Each week, amateur astronomer Zach Kagan watches the skies for signs of SCIENCE. We present here this week’s findings—a special offering that highlights the search for dark matter.
Dark Matter is a term that’s thrown around a lot when people talk about unsolved mysteries in astrophysics. You’ve probably heard about the stuff, but you may not know what it is, which is fine because neither do the astrophysicists. The problem is that galaxies don’t rotate the way that they should. We predicted that outer stars should move much slower than inner cluster stars. However, they tend to move at similar velocities on the galaxy’s edge. That isn’t possible according to our understanding of gravity, so an explanation was developed: there must be much more mass in the galaxy than is observed. Much much more. Around 20 times more. And all that mass, which drives the rotation of outer stars, must completely be non-visible.
So what was this stuff? Two theories emerged. The first supposed that all this dark matter was made up of large, dim objects like black holes, rogue planets, and dense neutron stars. These objects were collectively called MACHOs (Massively Compact Halo Objects). The second theory supposed that dark matter was made up of Weakly Interacting Massive Particles, or WIMPs because scientists have a sense of humor too. Over time, as experiments found negative results for MACHOs, WIMPs became the popular theory for the nature of dark matter, even though no one was sure what WIMPs were anyway.
But WIMPs are elusive, as Columbia astrophysics professor Elena Aprile has found. Prof. Aprile led a team of scientists at Italy’s Gran Sasso laboratory on a 13 month search for the most promising WIMP candidates. The Columbia team built a sophisticated device called the XENON100, which uses ultra dense liquid Xenon to sense rare collisions with the faint WIMP particles. The device is brought deep—roughly 5,000 feet—underground in a chamber lined with copper and lead to filter out the hailstorm of particles we normally experience on the surface (such as cosmic and background radiation). However Aprile’s team didn’t find anything, not even after 225 full days of data gathering by XENON100.