Physical Cosmology

Chris Clarke

In the earliest days of General Relativity theory, Einstein (in 1917), Friedmann (in 1922) and others used the theory to construct mathematical models of the entire universe. These Friedman-Robertson-Walker (FRW) models treat the matter in the universe as if it was distributed uniformly, showing only the very large-scale features. In the early days the main set of data was the observation that light from distant galaxies was received at a lower frequency than that at which it was presumably emitted (the Cosmological Red Shift). Hubble, who discovered this effect in 1926, had accounted for in terms of the galaxies moving away from us, whereas the FRW models accounted for it in terms of General Relativity: a geometrical theory of space and time. In these models space is closed (i.e. it is a higher-dimensional analogue of a sphere) if the density of matter is large enough. Successive discoveries refined and elaborated these models. Further confirming data came from observations of the numbers, brightness and apparent sizes of galaxies at different supposed distances.

Progress in nuclear physics, and in understanding how the chemical compositions of stars changed over cosmological time, led to the Hot Big Bang versions of the FRW models. In these, the past history of the universe occupies a finite span of time, as time is usually measured. So there is a "start time," even though there is no first moment. The nuclei of the elements Hydrogen, Helium and Lithium are synthesised from a hot dense charged gas of protons and electrons in the first few minutes of the universe, and all the other elements are synthesised later in the interiors of stars.

The first direct confirmation of a hot early phase to the universe was the discovery of the Cosmic Microwave Background Radiation (CMBR): uniform electromagnetic heat-radiation detected by radio receivers, left over from 300000 years after the start time, when the nuclei and electrons condensed into a transparent gas through which the radiation could freely travel. This confirmed by observation that the universe at that time was almost completely uniform. Recent observations by a satellite called COBE have detected variations of one part in 10000 in this uniformity, consistently with more modern theories. These variations grew to produce galaxies and stars.

Observations of the Binary Pulsar (a rapidly rotating star made of matter at the density of an atomic nucleus in orbit round a normal companion star) confirmed the accuracy of General Relativity as a theory of gravity to an unprecedented accuracy, essentially ruling out other competing theories of gravity except in very extreme conditions such as the first fraction of a second of the universe. This leaves the Hot Big Bang/FRW picture as the only serious contender for a description of the universe from the first seconds onwards. It has the feature that the average density of matter in the universe (needed to exert enough gravitational force to account for the overall dynamics of the universe) seems to be larger than the observed density of matter in stars, suggesting the presence of a lot of Dark Matter not in stars (e.g. planets, rocks, gas, dust etc).

Recent work has concentrated on more speculative theories of the earlier phases (not directly observable) and on explaining a number of coincidences in the early conditions, particularly why space on a large scale has almost exactly the geometry that Euclid said it had (technically "space is flat" - not quite closed) and why the matter is distributed so uniformly. One explanation is the Anthropic Principle which deduces these coincidences from the fact that we exist to observe the universe, and hence the universe must be such that life can evolve. Most cosmologists attribute these two particular coincidences, however, to a phase of Inflation, preceding the Big Bang phase, in which a very large expansion driven by quantum mechanical processes left the universe in a uniform flat state prior to the appearance of normal matter. An entirely evolutionary picture is being dimly suggested in which primordial physical processes create space, time, matter and the detailed laws of matter from an initial quantum state outside space and time.

Prof. Chris Clarke is visiting Professor in the Faculty of Mathematical Studies, University of Southampton. He is the author of 'Reality Through the Looking Glass' (1995).

Key texts

I have not yet been able to locate any reliable popular book on the very early (fractions of a second) universe. Most aim to wow the reader with imaginative illustrations of the latest unfounded speculations. The following give a good coverage of the early and later universe.

Couper, Heather and Henbest, Nigel (1999). Universe. London: Macmillan. ISBN 0 7522 1712 7.
A well presented Channel 4 Book lavishly illustrated with Hubble Space Telescope photographs. Unquestionably the best place for the beginner to start. Part 1 covers cosmology while the rest gives the context and conveys the power and beauty of our universe. Fairly reliable though some inconsistencies in presentation of the latest data.

Narlikar, Jayant V (1999). Seven Wonders of the Cosmos. Cambridge: Cambridge University Press. ISBN 0 521 63898 4.
The best popular "middle-brow" book I have found: no maths but plenty of solid information and arguments. Covers the theoretical background of relativity and astrophysics and includes current thinking about the significance of the CMBR. Totally reliable. The best book for the reader prepared to put in a bit of work.

Delsemme, Armand (1998). Our cosmic origins. Cambridge: Cambridge University Press. ISBN 0 521 62038 4.
Ties cosmology into the origins of life on earth: valuable background for "Gaia" ideas of the planet as a cosmic being, though this book is entirely mainstream science.

Swimme, Brian and Berry, Thomas (1992). The Universe Story. London: Penguin Books. ISBN 0 14 019472 x.
Similar material to Delsemme's book, but expressed poetically rather than in scientific detail, and bringing out the spiritual significance of modern cosmology, presented as a "modern myth" - a story to live by. Sometimes difficult to penetrate to the facts behind the poetry, but an inspiring presentation from the viewpoint of Creation Centred Spirituality.

Barrow, John D and Tipler, Frank J (1988). The Anthropic Cosmological Principle. Oxford: Oxford University Press. ISBN 0 19 282147 4.
A mine of information about the coincidences that underlie the nature of the universe as we know it. It requires a little numeracy to tackle this, but not much prior knowledge. The interpretations are highly controversial!