Astronomers have to be extra clever to map out the invisible dark matter in the universe. Recently, a team of researchers have improved an existing technique, making it up to ten times better at seeing in the dark.
Dark matter is frustratingly difficult to measure. It’s completely invisible: it simply doesn’t interact with light (or normal matter) in any way, shape, or form. But we know that dark matter exists because of its gravitational influence on everything around it – including the normal matter that makes up stars and galaxies.
As an example of this, take a look at gravitational lensing. A massive object, whether made of dark or normal matter, will bend the path of any light that passes close by. It’s usually an incredibly tiny effect, but definitely measurable. We can see lensing of starlight around the sun, for example, which is how we knew that Einstein’s theory of general relativity must be correct.
When light from a very distant galaxy passes through or a near a slightly-less-distant massive object, like a galaxy cluster, that light gets bent. We can use that distortion to measure the amount of dark matter in the galaxy cluster, and is one of our primary ways of mapping dark matter in the universe.
But we usually don’t get this sort of lucky alignment, so instead we have to turn to a different technique: weak gravitational lensing. To make this work, astronomers take a survey of a whole slew of galaxies and look for tiny little distortions in each one. Individually, it’s not much, but taken together it can provide a map of a large region of dark matter.
Recently, a team of astronomers improved on this idea by adding in the rotation of the observed galaxies, as reported in a recent paper published in the Monthly Notices of the Royal Astronomical Society.
Since we know how galaxies ought to rotate, we can combine the distortions in their shapes with the distortions in their internal movements to get a lot more bang for the observational buck, yielding a technique to map dark matter up to ten times more powerful than shapes alone.