General description

One of the major mysteries of the Universe is the inhomogeneous distribution of matter throughout space. Another major mystery is the homogeneous distribution of matter throughout space. This apparent contradiction–puzzle–paradox typifies our uncertainty about the origin of structure in the Universe. Both statements are true. This may make them a ‘great truth’ in Niels Bohr's sense of the phrase: a statement whose opposite is also true.

To measure the inhomogeneities of matter we need only look around us. Indeed, the closer we look the greater the density contrasts usually seem to be. We, ourselves, mainly made of water, are about as dense as our planet and a little denser on average than our sun. Each of these is about 1023 times as dense as the average smoothed out density in the solar neighborhood, which is a typical average density within large galaxies. In turn, the average density within large galaxies is about 1000 times that within large clusters of galaxies. If we average over regions containing many large clusters the density drops by another factor of 1000. These relationships are illustrated by Table 2 and Figure 22, both from excellent surveys by de Vaucouleurs (1970, 1971). Although the masses and radii astronomers obtain for large systems are often uncertain by factors of at least two, such uncertainty is hardly visible over the enormous range of these logarithmic scales.