Introduction

The purpose of this chapter is to give an overview and summarise several observations that are of cosmological relevance. Many of the results (like the existence of dark matter) described here were taken for granted in the earlier chapters, and we shall make an attempt to provide the description of the evidence in support of these results. This chapter also provides a summary of the current knowledge of different parameters of cosmological significance.

Cosmic Distance Scale

The measurement of distances to different celestial bodies is of primary importance not only in understanding their properties but also – for objects at cosmological distances – in determining the geometry of the universe and the cosmological parameters. Obviously the technique used to measure the distance will depend on the properties of the object that is being studied; we shall concentrate on the measurement of extragalactic distances.

The procedures used for distance measurements can be divided into two natural classes. The first one uses what could be called the absolute distance estimator. These estimators are certain properties of (or features in) an object that can be used to directly measure the distance to the object. The second approach uses a relative distance estimator that allows us to determine the ratio between the distances to two different objects. By choosing a wide class of overlapping relative distance estimators, we can build what is known as a cosmic distance ladder.