The Golden Age of Cosmology

The Golden Age of Science Fiction is 14.    --Anonymous

A look at the history of the subject suggests that actually our time is not so special.  It seems to be a feature of cosmology over the last five centuries at least that people always believe they are on the edge of the final answer.  In 1500 Columbus surely saw the precise size and shape of the Earth as the major remainly problem in understanding the world, and must have been confident that it could be solved in the near future.  By 1600, thanks to the great voyages of discovery, the Earth was measured pretty well, but philosophers like Kepler argued about the correct geometry of the solar system and whether the stars where spread throughout infinite space or just filled a finite region around the sun.  By around 1700, the work of Tycho, Kepler, Galileo, and others had decisively settled the structure of the solar system, and most philosophers believed in infinite space.  But just at this time major cosmological issues were raised in the correspondence between Leibniz and Newton's followers (which I have discussed elsewhere). Furthermore, perceptive astronomers realised that the Milky Way found no explanation in the cosmology of the time.  By 1800, the universe had changed again; astronomers led by William Herschel had a clear picture of the Milky Way as a disc of stars in which the sun is embedded,  and were actively working on the major cosmological problem of determining its structure. But a new issue had arisen, the nature of the nebulae: were these "island universes"; other Milky Ways spread through infinite space, as had been suggested by Immanuel Kant and Johann Lambert, or merely gas clouds, maybe forming solar systems as in Laplace's "nebular hypothesis", within a single large but finite system of stars? By 1900 it seemed clear to most astronomers that the nebulae were no island universes, and attention focussed on determining the size of the Milky Way, seen as the entirity of the universe. By the start of the second world war, the size of the Milky Way was pinned down, but in the meantime everything was changed by the work of Slipher, Hubble, Einstein, and their associates, showing that the spiral nebulae were indeed outside the Milky Way and receding from it.  The major problems were seen as determining the deceleration parameter of the universe, and explaining the apparent discrepancy between astronomical and geological timescales.

After a short pause for the war, cosmology started moving significantly faster than before. Nevertheless, since 1500 one has had to be reasonably unlucky to have lived through a period when the received idea about the universe has not gone through a fairly substantial change.  Hopes that the outstanding problems in cosmology are about to be solved are usually justified, but by the time the solution arrives, cosmology has moved on, to worry about questions on a larger scale.

Post-war cosmology has moved so fast that now most people have lived through two or three major revisions of the universe.  Attempts to measure the deceleration parameter were foiled by the realisation of the evolution of galaxy luminosity over cosmic time, but the timescale problem was resolved by the early 1960s.  By that time the new Steady State cosmology convinced many that yet another major revolution in cosmology had arrived, but this fell by the wayside with the discovery of the cosmic microwave background in 1965. The CMB focussed attention on the big bang and nucleosynthesis in the first few minutes, and by the early 1970s nucleosynthesis calculations gave a matter density that was clearly too low to close the universe, leading to the confident claim that the universe was open and would expand forever. But now nagging problems with missing mass in clusters of galaxies and the newly-measured flat rotation curves of spiral galaxies hinted that there was far more matter in the universe than was visible to telescopes.  The non-detection of fluctuations in the microwave background by the end of the 1970s, expected to account for galaxy formation, forced a new revolution in cosmology associated with inflation and non-baryonic matter, in which the observable universe is seen as sort of baryonic froth carried on an ocean of undetected particles predicted by speculative theories in particle physics.  For a decade, the universe was assumed by almost all astronomers to have the critical density predicted by inflation. Aspects of the new picture were brilliantly confirmed by the 1992 discovery of CMB fluctuations by COBE, but at the same time accurate measurements of the large-scale distribution of galaxies seemed to rule out the simplest cold dark matter model which had established itself almost as an article of faith during the 1980s.  Numerous modifications of this picture were developed in the 1990s, but the apparently clear measurement, at last, of the deceleration parameter in 1998, from observations of supernovae, caused a sensation when it turned out that the universe was accelerating.  Very quickly several other pieces of evidence turned up supporting this result, leading to a new revival of the cosmological constant, and hence a very significant revision to the inflationary cosmology framework.  In 2000, high resolution observations of the CMB fluctuations by the Boomerang experiment and several others again provided support for the basic picture of inflation, but not without throwing doubt on the baryon density yielded by big-bang nucleosynthesis calculations.  The significance of these latest results is far from settled.

From this account it certainly is true that the turnover of cosmological models has been accelerating throughout the last century.  But this is hardly surprising given the growth of astronomy as a professional activity.  In terms of the man years (and money) devoted to cosmology, far more effort is now required to make a significant change to our view of the universe than in the days of Kepler and Galileo.  Perhaps this does mean that we are converging rapidly on the final answer.  A second argument for the special nature of present-day cosmology is that in studying the CMB we have reached an absolute limit to observing the universe: no larger-scale structure can emerge because even in principle we cannot see significantly further; light from more distant parts of the universe has not had time to reach us yet. But in fact theorists already speculate about structure on vastly larger scales than our observable universe. In inflation theory the observable universe is a negligible region in a much larger, perhaps infinite structure, and in André Linde's eternal inflation, even that larger scale is just one of infinitely many universes constantly budding from each other.  Although it seems that these speculations are pure metaphysics, they do have testable, if indirect, consequences, and so it seems unlikely that the expansion of our world-picture will cease in the forseeable future.