Dark Matter

Dark Matter Map with Moon

A group of us at the Jodrell Bank Centre for Astrophysics are members of a large international collaboration called the Dark Energy Survey (DES). The goal of DES is to study the unexplained acceleration of the universe nicknamed “Dark Energy”.  We’re doing this with one of the largest sky surveys that’s ever been made, which will map out a huge 5000 square degrees, or about 1/8th of the entire sky, out to very large distances in the Universe over the course of five years using the DECam camera on the Blanco Telescope in Chile.

There are many different ways we can use DES images to probe cosmology – we can track the expansion of the universe with distant supernovae, or look for the largest galaxy clusters in the universe, for example.  But at Manchester we’re particularly interested in one of the newest kinds of observations that you can make with a telescope as powerful as DECam: weak gravitational lensing.

As the light from very distant galaxies crosses the universe to reach us it (surprisingly) doesn’t travel in a straight line.  Just like everything else, light is affected by gravity, and so the light rays are bent and distorted from their original paths.  We call this “gravitational lensing”, because the matter between us and the object acts like a giant lens, magnifying (and distorting) the object.  In a few cases we can see this effect very clearly, where the light is bent into giant and beautiful arcs – do a web search for “Strong Lensing” to see some fantastic images.

But we are interested in a much subtler effect, called “Weak Lensing” – where the distortion is much smaller, and not something you can see just by looking at a single galaxy.  But because every galaxy in the universe is lensed at least a little, if we can measure tiny distortions to galaxy images across a wide area of sky we can make a mass map, a projected 2D map showing how matter is distributed across the universe.  Before maps like this the only things we could detect with our telescopes were objects bright enough to be seen across cosmic distances – the magic of this technique is that it shows us everything regardless of how bright it is, including the strange Dark Matter that dominates cosmic structure.

That’s what DES has just released – a map of a small chunk of sky that we took in our first year of data.  I say small, in fact it covers hundreds of times the area of the full Moon and involved us measuring the shapes of about two million galaxies! Even so it’s just a taster for the much larger maps, thirty times bigger, that we’ll be making over the next 5 years or so. And as we analyse these maps we’ll learn about the evolution of the universe – how the great clusters and superclusters that make up the largest scales in the cosmos have grown across the last five billion years.  And hopefully, eventually, this can help us unlock the secrets of Dark Energy, one of the biggest mysteries in the whole of science.


Blogger details – Dr Joe Zuntz is a Research Associate at the School of Physics and Astronomy, the University of Manchester

Twitter: @joezuntz https://twitter.com/joezuntz

Web: http://www.jb.man.ac.uk/~zuntz/