It is fair to say that our Universe is on the whole well represented by one or other of these simple homogeneous and isotropic cosmological models. They certainly provide a basis for understanding the gross features of the Universe. But what about the galaxies and stars that are the basic building blocks of the Universe, […]
The simple model universes were selected for discussion because they provide a straightforward explanation for the origin of both the element helium and the cosmic background radiation, and because of their lack of complicating features. Nevertheless, it is by no means a simple task to discover all of their properties and construct cosmic histories which […]
How can this elementary yet profound deduction be checked ? It is at this point that the theoretical cosmologist enters the picture. His task is to exploit the assumption that the force of gravity dominates the evolution of the Universe. He uses Einstein’s theory of general relativity to map out the history of the competition […]
Can we reconcile this overall view of the Universe with the known laws of physics ? It is here that we leave the domain of observed fact and enter the realm of speculation. If the observed Universe can indeed be understood in terms of the laws of physics, then our confidence in our Earth-based laws […]
By 1950, there were two rival interpretations of Bubble’s observations : the BIG BANG and the STEADY STATE theories. Although we shall have more to say about theories of the Universe later on, it is appropriate to mention this famous controversy at this point since it was instrumental in motivating many important observations bearing on […]
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 […]
Hubble’s most remarkable and important discovery came soon after his establishing that many of the nebulae were extragalactic systems. He found a systematic increase in the redshift of their spectral lines with increasing distance. Adopting the most straight¬forward interpretation of the redshift as being a Doppler shift due to the recession of the source, we […]
With the passing of time, Man has been able to look increasingly further into the depths of space. Each step has produced new in¬sight into the nature of the Universe and our position within it. To the Greeks and Romans, the Earth was the centre of all things. The furthest they could see was the […]
Our Survey of active galaxies and quasars looks rather like a zoo of various cosmic beasts .In each compartment we have a type of galaxy ,frequently named for its discoverer (e.g. lacertid) ,or the technique of discovery (e.g. radio galaxy).What systematic trends emerge when we study these species collectively ?One recurrent theme is that of […]
Observations made with great care and skill by Allan Sandage and collaborators at the Hale Observatories showed, in the mid-1970s, that in the case of the brightest galaxies in rich clusters, the Hubble law, linking redshift with cosmic distance, probably holds good to a redshift of at least 0.46. Beyond the distance correspond¬ing to that […]