The Dark Universe
One of the examples of mega-surveys that are forming a trend in astronomy are weak lensing surveys which can be used to look for dark matter. To find out more, Dominic Ford asked Catherine Heymans, from the University of Edinburgh, how we know that dark matter exists...
Catherine - So, let's think about our own Milky Way galaxy. So, we've got stars that are spinning around in our own Milky Way galaxy and you can look how fast they're moving around and you can do this in lots of galaxies as well. If you imagine a piece of string and a ball on the end of a piece of string, you can swing that ball around. Now, the faster you swing that ball around, the tighter you've got to hold on to the string. Okay, what does this mean for our galaxy? Well, the ball is now your stars and there's no string that holding the stars as they go round our galaxy. It's gravity. You get lots of gravity if you've got lots of mass, and we can see how fast the stars are moving around. We can roughly count how many stars there are in a galaxy. We know roughly how much a star weighs. And there just simply isn't enough gravity in our galaxy to keep those stars going round. They're just moving too fast and so, that means there must be extra mass there which we call dark matter to keep those stars going around. If there wasn't a big halo of dark matter around our own Milky Way galaxy, all the stars would just simply fly out into the universe.
Dominic - Now, I know you're using a technique called weak gravitational lensing to probe dark matter. How does that work?
Catherine - Einstein had a wonderful theory of general relativity that told us that mass bends space and time. Now, what that means is that if we look at light from the very distant universe, as it comes towards us on earth, it doesn't travel in straight lines. It gets bent by gravitational potential of the matter. So, we've got lots of dark matter in between us and the distant universe. The light's coming towards us from the distant universe, it gets bent and distorted by the dark matter, and what that does is it leaves an imprint on the images of very distant galaxies. You can imagine it like dark matter just leaving a signature on images of the distant universe, "I was here." What it looks like to us on earth, is it looks like the galaxies are kind of lining up with each other. And so, that's the signature that we look for and when we see lots of galaxies all lining up with each other, what we infer is that there's been some dark matter in between us and them that's bent the light to make it look like these galaxies are lining up. We can use this to tell us where the dark matter is and how much of it there is, and that's the technique that we've used to map out dark matter on very, very large scales in our universe.
Dominic - So, is the idea here that if these galaxies were all lining up in the same orientation, there's got to be either some cosmic conspiracy which means those galaxies were all in the same orientation or there's some physics that's making them look that way.
Catherine - So, there is an effect that if you have two galaxies that are actually physically close to each other in the universe, when they form, they will tend to line up with each other, and we have to account for that as well. But what we do is we look at galaxies that even though they look like they're close to each other on the sky, they're actually physically very distant. And then we know that they can't possibly know about each other, so they can't possibly be lined up with each other. So, if we see an alignment, we know it has to have been caused by some intervening dark matter in between us and those galaxies.
Dominic - Dark matter has been in cosmology for what? It must be 80 years since the work of Fritz Zwicky. This must be quite a well-tested theory by now.
Catherine - It is a very well tested theory and there is lots of evidence out there to show that there is dark matter in our universe. Some would say an overwhelming body of evidence supporting this theory of dark matter. The question is now, what on Earth is it? So, we know that it's weakly interacting, we know that it's cold, but nobody has detected the particle yet. So, there have been some exciting new developments in the UK recently where a new consortium of all of the particle physicists who were involved in this type of research have joined together to support a new project called the LUX ZEPLIN detector. It's going to be 9 tons of xenon deep under the ground, I believe in South Dakota, 5,000 feet underground and the hope is, that one of these weakly interacting mattered particles will collide with a nuclei in this big detector, some electrons will be emitted and they'll be detected. And that's the hope that we'll actually be able to detect one of these particles and construction on that project begins next year.
Dominic - So, that's what they call astroparticle physics where you're looking for the particle which makes up this mass that we can't see, this dark matter.
Catherine - Exactly. So, as astronomers, we work on mapping the dark matter and using simulations of what the dark matter particle might be to predict what our observations should look like. At the moment, this theory of the cold dark matter particle, the simulations that we have of the universe with that theory match our observations. Now, we know that we've observed this type of dark matter, can we actually go and detect it? And it's really, really tough. As astronomers, we detect it through its gravitational effects. The particle physicists are trying to detect it through an actual reaction which is really challenging to do.
Dominic - And I guess on the astronomy side, are you looking to push these weak lensing surveys further?
Catherine - Exactly. So, we've just finished a big project called the Canada-France-Hawaii Telescope Lensing Survey. So, you can check it out on www.cfhtlens.org and you can see all of the exciting results that we've done with that survey, mapping out dark matter on the largest scales ever seen before. And now, we're moving on to a new project. It's a European Southern Observatory Project called the Killer Degree Survey or KIDS and that's going to be 10 times larger than the current project that we've worked on. And that's going to allow us to map out even more dark matter and see how dark matter is affecting the galaxies in our universe. We believe that dark matter dictates where and when the galaxies form, but we want to understand more about that.