Magazine article UNESCO Courier

The Structure of the Universe

Magazine article UNESCO Courier

The Structure of the Universe

Article excerpt

The most important feature of the physical world around us--the Universe--is its structure. Rather than being scattered haphazardly, the matter that surrounds us is, to a large degree, assembled in various formations or structures. Structures of one type constitute the building blocks from which are formed structures of a second level that are considerably larger in spatial dimension. These second-level structures are in turn assembled into third-level structures and so on, but, as we shall see later, not ad infinitum. In other words, there is a hierarchy of structural levels.

If we confine ourselves to astronomical scales, any enumeration of these structures must necessarily start with the Earth and the other planets of the solar system whose centre is the star we call the Sun. Hundreds or thousands of millions of stars are grouped in galaxies like our Galaxy, the Milky Way, or the nebul in Andromeda. This is the second astronomical structural level. Galaxies, however, are unevenly distributed in space and form huge clusters or superclusters. It is these structures that are of interest in cosmology--the study of the Universe itself.

For the non-expert in astronomy the unit of distance of which he can most easily form a mental picture is the light-year--the distance travelled by light in one year. A light-year is approximately equivalent to ten million million (10.sup.13) kilometres. For comparison let us recall that it takes light eight minutes to reach the Earth from the Sun, but approximately four years from Proxima Centauri, the nearest star. Proxima is therefore four light-years away.

The diameter of a typical galaxy may be as little as ten thousand or as much as one hundred thousand light-years, and the distance from the Earth to the nearest giant galaxy, Andromeda's nebula, exceeds two million light-years. Galaxies are grouped together in clusters, and the diameters of the big clusters of galaxies are of the order of ten million light-years. Clusters themselves group together in superclusters that are between one hundred million and three hundred million light-years across.

Big clusters of galaxies, containing thousands of galaxies and more, have been known for a long time because they stand out clearly in the sky. Figure (1) overleaf, for instance, shows the great cluster of galaxies in the direction of (but far beyond) the constellation Coma. But superclusters have only been unambiguously identified as a result of progress achieved in observational astronomy in the past decade.

The structure of the superclusters could not be determined as long as the photographs studied in stellar atlases showed only a projection of a galaxy's position on the celestial sphere. If galaxies seem to be alongside each other in photographs, it does not follow that they are actually near one another. It is quite possible that such galaxies are at different distances from the Earth and that it is only by accident that they lie almost along the same line of sight as viewed from the Earth. The true position of galaxies in space can be ascertained only by constructing a three-dimensional picture. To do this a third co-ordinate--the distance to the galaxy--must be added to the two angle co-ordinates which give the position of the galaxy on the celestial sphere (these are analogous to geographical latitude and longitude).

The distances to the distant galaxies are found using Hubble's Law, which tells us that the velocity at which a distant galaxy is receding from us is proportional to its distance from us. The coefficient of proportionality is called Hubble's constant (see page 16). The velocity of a galaxy can be determined from the Doppler effect on the light coming from that galaxy, that is to say by measuring its "red shift" (see page 16).

Red shifts have now been measured for more than ten thousand galaxies. Using the distances to the galaxies thus obtained, tree-dimensional pictures of the distribution of the galaxies in space have been constructed with the aid of computers. …

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