When you look up at the night sky, you might suppose that all the galaxies are randomly scattered across the Universe. Like raindrops falling on the ground. But you’d be wrong. When mapped out, they look like dew drops on a spider’s web.
This is just one of the discoveries made possible by increasingly detailed maps of the Universe. Our Professor of Cosmology, Dr David Bacon, is involved in making these maps. He looks at how light and radio waves travel through the Universe, using optical and radio astronomy.
I would like, in my lifetime, to have helped make a complete map of the observable Universe
There are hundreds of billions of galaxies, each made of hundreds of billions of stars. David explains:
‘With numbers that big it’s hard to imagine that mapping them is possible. But that’s just what we’re doing. We can map in 2D just by looking at where they are in the sky. But we can then work out how far away they are and make the map 3D.’
Mapping the Universe
It’s this 3D mapping which revealed the web-like pattern of galaxies. But what causes the pattern?
Mapping the galaxies tells cosmologists a lot about the distribution of matter and energy. And it turns out that there is an underlying web of material. But it’s not made of atoms, like humans or the galaxy. It is dark matter.
Unlike with atomic entities, light can travel through dark matter. But dark matter is heavy, so the light bends. And there’s much more dark matter than normal matter out there. All of it bending the light.
David and his colleagues have been mapping dark matter. Using optical astronomy, they measure the shapes of distant galaxies. They’re looking for distortions caused by the light being bent on its journey to Earth.
‘We know what galaxies normally look like. So I can make a map of the dark matter that’s bent the light, revealing the web shape. Overlaying this with a map of galaxy clusters reveals that the galaxies typically lie like dew drops along that web.’
Tuning in to radio
Radio telescopes are now being used too:
‘We’re trying to make an image of the web as it glows in radio. At first it will be quite low resolution, because we’re trying to do it for the first time. But over time we’ll get a much clearer image so we can fine tune the maps.’
It’s at the point of fine tuning that radio astronomy will come into its own. Making 3D maps with optical telescopes is incredibly time consuming. Each section of the sky has to be observed separately. And the light from each galaxy has to be broken down into a spectrum and measured, to work out how distant that galaxy is.
Radio sidesteps these issues:
‘We take an image of how intense the radio waves are in a very thin range of radio frequency. A very thin slice of distance away. And we can do this for multiple frequencies, all in the same radio image. So we get this block of radio information all at once. It’s coarser but it’s really fast at getting the whole 3D map as a block. It’s much more efficient, just not as advanced as optical.’
Filling in the gaps
At the moment this map of the Universe is far from complete. Until recently it was only possible to see the really far Universe and the near Universe but not what’s in-between.
Cosmologists can see far away, near the time of the Big Bang, because everything was extremely hot and so can be detected. And what’s nearby can be seen with optical telescopes.
But in the middle, everything is much cooler. So how do you see this?
It turns out that the neutral gas here is emitting slight radio waves. As radio telescopes become sufficiently sensitive, they will reveal these ‘dark ages’. And this data will enable David to further map the sky.
‘I would like, in my lifetime, to have helped make a complete map of the observable Universe.’
This mapping is proving to have other applications. The techniques of correlating features of different maps can help make risk maps - vital for small island developing states, like Fiji, at risk from flooding and tsunamis.
These methods of analysing big data sets are helping answer the big curiosity questions of the cosmos. But also having a positive impact back on Earth.