At last, after the 12 chapters which comprise the whole of Volume 1, we leave the Solar System behind and enter the astronomical domain, where we can no longer detect high energy particles directly but can only infer their presence from the radiations they emit. High energy processes are now known to be important in essentially all classes of astronomical object, and so we begin our study with a survey of the contents of the Universe – this will provide the astrophysical context for our study. This will be a very broad-brush description, and should be supplemented by the more specialised texts listed in the Further reading and references section.

The large-scale distribution of matter and radiation in the Universe

The modern picture of how matter and radiation are distributed in the Universe on a large scale is derived from a wide variety of different types of observation.

The isotropy of the Universe as a whole

On the very largest scale, the best evidence for the overall isotropy of the Universe comes from measurements of the cosmic microwave background radiation. This is the intense diffuse radiation observed in the centimetre and millimetre wavebands discovered by Penzias and Wilson in 1965. It is wholly convincing that this radiation is the cooled remnant of the very hot early phases of the Big Bang. The radiation decoupled from the matter when the Universe was only about 1/1000 of its present size, and provides direct evidence for the isotropy of the matter and radiation content of the Universe.