Expansion of the Universe

Abstract

AT the Friday evening discourse on November 25 at the Royal Institution, Dr. Knox-Shaw discussed the observational evidence for the expansion of the universe. The nebulæ lying beyond our galaxy stretch away into space farther than our present limit of penetration. Most of them can be studied only with our largest telescopes, and for our knowledge of their distances and motions we are indebted largely to the work of Dr. Hubble at the Mount Wilson Observatory. In some forty nebulæ he has been able to detect individual stars, and in a few cases to identify them as belonging to types already known in the galaxy. From the apparent luminosities of these stars he has derived distances for the nebulæ in which they are involved. In all other cases the distances are based on the apparent brightness of the nebulæ themselves. The scale of distance thus constructed is still very uncertain. The absorption lines in the spectra of the extra-galactic nebulæ are shifted towards the red in a way that suggests that they are all moving away from us with velocities which increase with the distance. Whether there is an alternative explanation of these shifts is a question for the physicist rather than for the practical astronomer, but if we assume that they actually indicate motions of recession, we find that the velocities of the nebulae are proportional to their distances from us, as would be required in a uniformly expanding universe. Hubble's value for the rate of expansion, an increase of 560 km. a second for each million parsecs of distance, must be regarded as liable to revision as further observational material becomes available. A cluster of very faint nebulse in Gemini, so remote that its light has taken some 135,000,000 years to reach us, has recently been photographed at Mount Wilson, and seems to be moving away from us at the immense speed of 24,000 km. a second.