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.
That doesn’t mean that dark matter doesn’t exist. Certainly Prof. Aprile believes that dark matter is still out there because her team is building an even more sensitive instrument, the XENON1T, to try to observe weaker and weaker interactions between WIMPs and Xenon. And the majority of the physics academic community agrees with Prof. Aprile that dark matter exists in some form. But what that form is seems to be more and more mysterious. The only concrete evidence scientists have ever found for dark matter is in its gravitational effects on other masses. For example, when two galaxies colide (a bullet cluster in astronomy lingo) extra, non-observable mass seems to be affecting how the collision unfolds. There’s no radiation or particle interactions that give us an idea on what that mass might really be. That’s caused some physicists to develop theories for gravity that modify Newton’s gravitational equations to explain the problem of galatic rotation. But while Modified Newtonian Dynamics (MOND) and its variations do a good job of modeling galatic rotation, they don’t get us closer to understanding why they behave that way.
Dark matter has also retained support because of its usefulness in creating simulations of our cosmological history. A great example of this is study done by Eli Visbal and his Harvard team this summer. They created a mathematical model that predicts the development of early galaxies and clusters within a 1.3 billion lightyear cube of space. By filling this cube with dark matter, as well as the mixture of gasses that is known to be crucial for the creation of stars, the team was able to see that incipient galaxies formed around and within clumps of dark matter, while vast amounts of void would appear between galaxies. Visbal’s team determined that these dense, hot clouds of gas in the universe’s early proto-galaxies would would emit low radio signals, signals that we could theoretically measure from Earth. If such signals are found by a team of experimentalists then it would lend a lot of support to the dark matter theory.
Full disclosure: this Bwogger is hoping that it turns out that dark matter doesn’t exist. Why? The super scientific reason that a certain bet was made with a certain suitemate freshman year…
Science via Wikimedia Commons