What if I told you that while you can\u2019t see dark matter, maybe you can hear it? I know, I know, it sounds crazy\u2026and it is crazy. But it\u2019s crazy enough that it just might work. It\u2019s a real life experiment, called the\u2026let me see here\u2026the Cryogenic Rare Event Search with Superconducting Thermometers, or CRESST \u2013 that\u2019s a double s in case you didn\u2019t catch that. Look it\u2019s not the greatest of acronyms but we\u2019re going to just go with it.<\/p>\n
The experiment is buried deep under the Gran Sasso Mountain in central Italy (which, by the way, gran sasso is Italian for \u201cgreat rock\u201d in case you need some bonus trivia on your next date night), and it consists of a giant crystal of calcium tungstate, also known as Scheelite. The crystal is cooled all the way down to just a few millikelvin, right on the edge of its superconducting state. The idea is that if a dark matter particle happens to intersect with the detector, it will cause the crystal to vibrate (the energy has to go somewhere, after all), which will mess up the superconducting state and produce a detection.<\/p>\n
CRESST has been operating for years and has yet to make a confirmed detection of a dark matter particle. But in science, an experiment is only a failure if you don\u2019t learn something, and we\u2019ve definitely learned about dark matter with the CRESST experiment. Specifically, we\u2019ve learned what dark matter isn\u2019t. We have some limits from our observations of the wider universe, and we\u2019ve been able to eliminate a lot of possibilities, like neutrinos. But after that it\u2019s fair game. That means we have two jobs. On one hand we need to come up with reasonable and well-motivated candidates, which involves a lot of creative brainstorming, theorizing, and guesswork. And on the other hand we need to come up with experiments that can rule out specific candidates, or failing that put some more precise limits down on what kinds of properties the dark matter can have.<\/p>\n That\u2019s how we end up with graphs like that, which are showing how that particular experiment ruled out certain mass ranges and interaction strengths of the dark matter. By simply running the experiment and not seeing anything after a while, you can confidently say which properties of dark matter are excluded: you can say that the dark matter CANNOT be this particular mass with this particular interaction strength. And then you do it again, and again, and again.
\nIn published papers about dark matter detection experiments you\u2019ll often find charts and graphs that look like this<\/a>. Remember that we don\u2019t know the identity of the dark matter particle. We have some guesses based on various theories and concepts, but at the end of the day we don\u2019t know the precise properties of that particle. Specifically, we don\u2019t know its mass or how easily\/rarely it interacts with normal matter.<\/p>\n
\nAnd of course you don\u2019t run just one experiment. Remember, the dark matter could have a wide variety of masses and interaction strengths, and no single experiment is able to cover the entire range. <\/p>\n