Cosmology and a matter outstanding

December 21, 2001

Over the past half century, the origins of the universe have begun to unfold. But Antony Hewish argues that there are still many questions left unanswered.

Amazing progress has been made during the past 50 years in our understanding of the origin and evolution of the universe. New discoveries, especially in radioastronomy - a field that has seen six Nobel awards to date - have unveiled features of immense significance and we can now trace, with some realism, how the stars and galaxies emerged from the cosmic inferno that existed soon after the beginning of time.

To illustrate the power of current observations, it is helpful to think in terms of a human lifetime. On this timescale, if our present universe is likened to a mature adult, radio telescopes can now provide images showing how it looked in its early history, just one day after conception, when it was an almost featureless, incandescent gas. The present challenge is to trace how this evolved into the population of galaxies that we see in the sky today. Combining several kinds of observations points strongly towards the existence of a new type of substance, "dark matter", that pervades all space and manifests its presence only through gravitational forces.

In the early 1950s, it was not known that the universe had a beginning. The observed recession of the galaxies certainly points to an origin, about 14 billion years ago, when all matter would have been compressed into a tiny volume, but this conclusion is not inevitable. If matter is steadily created everywhere in space so that new galaxies form as the older ones move apart, a temporal beginning can be avoided. Such a steady-state universe would have the same general appearance at all times. This theory avoided the conceptual difficulty of a temporal beginning.

Observations of distant galaxies provide a new view into the past, since light travels at a finite speed. But in the 1940s telescopes could not probe far enough to check whether the universe was, or was not, evolving with time. The situation changed in 1952 when a radio-emitting galaxy was discovered that provided a look-back over 1 billion years. As larger radio telescopes came into use and more radio galaxies were located, evidence grew that the early universe contained substantially more of these objects than it does now, thus indicating an evolutionary history incompatible with a steady state.

Stronger evidence for evolution came in 1965 when radio astronomers discovered a faint background of thermal radiation coming from all over the sky. The radiation indicated a very low temperature, just a few degrees above absolute zero: its possible existence had already been predicted for the big-bang universe. In the aftermath of the initial explosion, when the temperature had fallen to about that of the Sun's surface, the first atoms could form. Before this, the cosmic gas was opaque due to scattering by freely moving electrons, but it would have become transparent when electrons were trapped by atoms. Radiation could then travel freely through space. After propagating for about 14 billion years, radiation arrives, hugely red-shifted, so that the apparent temperature is reduced almost to zero. This distant view of the universe at less than 1 million years from the beginning raises a startling problem concerning the origin of galaxies.

Galaxies must have condensed out of the expanding cosmic gas after the gravitational collapse of primordial clouds of higher than average density. These clouds should be detectable in the background radiation as localised areas at least 0.1 per cent brighter than their surroundings if collapse is to be fast enough to spawn galaxies within the known age of the universe. But observations show a pattern of brightening ten times weaker than this. At first sight, it seems that galaxies have no right to exist. Fortunately, there are convincing reasons to suppose that there is far more matter in space than that directly observed.

Studies of the rotation of galaxies, for example, show that they must contain substantially more mass than that observed as stars and interstellar gas in order to be held together by gravity. Similarly, large clusters of galaxies are bound by gravitational forces far stronger than those estimated from the mass observed. This unseen "dark matter" cannot be ordinary matter - for example, cold stars that do not emit radiation. Any such material would have been present during a brief era of nuclear fusion a few minutes after the big bang and its existence is ruled out by the observed abundances of simple nuclei such as helium and deuterium. Any significant increase in the density of the cosmic plasma during the fusion era would have led to much less deuterium than is found. Various estimates indicate that there must be about ten times more unseen matter compared with that which is observed, and collapsing clouds of dark matter can gravitationally drag ordinary matter with them to form galaxies.

Observations give the most encouraging support to the dark-matter theory. It is widely held that the universe must have undergone a huge and rapid inflation during a tiny fraction of a second after the beginning. Once over, quantum theory predicts that the cosmic gas should contain clouds of all sizes, but the smallest begin to collapse first because gravitational forces travel at the speed of light and longer times are needed for gravity to embrace the larger clouds.

In this opaque phase of the cosmic gas, radiation and matter are strongly linked and internal pressure prevents the steady collapse of clouds. Instead they rebound and pulsate periodically, like acoustic oscillations, until the cosmic gas becomes transparent. At this stage, the background radiation should show very characteristic patterns on different scales, the brightest patches corresponding to the largest clouds that could be gravitationally embraced at the onset of transparency.

Observations of the background radiation indicate remarkable agreement with the predicted pattern and it is hard to see how this could be fortuitous. Burning questions remain. What is dark matter? Why does rapid inflation occur? How can gravity be incorporated into quantum theory? But it is these questions that make the future of astrophysics so exciting.

Antony Hewish was awarded the Nobel prize for physics in 1974.

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