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Telescopes and space missions

Telescopes and space missions

Dating the universe

22 Feb 2011

How Old is the Universe?
David A Weintraub
2011 Princeton University Press £20.95/$29.95hb 380pp

The fusion issue

Carl Sagan famously said that if you wanted to make an apple pie from scratch, you first had to create the universe. The deceptively simple title of David Weintraub’s latest book invokes a very similar philosophy: if you really want to know the age of the universe, then you too have to start from scratch. How Old is the Universe? places the question in its proper historical context and explains what has gone into answering it. Although other astronomy books have explained some of the methods here, Weintraub’s brings everything together into one narrative. Such an approach is sorely needed, as the universe’s age lies at the heart of modern cosmology.

In the first chapter, some main sources of evidence – such as meteorite samples, globular clusters, Cepheid variables and white dwarfs – are introduced and briefly explained. This gives a nice overview, before each item is discussed in depth later.

Weintraub’s narrative begins in the 17th century with James Usher, an Irish bishop who calculated the age of the Earth using biblical chronology (concluding that it started in 4004 BC.) Today, his methods are often ridiculed, particularly with the resurgence of young-Earth creationism. However, Weintraub shows that he was neither the first nor the last person to use the Bible to date the Earth. Moreover, Usher did the best that he could with the information available to him. Surprisingly, perhaps, the Copernican revolution two centuries before had contributed little to the 17th-century understanding of the universe’s age. But by attempting to calculate the Earth’s age, Usher and his contemporaries were at least on the right lines: if you could know the age of the Earth with precision, it would serve as a vital “stepping stone”, a lower bound on the age of the universe.

The next figure to make an impact was Johannes Kepler, who, in his mathematically rigorous fashion, proposed a more astrophysical approach. But even he came up with a figure of 3993 BC. Surely Sir Isaac Newton could do better? No. Newton’s approach was unscientific, and the figure that he arrived at was close to those put forward by scientists, bishops, rabbis and other great thinkers of the time. In this entertaining way, Weintraub shows that scholarly consensus does not always equate to fact – an important lesson for all scientists. It was not until the discovery of radioactivity at the end of the 19th century that the Earth’s age could be estimated accurately.

The Sun provided the next stumbling-block. Weintraub shows how scientists in the 19th and early 20th century did their best to try to explain the Sun’s age, especially in terms of the Earth. Geological and evolutionary evidence suggested that the Earth had been around for at least a billion years, perhaps longer. But if the Earth was so old, countered the physicists, how could the Sun have remained luminescent for so long without consuming all of its fuel? The big gap in their knowledge was nuclear fusion, and the “old Earth, young Sun” paradox could not be resolved until that particular breakthrough in physics had occurred.

The major headaches, however, were stars and star clusters. As Weintraub says, “Not all stars are the same.” He does a good job of conveying the interrelated problems of estimating a star’s apparent brightness, distance and luminosity. Some scientists found that their calculations were erroneous – again, because “not all stars are the same”. But some of these dissimilarities led to opportunities: the intrinsic brightness of Cepheid variables, for example, was found to relate to their luminosity period. Hence, by observing Cepheid variables in a galaxy, combined with redshifted spectra, astronomers could measure values for the Hubble constant, and thus the universe’s expansion rate. The rate of white-dwarf formation was also calculated, so that observations of their total number gave another way of placing an age on the universe. The book describes all of this in detail but, curiously, the proof copy that I read never mentions the terms “standard candle” or “distance ladder” when discussing objects with well-established intrinsic brightnesses that are used to measure the universe’s size (and hence its age). These two terms crop up time and time again in astrophysics, but possibly their omission will be remedied in the final version.

The next challenge came with galaxies. As late as the early 20th century, no-one knew what they were. It would take the efforts of many scientists to work out their dynamics, structure, composition and nature. This work paid off: investigation of galactic spectra would produce a paradigm shift in our view of the universe, leading scientists to conclude that it is expanding – and far bigger and older than previously thought.

As an astrophysics graduate and as someone who writes astronomy articles, I found How Old is the Universe? to be a satisfying, necessary and timely book. It should appeal to anyone wanting to learn about cosmology and astronomy in its broad context, but it would be especially good for astrophysics undergraduates because it assumes some physics knowledge, and has a good smattering of graphs, spectrograms, diagrams and images. University departments should ensure that they have some copies to hand.

In the book’s final section, Weintraub brings the journey right up to date by discussing supernovae, the cosmic microwave background, dark matter, dark energy, the Big Bang, inflation and quantum physics. He pulls together all of the relevant facets of scientific investigation from a variety of different fields, including geology, palaeontology, astronomy and physics, to ultimately arrive at the current best estimate for the universe’s age: 13.7 billion years (give or take 100 million years or so). We often take this figure for granted, along with the fact that it is known to within 1%. What this book shows is how deduction, dedication, care and persistence in many fields have led to the figure we have today. It is the story of a scientific triumph.


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