Hubble also embarked on some deep surveys of the distribution of galaxies in the Universe, as far out as his telescopes and the then-available photographic plates would allow. Two things struck him about these surveys. Firstly, notwithstanding the manifest cluster¬ing of galaxies on scales of a. few megaparsecs, the Universe when viewed on very large scales looks HOMOGENEOUS. There is no sign of the number of galaxies significantly thinning out as we approach the limits of the Universe accessible to our most powerful optical telescopes. Secondly, the Universe looks very much the same in all directions, and moreover, the cosmic expansion is proceeding at the same rate in all directions. Astronomers describe this by saying the Universe is ISOTROPIC. An important consequence of the apparent homogeneity and isotropy of the Universe is that there is no meaningful centre. Our Galaxy, the Milky Way, does not occupy a privileged position. It has taken less than five hundred years to dis¬place mankind from the very centre of all things to a small planet orbiting a rather average star of a galaxy that is typical of millions of others.

Recently, radio astronomers have been able to put stringent limits on the isotropy of the Universe. By observing at centimeter wavelengths, they have discovered a component of radiation that is not due to either known radio sources or noise within their receiving systems. Moreover, this residual radiation is seen to be isotropic to better than 0.1 per cent, that is, any deviation from the radiation being the same in all directions is likely to be less than this amount. The high degree of isotropy excludes the possibilities of the radiation originating in the Solar System or in the Galaxy. It is therefore concluded that this radiation must be of cosmic origin, and it is referred to as the COSMIC BACKGROUND RADIATION. We shall have more to say about this radiation field later: it is of central importance in understanding our Universe. Here we are only concerned with its isotropy, for if the radiation is indeed of cosmic origin, then this is the most stringent constraint on the iso¬tropy of the Universe. (Indeed, this isotropy measurement is prob¬ably the most accurately known parameter that characterizes the Universe). Unless we believe that we are in a privileged position at the centre of the Universe, this radiation also tells us that, in the large, the Universe is homogeneous.

Hubble’s deep surveys probably included large numbers of galaxies receding from us at about one-third the speed of light, and whose distance from us is to be measured in thousands of megaparsecs. However, the galaxies which are so typical of the ones observed by Hubble emit very little light and radio-frequency radiation as compared with the quasars and radio galaxies. Hence these latter objects allow us to probe the Universe to greater depths and at the same time look a long way into the past. Indeed, the most distant quasar yet observed has a redshift factor of about 3.5, some four times the redshift of the furthest galaxy for which a redshift has been measured. The significant fact that has emerged is that the actual number of quasars and radio galaxies was much greater in the past than now. It was this fact that first established that the Universe was different in the past than now. Not only is this conclusion gratifying in providing support for the most straightforward interpretation of the cosmic expansion, but it also poses a serious problem for cosmological theories such as the Steady State theory which proposes that the Universe looks the same at all times. Of course, it might be argued that we do not properly understand the nature of either quasars or radio galaxies, but notwithstanding such arguments, it would take a fairly serious step, such as abandoning the conventional interpretation of the redshift, to fault the conclusion that we live in a Universe that changes with time.

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