It is possible to see about 6,000 stars with the naked eye, and using a small telescope as many as 600,000 become visible. More powerful telescopes reveal, in addition to individual stars, many thousands of galaxies. Each of these galaxies contains as many as one trillion stars. The most powerful telescopes can probably detect 1 billion galaxies. Yet despite the immensity of these galaxies, they appear as the tiniest blotches of light surrounded by vast reaches of empty space. The universe, which by definition includes everything in existence, is unimaginably large. To study it, it must be broken down into its constituent parts.
The basic structure of the universe
An analysis of stars shows that many are part of clusters of as few as 10 stars to as many as 1 million stars. All revolve as a group about the center of the galaxy of which they are a part. This hierarchy continues to build up, with many galaxies being members of galactic clusters, bound together by their mutual gravitational interaction. The local group of about 20 galaxies, including our own, is one example of such a cluster. These clusters are typically tens of millions of parsecs across and contain several hundred galaxies. There is some evidence to suggest that clusters of clusters exist, or “super clusters” of galaxies.
Despite the apparent random distribution of stars, it is generally believed that the universe takes on a uniform appearance when viewed on scales greater than about 1 billion parsecs. It is said to be homogeneous and isotropic on these scales—that is, it looks the same from all points, in all directions. That the universe should have a structure on this large scale is surprising in view of its violent beginnings (according to the big bang theory). Even so, the structure makes it much easier to try to calculate its properties.
The expansion of the universe
In the 1920’s the universe as a whole was discovered to be in a state of expansion. This finding was revealed in the recession of the galaxies: whereby the farther a galaxy is from the earth, the faster it appears to race away. This phenomenon is expressed by Hubble’s law, which states that the recession speed of a distant galaxy is proportional to its distance. According to this law, there is a distance from earth at which a galaxy recedes from it at the speed of light. Because it is impossible to see this or any other more distant galaxy, the corresponding distance can be taken as representing the radius of the visible universe. Hubble’s constant (Ho) relates the distance of a galaxy to its speed of recession. Determining the actual value of Hubble’s constant is made difficult by the uncertainties involved in measuring the vast distances to the farthest galaxies. Estimates for the Hubble constant range from 40 to 100 kilometers per second per million parsecs.
The observation that all the galaxies that obey Hubble’s expansion law seem to race away from us might lead one to think that the earth is the center of expansion. But the universe expands uniformly at every point, so we see only a small area of its expansion.
The present expansion of the universe suggests that at an earlier stage, the galaxies were closer together. It is thought that there may have been a time when all the matter in the universe, and the space that contains it, was merged into a single globule, which exploded to start the expansion. This is the big bang theory of the origin of the universe. Hubble’s constant (Ho) is also used to indicate how long ago this initial explosion took place, flinging all matter outward to condense later into the galaxies we now see. But because of disagreement about the value of the Hubble constant, estimates of the age of the universe range from 7 to 22 billion years.
The big bang would have generated vast quantities of radiation from the enormously high temperatures involved. This radiation, now much cooled and diluted by the expansion, would be—and still is—observable in the form of background microwave radiation, corresponding to a temperature of 3 K (—454° F.; — 270° C), just above absolute zero.
Stars and galaxies also contribute to the total radiation background, which covers all known frequencies of the electromagnetic spectrum. The universe is, at present, dominated in its evolution by the matter within it. In the first 10,000 years of its existence, however, the universe’s development was governed by its radiation content.
The density of the universe
The density of matter in the universe can be calculated once the number of galaxies per cubic mega parsec and the mass of each one is known. Counts of the numbers of galaxies exceeding a certain luminosity show that there is roughly 1 galaxy per 50 cubic mega parsecs of space. Taking an average galaxy mass as 10’1 solar masses, the density of space averages only 1 atom per 10 cubic yards (7.6 cubic meters). This figure for the density is considered by some astronomers to be an underestimate. They argue that intergalactic space may contain large amounts of dark matter in the form of dust, dead stars, black holes, and cold gas. Such material would be extremely difficult to detect but, if found to exist, could have very important consequences. A high enough density of matter could halt the expansion of the universe and cause it to collapse again, thus resulting in another big bang.