 So for a hundred years we've known that the Milky Way is filled with very, very high energy particles, ultra-relativistic particles, energies beyond what you can accelerate at the large Hadron Collider at CERN. In fact, we know that other galaxies have these particles as well and there's been lots of speculation and also many, many attempts to pin this problem down experimentally. So where are these particles accelerated? How do they propagate? How do they influence galaxies and the clusters of galaxies? And it's been very difficult to make progress on this issue because of a lack of pretty good observational data. So really what we want to do is to take this field of non-thermal astrophysics, the astrophysics of these particles which somehow managed to get an unfair share of the energy. They're not in thermal equilibrium, they achieve these very, very high energies. To put these particles to understand in a quantitative way how these particles influence astrophysical processes and to really establish this field on the same level as our understanding of the thermal universe, which has been very, very thoroughly explored in many wave bands and that has very detailed modelling. This non-thermal astrophysics is like behind, but we believe that it could be equally important in many aspects and that our picture of the universe is incomplete and our understanding of the way that, for example, that galaxies evolve is not going to be complete until we can really put this aspect of astrophysics on the same footing as thermal astrophysics. The big question is by what processes are these particles being accelerated? Where? And then what do they do? What is their impact? And potentially then by looking at the universe and these high energies we can find out, get a completely new perspective on the way the universe works and may have to rethink many things. The method we use is to look for very high energy gamma rays. So these gamma rays are produced in the interactions of accelerated charged particles and travel in straight lines towards the earth. Now gamma rays, luckily for us, don't reach the surface of the earth. They interact in the earth's atmosphere. And for low energy gamma rays, this means that they're basically absorbed and they're gone. But at these terror electron volt energies, we're talking about photons that are a billion times more energetic than x-rays, what happens is that the interaction of the gamma ray produces a cascade of particles in the earth's atmosphere and then individual particles in this cascade can be travelling faster than the speed of light. Of course this sounds implausible, but this is faster than the speed of light in air which is a little bit slower than the speed of light in a vacuum. So these are particles that are ultra-relativistic and they go faster than light in air. And then there is an effect called the Cherenkov effect which is basically producing a blue light in the optical band. And these cascades of particles therefore make a flash of blue light which is beamed in the direction of the shower. And if you put a very, very sensitive telescope on the ground and look for a few nanoseconds long flash of light, a few billionths of a second, you can detect these gamma rays. And if we have several different telescopes all looking from different directions, we can very well reconstruct the direction from which the original gamma ray came. So it's a very indirect way if you like. We use the whole atmosphere of the earth as part of our gamma ray detector. We use these many views of the shower from different directions to reconstruct the direction of the individual gamma ray. And then we have one little point on our gamma ray map of the sky and then we collect many thousands of such events per second. And over time we build up a view of what the high energy sky looks like in this terra-electron bomb domain. When we have recorded individual gamma rays, we reconstruct their direction and their arrival time and their energy. From the cascade we can calculate the energy of each individual photon. So then what we can do is make a map, an image of the whole sky or parts of the sky that we survey. And then we will see individual sources of gamma rays. And for each source we can make the spectrum of gamma rays. We know the energy distribution. And so by exploring the morphology and the spectrum of the gamma ray emission from these sources, we can learn something about the way that they're accelerated. What we find when we look at the universe in these terra-electron photons is that the process of acceleration of particles to these extreme energies seems actually to be rather common in nature. So originally people thought, well, there'll be one particular special source of these cosmic rays that we see at the earth. And people are probably supernova explosions. And indeed we see the shells of supernova explosions. We see that in these blast waves, expanding shells around supernova explosions, that particles are being accelerated unambiguously. But we see many other things as well. So the universe seems to be able to play this trick of particle acceleration in many, many places. It's not a sort of strange and unusual phenomenon. It's a normal astrophysical process. So we see this in the colliding winds from stars, and it's blown by stars. We see close to pulsars and in the nebulae around pulsars in the jets of active galaxies and emission around stellar clusters. So it seems that this is something we can't ignore here, that these non-thermal particles, particles with an unfair share of the energy are everywhere in astrophysical environments. And we really now have to try and understand what they're doing there, what their impact is. And one recent example is the discovery that in the very central part of our own galaxy, in the so-called central molecular zone, we have diffuse gamma ray emission from all of the molecular clouds that we see. And from this gamma ray glow of these clouds, we can infer the density of relativistic particles. And we see that it peaks at the very center of our galaxy. The profile is consistent with a diffusion of particles away from a source, which is basically at the position of the supermassive black hole, Sagittarius A star. And furthermore, the spectrum of the gamma rays indicates that particles are being accelerated close to the supermassive black hole to PV energies, 1,000 tera-electron volts. And this was really unexpected. So in fact, this is the only place in the galaxy we see acceleration up to these energies. And it could be that all of the PV particles we see at the Earth are actually accelerated at the supermassive black hole, which is really against the conventional wisdom. We have for the first time now a real picture of what is happening at TV energies in the cosmos. And there is, from this clear experimental picture, a growing appreciation of the importance of these relativistic particles in astrophysical environments. So for many years, there weren't good measurements. And it was very difficult for people in their cosmological models or their models of the way that astrophysical systems evolved. It was very easy for them to ignore these things or very difficult if they actually wanted to include them because there were no good data. So now I think people don't feel like they can ignore these relativistic particles anymore. So for example, at recent simulations of the way that galaxies evolve, including for the first time in a self-consistent way, these relativistic particles show that galaxies look completely different if you evolve them with cosmic rays in. And also the process of feedback on scales, even of galaxy clusters, we believe now may be mediated to a significant extent by these relativistic particles. We're still lacking experimental evidence there and we need their more sensitive instruments. But there is a growing appreciation, I think, of the importance of these relativistic particles and of the helpfulness of these TV signatures in probing these very extreme environments often and understanding where and how particle acceleration is happening to these energies. The relevance of this is it's one piece of the jigsaw of our understanding of the universe and it's a sort of neglected piece. So putting this piece in has the potential to solve lots of puzzles in astronomy and also change lots of ideas about the way that many systems work or evolve and hence the impact on cosmology and the way we view the history of the universe. So beyond understanding the impact of relativistic particles with TV photons in the future, we hope to search for the annihilation of relic particles left over from the Big Bang, dark matter particles. This is actually a good chance to see a gamma ray signature of such particles and learn about the particle nature of dark matter. We can also look for new exotic particles like axions and look for deviations from Einstein's theory of relativity that might occur because of quantum gravity effects. So because we're working with these very high energy photons, we have lots of potential to explore particle physics questions, if you like, in addition to understanding this astrophysics. So it's clear that if we want to make real progress in this field then we need better instrumentation. We've more or less done what we can do with current instruments and we have a very ambitious plan, which is to construct something called the Cherenkov Telescope Array, which is a big international project, 32 countries, over a thousand scientists are involved. And CTA will be more than a hundred Cherenkov telescopes built on two sites in the desert in Chile and in La Palma. And this instrument will be much more precise, much more sensitive and also cover a much wider energy band. So giving us access for the very first time to energies of hundreds of TV. And importantly, it will also be for the first time a true observatory and the sense that you have in other well-established wavelengths. Not an experiment or some cutting edge and a small team of experts doing it. But we want to have with CTA key science projects to tackle key things but also the opportunity for interested scientists from around the world I want to look at such and such a source, I want to look at Itercarina, I want to look at Hydrae, people will get their data from the observatory. So it's a sort of service mode of observation and we want to open the community up, basically put the TV waveband into the astrophysical mainstream and have people using TV data together with X-ray data and radio data in a routine way. So this is a sort of coming of age if you like of this field of astronomy where we go from just proving that it works if you like and proving that it's interesting to really having it used by a very wide community.