# The invisible universe

Only a fraction of the universe is visible to the naked eye: how do we observe that which can't be seen?
28 September 2021

## Question

Why do we need to observe the stuff in space that can't be seen?

Matthew Bothwell, Camrbidge University's public astronomer, brings space science down to Earth...

Matthew - The quick answer is that there is an awful lot more that we can't see. I'll give you an analogy. The difference in wavelength between the reddest and bluest light we can see is roughly a factor of two. So, the bluest light we can see is around 380 nanometers and then the reddest light we can see is about 740 nanometers. We have this factor of two in wavelength, which is our window that we use to see the world. There's a nice coincidence. A factor of two in wavelength also has meaning in terms of sound. A factor of two in wavelength is one octave. If you think of red light as being middle C and then the bluest light is the C one oxidative higher. That octave is the window that we use to see the world. The full spectrum of light that arrives from the universe is about 65 octaves. That's nine grand pianos standing in the line, right? If you imagine all those pianos being played at once and you could only hear one central octave you would miss almost all of the music. That's the problem with astronomy. If you're only looking with our eyes you're missing almost all of the information that's out there

Chris - What's the solution?

Matthew - The solution is to build instruments that can see in different wavelengths. I'll give you an example from my research. There's a species of galaxy in the early universe that is completely invisible if you look using the light we can see with our eyes, what we call optical light. These galaxies are amazing. They're some of the most powerful factories for stars in the whole of the universe, but they're all cocooned away behind shrouds of dust. No light leaks out, but we go to long wavelengths, like infrared and radio waves and we can see through the dust and discover these amazing galaxies. If you were a biologist and you could just strap on a pair of infrared goggles and see brand new invisible species, that would be the discovery of a lifetime. That's kind of what we can do with astronomy. We can go to these other wavelengths and see things in the universe that we just didn't know existed.

Chris - Is this big business now, is this really the next step forward? People are beginning to explore new wavelengths, new regimes, to see through these dusty shrouds that we couldn't penetrate previously.

Matthew - It is absolutely big business. Yes. All of modern astronomy is this multi-wavelength affair. The job of a modern astronomist is to collect all of the data from across the spectrum and combine it all into one big synthesised picture of what the universe is doing. There's even stuff beyond that. Modern cosmology tells us that most of the universe is invisible, full stop, it's made of dark matter dark energy that we can't see in any wavelength. It's big business to understand the things that we can see and then it's even bigger business in the future to figure out the rest of the universe that is, at the moment, completely baffling.

Chris - Is it not also true that a sizable chunk of the universe is off limits to us anyway? It's growing and growing away from us faster than we could ever reach. Even if we could travel at the speed of light we could never get there and enjoy the restaurant at the end of the universe because it would have invented more universe by the time we got there.

Matthew - Yeah. That's exactly true. There's this thing called the cosmic horizon, which is a bit like the horizon on earth. It's the bit that you can't see. It's a very long way away. It's about 46 and a half billion light years to the edge. Beyond that we just don't know, right... The universe might be infinite or it might have an edge. We might never know because we are trapped inside our cosmic horizon.