Let me begin with two items which are the source of great pleasure in attending this symposium in honor of Riccardo Giacconi. The first is the obvious one that we should celebrate Riccardo's enormous contributions to the undoubted success of the Space Telescope Science Institute. This has been a complex and difficult task but one which is now showing the just rewards for a really huge effort on the part of many people.
The second great pleasure is the fact that Rashid Sunyaev has been able to attend this symposium. It was more than 20 years ago that Rashid and I produced our spectrum of the extragalactic background radiation at all accessible electromagnetic wavelengths and I reproduce our spectrum in Fig. 1. The remarkable thing about Fig. 1 is that it is still a reasonably accurate representation of the overall spectrum of the background radiation. The reason for this is that in those wavebands in which the background is reasonably easy to detect, background radiation was amongst the earliest observations to be made by telescopes with low angular resolution. In contrast, in those wavebands in which the background was swamped by the contribution of discrete sources, this has by and large remained the same.
FIgure 1. The spectrum of the extragalactic background radiation as is was known in 1969 (Longair and Sunyaev 1971). The solid lines indicate regions of the electromagnetic spectrum in which extragalactic background radiation had been measured. The dashed lines are theoretical estimates of the background radiation due to discrete sources and should not be taken very seriously.
I must confess that my heart sank somewhat when I saw that the topic I have been asked to discuss is one of the least popular and exciting aspects of the background radiation - The Radio Background - Observations. Essentially nothing has happened in this area for about 25 years and most of the exciting topics will be covered by John Peacock who will discuss the interpretation of the background radiation and, in particular, what we can learn about the cosmological evolution of the radio source and quasar populations.
I will therefore do two things. First, I will talk about what I am meant to talk about - the classical extragalactic radio background radiation and then I will turn to some new work which Andrew Blain and I have been doing concerning the millimeter and sub-millimeter background radiation which we believe can tell us a lot about the early evolution of galaxies.