What’s the Matter?

Abstract

But in spite of the outstanding successes outlined above, we now find that we do not know what 95% of the universe is made of. The average density of the universe determines many of the overall properties of the universe. The density to which all measurements are compared is the so-called critical density. It is a tiny 10–29 grams per cubic centimetre (about five hydrogen atoms per cubic metre), but it is a central number in cosmology. It is the dividing line between the three main possible geometries of the large-scale universe. If the actual density is less than this, the universe is called ‘open’: it is negatively curved like a saddle, and parallel light rays diverge from each other over large distances (note that this is a two-dimensional analogue of a three dimensional space, but you get the idea). If the actual density is greater than the critical density, the universe is ‘closed’: it is positively curved like the surface of a sphere, and parallel light rays converge. But if the actual density is exactly equal to the critical density, the universe is ‘flat’: parallel light rays remain parallel, as in our ordinary conception of space.