2 February 2011


David A. Weintraub. How Old is the Universe? Princeton University Press, 2011

There is a consensus among astronomers that the answer to the question posed by the title of this book is about 13.7 billion years. Professor Weintraub tells us how this figure was arrived at, beginning with Aristotle, who dealt with the problem by asserting that the universe had always existed and would always continue to exist. It was not until the 18th century that natural philosophers started to make the first scientific attempts to estimate the age of the Earth, instead of relying on interpretations of the Old Testament. However, real progress had to await the discovery of radioactivity towards the end of the 19th century.
For example, the study of the properties of radioactive elements in rocks can be used to determine the time which has elapsed since they solidified. In the case of the decay of potassium 40 to argon 40, the argon will escape into the atmosphere until the rock solidifies, when it will become trapped as it is formed. From this time the ratio of argon 40 to potassium 40 increases from zero, so that the ratio when the rock is examined will be a measure of the time since solidification, as the half-life (the time taken for half of the atoms of a particular radioactive substance to decay) of potassium 40 is known.

The oldest rocks found on Earth are about 4.4 billion years old, and the oldest known meteorites in the solar system are 4.56 billion years old, so the universe must be much older.

Weintraub describes the various methods which have been developed to calculate the ages of the stars and thus to find which are the oldest. The universe must obviously be older than these.

The discussion of the expansion of the universe will probably be of great interest to many readers. Edwin Hubble's project to measure galactic distances, which he began in 1929, eventually led to Hubble's law which says that there is a linear relationship between the red shift velocity of a galaxy and the distance to the galaxy.

There were two possible interpretations of this; the obvious one that the universe began with an explosion and the galaxies are flying through space, and the theory that the galaxies are fixed in space, held together by gravitational attraction, and that it is space itself which is expanding.

The second interpretation is accepted because the first would have the distant galaxies travelling faster than light, and nothing can travel through space faster than light. However, this restriction does not apply to the rate of expansion of space itself.

This book is straight science without the - often amateurish - theological or philosophical speculations which are often to be found in popular writings on cosmology. It should be of particular interest to anyone who is thinking of embarking on a serious study of physics or astronomy. -- Reviewed by John Harney

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