions in an underground detector[20], or for gamma rays that can be seen in a special gamma-ray telescope[21].

In either case, a signal from dark matter would likely be very faint, resulting from an interaction that can’t be explained any other way, or a signal that doesn’t seem to have any other possible source. Even if the effect is faint, it might still be possible to observe, and any such signal would be an exciting step forward in being able to see the dark matter more directly.

In the end, it may be a combination of signals from experiments deep underground, in particle colliders, and different types of telescopes that finally lets scientists see dark matter more directly. Whichever technology ends up being successful, hopefully sometime soon the matter that makes up our universe will be a little less dark.

Hello, curious kids! Do you have a question you’d like an expert to answer? Ask an adult to send your question to CuriousKidsUS@theconversation.com[22]. Please tell us your name, age and the city where you live.

And since curiosity has no age limit – adults, let us know what you’re wondering, too. We won’t be able to answer every question, but we will do our best.

References

  1. ^ Curious Kids (theconversation.com)
  2. ^ CuriousKidsUS@theconversation.com (theconversation.com)
  3. ^ Dark matter (theconversation.com)
  4. ^ I’m a physicist (scholar.google.com)
  5. ^ looking for signals (theconversation.com)
  6. ^ dark matter (science.nasa.gov)
  7. ^ Fritz Zwicky (www.britannica.com)
  8. ^ Coma Cluster (science.nasa.gov)
  9. ^ Vera Rubin (theconversation.com)
  10. ^ spiral galaxies (science.nasa.gov)
  11. ^ NASA, ESA, and the Hubble Heritage Team (STScl/AURA); Acknowledgement: K. Cook (Lawrence Livermore National Laboratory) (science.nasa.gov)
  12. ^ optical telescopes (letstalkscience.ca)
  13. ^ gravitational lensing (science.nasa.gov)
  14. ^ X-ray telescopes (www.britannica.com)
  15. ^ clusters of hot gases (theconversation.com)
  16. ^ NASA, ESA, CXC, M. Bradac (University of California, Santa Barbara), and S. Allen (Stanford University) (science.nasa.gov)
  17. ^ the weak force (www.energy.gov)
  18. ^ atom’s nucleus (www.energy.gov)
  19. ^ it can change (www.sciencedirect.com)
  20. ^ in an underground detector (theconversation.com)
  21. ^ gamma-ray telescope (science.nasa.gov)
  22. ^ CuriousKidsUS@theconversation.com (theconversation.com)

Authors: David Joffe, Associate Professor of Physics, Kennesaw State University

Read more https://theconversation.com/how-do-scientists-hunt-for-dark-matter-a-physicist-explains-why-the-mysterious-substance-is-so-hard-to-find-269876

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