Unseen Cosmos: The Universe in Radio

  • Francis Graham-Smith
Oxford University Press: 2013. 9780199660582 | ISBN: 978-0-1996-6058-2

Francis Graham-Smith's Unseen Cosmos sets out the unique role of radio telescopes and observations at radio wavelengths in transforming our understanding of the Universe. The former UK Astronomer Royal describes the many important discoveries in radio astronomy and the techniques that made them possible. It is an extraordinary tour, from the rotating ultra-dense neutron stars known as pulsars and the cosmic microwave background left over from the Big Bang to powerful, distant radio-wave-emitting galaxies and the radio emission from molecules in galactic regions where stars are born.

Astronomy today is a multi-wavelength discipline. Observing astronomical objects and even the structure of the Universe at wavelengths from radio waves to gamma rays allows us to see different processes and often different parts of these objects. Observations in the infrared reveal cool galactic gas and dust; in the ultraviolet, hot young stars. At radio wavelengths, we spot neutral hydrogen gas and its motion, as well as synchrotron radiation (from electrons moving in a magnetic field at close to the speed of light) in galactic or intergalactic magnetic fields. X-ray telescopes detect very hot gas in and between galaxies, and optical wavelengths reveal the light from stars and ionized gas clouds. All of these data must be combined for a full understanding of objects.

Credit: MARTIN O'NEILL; BASED ON A PHOTO BY NEILLD/BIGSTOCK

Multi-wavelength observation is also needed because many astronomical phenomena are now known to be intimately linked. The evolution of galaxies and clusters of galaxies is a good example: there are complex, still little-understood relationships between phenomena such as radiation and jets from active galactic nuclei (AGNs, regions at galactic centres that emit vast amounts of energy, powered by supermassive black holes), accretion of gas, star formation and galaxy mergers. Observing galaxies at different epochs, stages of development and wavelengths is helping to clarify how energy is transferred between AGNs and the gas in and between galaxies, and how this affects the rate of star formation.

Against these new trends in astronomy, it is easy to forget radio astronomy's special role over the past 80 years. Graham-Smith reminds us that the existence of the Big Bang was confirmed initially by counting distant radio galaxies and radio quasars — remote, extremely luminous AGNs — and then by the discovery of the cosmic microwave background. He describes the beautiful experiments that measured the irregularities in this radiation and how they have transformed cosmology from a science based at least in part on aesthetics to one in which key parameters have been determined to an extraordinary level of precision. He details the discovery of pulsars by Jocelyn Bell and Tony Hewish and the extreme physics of these stars. The use of rapidly rotating pulsars as clocks has allowed astronomers to probe physics in very strong gravitational fields and has repeatedly confirmed the predictions of Einstein's General Theory of Relativity.

Credit: MARTIN O'NEILL; NASA/COBE SCIENCE WORKING GROUP

The new radio telescopes — such as the Square Kilometre Array (SKA) to be built in southern Africa and Australia, which will be the largest ever — also open up big possibilities. We could discover how the Universe was re-ionized by the first stars and/or quasars, detect the gravitational waves predicted by Einstein and possibly even detect extra-terrestrial intelligence. The SKA will be sensitive enough to see ambient radio emission (the equivalent of airport radar) from habitable planets orbiting stars in our vicinity, and is by far the most likely way to find ET.

The first radio-astronomical observations were carried out by Karl Jansky and Grote Reber in the 1930s, but the key technological advances took place after the Second World War. Astronomers such as Martin Ryle, John Bolton, Bernard Lovell and Graham-Smith himself were amazingly innovative in designing and developing new instruments, such as radio interferometers. I was lucky to be a research student at the University of Cambridge, UK, from 1970 to 1974, with access to the One-Mile Telescope and 5-km Array. This was a unique opportunity — everything observed was new, exciting and publishable. Unseen Cosmos describes this history. And the tradition of innovation has persisted: the technology challenges in designing and building the SKA are immense. They range from wide-field and wide-bandwidth receivers to innovative algorithms for calibrating and making images from observations. The vast data output will stretch researchers' capacity.

Although much of the history has been told before, I found Unseen Cosmos interesting and informative. Combining history with explanations of particular topics and their contemporary development has its limitations, however. And like most books that try to describe very complex physics in a simple way, this book succeeds in some places and not in others. I found the description of pulsars lengthy but hard to understand. I would also have welcomed more on current developments and what capabilities will be provided by the new radio telescopes, such as the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile and the SKA.

Because radio astronomy is developing rapidly, it is perhaps safer to write a book that includes a large dollop of history than to write one that could quickly become dated. Nonetheless, this book is a useful reminder of why we want to build huge, technically challenging and expensive radio telescopes like the SKA.