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Zeroing in on Dark Matter: MACHOs Vs. WIMPs

November 5, 2018


One interesting thing about science is that sometimes, some of the smartest people in the world can get something wrong - but it doesn’t make an experiment a failure. It’s just a null result, like some of history’s most important experiments, and it can still teach us a lot about the universe. And that’s exactly what happened at the world’s most sensitive dark matter detector. 


Back in 2016, astronomers announced that after twenty months of searching, they still hadn’t detected a single sign of dark matter. Everything that astronomers can see in the universe is made out of the same kind of matter that we are - stuff like protons and electrons. That’s why they tend to call it “ordinary matter.”


But astronomers have known for almost a hundred years that there is more than just ordinary matter out there.


By looking at how things like how quickly stars orbit the centers of galaxies, and how light gets distorted by gravity, they’ve realized that about 85% of the mass in the universe is what’s known as dark matter. Dark matter doesn’t emit any light - hence the name. So nobody has ever actually seen dark matter, but we know it’s there because of its gravitational pull.


Some scientists think that dark matter is black holes left over from the early universe, which is one of the things that gravitational wave detectors like LIGO are starting to test. Comparable to life on Earth, space classifies dark matter as one of two things: WIMPs and MACHOs (Weakly Interacting Massive Particles and Massively Compact Halo Objects). And dark matter could be made of lots and lots of light WIMPs, or fewer, but heavier WIMPs.


The Large Underground Xenon - or LUX - dark matter experiment is looking for these WIMPs.



LUX is exactly what it sounds like: a huge underground tank of xenon that’s designed to detect dark matter. See, WIMPs would interact with ordinary matter through the weak nuclear force - the force responsible for things like radioactive decay. And those interactions can leave traces, if you know where to look. So LUX used a tank with a third of a metric ton of liquid xenon, in the hope that a WIMP would come and crash into one of the xenon atoms every once in a while. That would make the xenon release a shower of particles that would work their way out of the tank to a collection of incredibly sensitive detectors.


Xenon is perfect for this kind of experiment because it’s really sensitive to the weak force, but it’s not so sensitive that it releases particles on its own, without being hit by a WIMP. And while there are other WIMP detectors out there, LUX is the most sensitive one in the world.


The scientists ran the project for twenty months, ending in May of 2016. And after weeding through all of their data, they’ve announced that they didn’t see evidence of a single WIMP.


But that doesn’t mean there aren’t any WIMPs out there.


LUX’s findings suggest that WIMPs, if they’re out there, must be hidden better than most people assumed. So even though it found nothing, the LUX dark matter experiment excluded a huge range of lighter WIMP models.


Null results like these can advance physics and astronomy just as much as the big groundbreaking discoveries that everyone hears about. 


LUX is shut down for now while it’s being upgraded. By 2020, the researchers hope to have 30 times as much xenon in the tank. And with more xenon and other improvements, LUX’s second run should be at least seventy times more sensitive than the first one was.


That’ll let it search for a much wider range of WIMPs, including those more massive ones. So in just a few years, we might know for sure whether dark matter is made of WIMPs. And if it’s not, then physicists will know that they should start looking somewhere else. Either way, we’ll know a whole lot more about the universe.


Thank you for choosing SkyFeed! Be sure to follow my social media (Instagram: @astrolia) and subscribe to get the best of space exploration delivered to your inbox every week.


Source: This story was originally published on SciShow Space. I am republishing a lightly edited version on SkyFeed in light of interest on the subject. Green, Hank. "Zeroing in on Dark Matter." SciShow Space, YouTube. 29 July, 2016. Web video. 

Additional: Carroll, Sean. "Dark Matter: MACHOs or WIMPs?" Great Courses Daily, California Institute of Technology. Web article. Accessed 3 Nov, 2018. Citation: Rovira, Lia N. "Zeroing in on Dark Matter: MACHOs Vs. WIMPs." SkyFeed. 5 Nov, 2018. Web article.

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