Introduction

From observations of the Cosmic Microwave Background (e.g., [491]) and from other, independent astronomical observations we know that, averaged over sufficiently large scales, our universe is highly homogeneous and isotropic. This means that at a given instant of cosmic time our universe shows the same basic properties everywhere, regardless of the distance and regardless of the direction in which we observe. Therefore, at first glance, it may not appear worthwhile to observe distant objects. However, due to the finite velocity of light, a look at distant objects is automatically a look into the past. In astronomy we never see an object in its present state. We always observe the properties which the object had a long time ago, when the light was emitted. Since we know very well that our universe is evolving with time, observations of distant astronomical objects allow us to study the history of our cosmos. In the light of the most distant objects we directly see the universe in its earliest stages of development. Thus, distant objects are interesting not because of their distance in space, but because of their distance in time.