The Light/Dark Universe

When one looks up at the night sky, many questions are evoked: why do stars twinkle? Why does the moon have so many phases? How many stars are there? Are we alone in the Universe?

However, as non-astronomers, the one question that we might consider too trivial to ask is actually one of the keys to understanding the Universe: why is the night sky dark?

The Light/Dark Universe by James Overduin and Paul Wesson describes a historical and mathematical journey that begins with the everyday naked-eye observation of a dark night sky and arrives at fundamental conclusions about the cosmos.

Why is the fact that the night sky is dark so remarkable? The premise is that in a large universe, no matter where we look in the night sky, our eyes should fall upon an object that is emitting light, so the whole sky should be as bright as the Sun itself. Clearly this is not true, and this is Olbers' paradox, described by Heinrich Wilhelm Olbers, a German astronomer, in 1823.

While the question about the reason for the existence of a dark sky was asked as early as the 16th century, an accepted answer took much longer to form. It was the poet Edgar Allan Poe and astronomer Johann Madler, Overduin and Wesson say, "who first connected the dark night sky with the idea of the birth of the Universe in time, and thereby laid down one of the cornerstones of cosmology".

Olbers' paradox was originally stated in relation to the brightness that our eyes can perceive, but the book takes us beyond the visible to consider the background glow that emanates at many other wavelengths of light, from radio, microwave and infra-red to ultraviolet, X-rays and gamma rays.

As we are led through numerous calculations, we realise that the ultimate answer to the paradox is a balance between the lifetime of the sources of light and their diminishing brightness due to the expansion of the Universe. In the case of the cosmic microwave background, "here the expansion is of paramount importance, since the source of the radiation ... was emitted at more or less a single instant in cosmological history".

Each wavelength provides a different set of clues to the nature of the Universe, enabling astronomers to construct the most likely and coherent cosmological model.

Combining both observations and theory in all wavelengths, Overduin and Wesson explain how the existence, description and candidates for dark matter and energy can be constrained.

My hope from the summary and preface of this book was that the authors were intending to describe, in a generally accessible manner, the resourceful and extraordinary way in which we can exploit the information-packed spectrum of extra-galactic background light.

The authors state that they "dedicate this book to the thinking reader who has looked at the glory of the night sky and wondered what does it mean?"

However, while some of the book is accessible to early undergraduate physicists and mathematicians, the majority is very definitely aimed at those fluent in general relativity and the mathematical specifics of cosmology. It reads somewhat paradoxically, with the beginnings of chapters going into explicative detail about familiar aspects of the subject, while only a few pages later, complex equations and calculations pass by with little explanation.

In its present form, I feel this book is fitting only for a graduate audience, but its theme and concept are inspiring and it describes a fantastic example of the power of observation. Furthermore, the conclusion - "we arrive by a combination of both (experiment and thought): the night sky in a general way is 'dark' because the Universe is young" - is plain.