Antony Hewish

Mapping the Primordial Universe


Abstract

The last five decades have witnessed a phenomenal advance in our understanding of the origin and evolution of the Universe. Cosmology is no longer a matter of mere theoretical speculation. Now it has become an observational science and we can seriously discuss what the physical conditions were like within one second from the instant when the Universe began. Much of this progress has come from radioastronomy. First, the discovery of radio galaxies at great distances provided a means of looking back in time over a substantial fraction of the history of the Universe. This showed that the most powerful radio galaxies were much more numerous when the Universe was young, providing clear evidence for temporal evolution which was inconsistent with a steady state system. Next came the discovery of the cosmic microwave background radiation, a relic of the initial explosion which confirmed the “big bang” theory and defined the temperature of the expanding cosmic gas long before stars and galaxies came into existence. But the microwave background raised a new and fundamental question.

In the rapidly falling temperature a few minutes after the explosion, nuclear fusion occurred and the relative abundances of hydrogen, helium and deuterium observed in the Universe today indicate what the gas density must have been at this time. The problem is that there was not enough matter then to allow the formation of galaxies within the known timescale of about 15 billion years set by the present age of the Universe. The microwave background presents an image of the Universe as it was about 300,000 years after the beginning, when the gas was cool enough for hydrogen atoms to form. Prior to this, photon scattering by free electrons caused the Universe to be opaque. For galaxies to condense out of the expanding gas requires the gravitational collapse of clouds having densities at least 0.1 per cent greater than the average. Similar variations should also be seen in the temperature of the background radiation but observations show fluctuations which are more than ten times smaller than this. It is therefore assumed that matter exists in some other form – ‘dark matter’ in which the necessary density variations do occur but the matter consists of some new and unknown particles which interact much more weakly with photons. Gravitational collapse of dark matter clouds would then occur on the required timescale and normal matter would be dragged in with them to form the visible galaxies. The gravitational effects of invisible matter are already evident in galactic rotation, the dynamics of clusters of galaxies and gravitational lensing. In fact, the observations suggest that the bulk of the mass in the Universe must be non-baryonic owing to the constraints on the baryonic content imposed by the abundances of the light nuclei. One of the main questions in cosmology today is to understand what caused the primordial density fluctuations of dark matter and the way this defined the effective blueprint of the Universe which ultimately controlled the large-scale structure of galaxies and their spatial distribution. Many possibilities have been suggested, including quantum fluctuations of dark matter and topological structures in spacetime resulting from phase changes in the earliest expansions. These should leave their characteristic imprint on the background radiation but reading this message is technically demanding and beyond the powers of conventional radio telescopes. Traditionally, the dominant requirement has been high angular resolving power requiring interferometric systems on the largest baselines. Now we need radio telescopes that can detect temperature differences of a few millionths of a degree on scales comparable to the angular size of the moon. The lecture will review this exciting new frontier of radioastronomy with emphasis on an instrument built at the Cavendish Laboratory, Cambridge, and now coming into operation on Tenerife.


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