ACBARSZ · Sunyaev-Zel'dovich studies of clusters of galaxies using the ACBAR instrument on the viper telescope at the South Pole

Final Activity Report Summary - ACBARSZ (Sunyaev-Zel'dovich studies of clusters of galaxies using the ACBAR instrument on the viper telescope at the South Pole)

One of the most remarkable things in nature is that simple laws of physics developed to explain phenomena on Earth apply throughout the observable Universe. Take the scattering of light for example. Everyone is familiar with the fact that the light, or radiation, coming from the sun can be split in a continuous rainbow of different colours. So why is the sky blue? It is blue because the blue radiation scatters more off the molecules in the upper atmosphere than does the red radiation. The red radiation gets straight to us from the sun, whereas the blue radiation bounces around and thus appears to be coming at us from all directions. The physics of this scattering is well understood, we can apply simple mathematical formulae to predict how radiation coming from the sun will be effected by the atmosphere. We can also write down formulae (albeit not so simple) to predict how light will scatter off free electrons. For this we need to consider light not as 'rays' or waves, but in terms of tiny massless particles called photons. In space, there are large regions where all the atoms have become ionized, either because of UV radiation from a nearby star or because they are influenced by enormous gravitational fields. When you have ionized atoms, you must also have free electrons, and thus photon scattering. On Earth, we are bathed in a photons coming from the Sun, these photons are only a few minutes old (it takes eight minutes for light leaving the surface of the Sun to reach us). So it may seem counter intuitive that almost every photon in the Universe is more than 10 billion years old. These ancient photons are the dying embers from the Big Bang. These ancient photons are very weak, they have lost almost all their energy over the passage of time. In physics, we can think of photon energy being related to temperature; the photons coming from the Sun have an equivalent temperature of 6 000 degrees, the photons coming from the Big Bang have a temperature of only three degrees. That is three degrees above absolute zero, not three degrees above freezing. To get down to those temperatures on Earth, we need to use complex vacuum refrigerators filled with an expensive isotope of Helium. So now imagine a photon from the Big Bang (we call them Cosmic Microwave Background (CMB) photons) travelling through an ionized region. The photon is 'cold', the electrons are 'hot', so during a scattering event, the photon will get a bit 'warmer' or a bit more energetic. Going back to our physics formulae, we can predict just how the spectrum of the CMB should change when CMB photons pass through the ionised gas that fills clusters of galaxies; clusters are enormous, semi-spherical, regions containing gas, galaxies and dark matter, all held together by enormous gravitational fields. During this European Union (EU) supported project, we have made observations of clusters of galaxies using a CMB telescope at the South Pole and using X-ray telescopes in space. These observations have allowed us to test whether the physics formulae developed on Earth really do apply in space; they do. They have also allowed us to measured how many free electrons there in the clusters we studied and how those electrons are arranged in the clusters. The telescope at the South Pole was the first to be able to make these type of measurements, but several more are currently being built. So our results will impact how these new telescopes will be operated. Our results are also useful in many areas of astrophysics and cosmology, as they represent the first steps in our understanding of this particular type of scattering known as the Sunyaev-Zel'dovich effect.

Data: CORDIS, © European Union