LAROUSE ENCYCLOPEDIA OF ASTRONOMY

 P. 422 - 423

Redial velocities of the galaxies, and the expansion of the Universe. - We have already learnt how the displacement of the lines in the spectrum of a celestial object can be measured by comparing this spectrum with that of a laboratory source, and that from this shift the body's radial velocity can be derived - i.e. its velocity of approach or recession in the line of sight. When this technique was first applied to the extragalactric nebulae, some 40 years ago, velocities of 1,000 m.p.s. and more were encountered, far surpassing the tens, or at most the few hundreds, of m.p.s. of the stars. But at that time no general law of extragalactic velocities could be deduced, since exact data relating to the distances of the galaxies were lacking. It was only in 1929 that Hubble, having determined the distances of a sufficient number of galaxies with adequate accuracy, was able to announce that an astonishing relationship links the distances and radial velocities of the extragalactic nebulae: when the Sun's motion due to the rotation of the Galaxy was allowed for, it was found that all the nebulae save two or three of the nearest are receding from us; and, furthermore, the greater the distance of a nebula, the greater its velocity - the radial velocities of recession increasing regularly by about 300 m.p.s. per megaparsec. The nebulae of the Virgo cluster, at a distance of 7 × 106 lightyears, appear to be receding at a mean velocity of 600 m.p.s. During the following years, from 1930 to 1936, this relationship was confirmed, and its scope extended, by the measures made at Mount Wilson by Hubble and Humason of the radial velocities of a further 150 or so nebulae; these included both isolated specimens in the general field and members of clusters, distributed throughout almost the whole region of space observable at that time. In order to obtain measurable images of the spectra of the remotest and faintest nebulae it was necessary to develop special short-focus (therefore very fast) spectrograph objectives, and to expose the plates for 10, 20, or more hours, spreading the exposure over several nights. These observations showed that throughout the whole observable region the radial velocities of recession continue to increase steadily with distance, attaining the fantastic value of 35,000 m.p.s. (one-seventh of the speed of light) at a distance of 2.4 × 108 lightyears - that of the remotest nebula for which a spectrogram could be obtained. It is this extraordinary phenomenon that constitutes the so-called 'expansion of the Universe'. For we cannot believe that the galaxies are really scattering away from us in all directions: such a state of affairs would put the Galaxy back in a unique position if the Universe, and furthermore make it the centre of a universal 'repulsive force' - something so inherently improbable as to be quite incredible. It must rather be supposed that space itself is expanding - a possibility that had already been suggested by the theory of relativity - and in doing so is carrying the galaxies with it. Hence the distances of every galaxy in the Universe from every other galaxy will be continuously increasing, and it would make no difference from which one we happen to make our observations. The result in every case would be the same: an apparent recession of all others. A clearer idea of this state of affairs may be given by the following well-known analogy. Suppose that the extragalactic nebulae are represented by droplets on the surface of a soap bubble which is being blown up: then it can easily be seen that as the bubble swells, the distances separating the droplets will increase; furthermore, they will increase with a velocity that is proportional to their separation. No matter which droplet we consider, all the others will appear to be receding from it; hence none has a more unique or central position than another. The 'radial' velocities of the droplets are of course those of their displacements measured across the surface of the bubble, whose expansion is taking place in space, about a centre which is situated in its interior. It is generally agreed that the apparent recession of the galaxies must result from an analogous expansion of space - the four-dimensional space-time of relativity theory. The observed phenomena have in fact been reasonably well accounted for by the theory of an expanding Universe as developed some 30 years ago by the Belgian cosmologist, the Abbé G. Lemaitre, as a solution of the fundamental equations for the structure of cosmic space deduced from Einstein's general theory of relativity.