In the past decade, many papers on astronomy and cosmology have topped the Thomson Reuters Science Watch league table of the most highly cited research in physics. Citation analysis shows that astronomy surveys, principally the Sloan Digital Sky Survey, score very highly indeed, as do missions to prise ever greater detail from the cosmic-microwave background. That's because the mapping projects produce three-dimensional maps of the structure of the Universe, showing the distribution of ordinary matter and mysterious dark matter, and the microwave background has embedded within it acoustic signals that inform on the origin of structure. Enormous progress has taken place in our ability to observe the past evolution and present state of the Universe.
In addition to learning a great deal about the construction of the Universe, we also have had the good fortune to be able to refine our knowledge of its cosmological parameters. For half a century from the 1930s onwards, observational cosmology was a search for two numbers: the expansion rate of the Universe and its energy density. The search used two telescopes situated in California, the 100-inch at Mount Wilson and the 200-inch at Palomar, and was conducted by two men, Edwin Hubble and his successor Allan Sandage.
The Hubble Space Telescope started life in 1990 as a Nasa key project to measure the rate of expansion. Today, thanks to a series of stellar successes in space and on the ground, the cosmological parameters include the density of dark matter and dark energy, an agreed age of the Universe (13.7 billion years) and a rate of acceleration. By turns these achievements have led to the emergence of a consensus cosmology, in which the key numbers are no longer matters for wordy debate. Even though we do not know what dark matter and dark energy are, cosmologists largely agree on what kind of Universe we live in.
Brian Greene has produced a captivating account of developments in theoretical cosmology, fundamental physics and philosophy that stretch far beyond the homely Universe we now think we understand. He takes us on an incredible journey through a landscape of parallel universes that we cannot reach. We explore the multiverse, an intriguing fusion of ideas from cosmology, quantum physics and computational creativity.
The multiverse proposal is that our Universe may be just one among many, or even an infinite number of, universes. Surprisingly perhaps, the intellectual puzzles raised by the multiverse have not only become mainstream for cosmologists, but also have a place in the academic disputations among scholars working in the field of science and religion.
This is Greene's third book devoted to giving a non-technical account of modern cosmology. Unlike his first two books, which have preliminary chapters on relativity and quantum physics, The Hidden Reality plunges straight into parallel universes, with background physics developed as needed using Greene's trademark style of metaphor, analogy and history in place of equations.
Observational cosmology became more than a search for two numbers in 1963 when physicists discovered that space is filled with microwave radiation, a relic of the energy that powered the Big Bang. That discovery led to the concept of inflation, a burst of enormously fast and accelerating growth right after the onset of the Big Bang. This brief inflationary era hugely expanded the cosmos and set the scene for theorists to investigate the Universe (or universes) that inflation has flung beyond our grasp.
There's no reason why inflation should be a one-time event, as Greene points out. Rapid stretching of space could take place at many locations in the cosmos, resulting in a Universe peppered with many widely separated regions.
Our Universe happens to be but one of many universes floating in a vast expanse. A century ago, astronomers went through an expansive rethink as it became clear that the misty nebulae, Immanuel Kant's "island universes", were indeed vast galaxies like the Milky Way. What's different today is the scaling factor: other universes are too far away to be detectable. Any intelligent life would think that their universe was the Universe.
The multiverse picture in which one huge cosmic domain is carpeted with universes is not a difficult concept to envisage. But for a more demanding mental workout, Greene brings on string theory, a product of attempts to unify the laws of physics, which treats nature's fundamental particles, quarks, as vibrating strings. String theory is a single framework that can handle both relativity and quantum mechanics.
Although the pioneers of string theory soon became dismayed by its mathematical flaws, they then discovered that the anomalies disappeared if the equations were written such that space has many dimensions. In addition to the three dimensions of our everyday experience, there are additional dimensions so compact that we cannot directly experience them. At first sight this is baffling, but Greene has written a fine chapter on unifying nature's laws through string theory. His is an exceptionally clear account of what string theory can and cannot do.
For cosmology, one attractive feature of string theory is that its many unseen dimensions open up a whole new family of parallel universes. In the late 1990s, theorists discovered that space with 10 dimensions permits objects other than strings. There are two-dimensional membranes, which Greene playfully refers to as "flying carpets"; three-branes (with three spatial dimensions) that could contain the whole of the Universe as we know it; four-branes; and so on. This braneworld scenario allows a separate universe to be hovering alongside our own, only a small distance along dimensions that we can't experience. In the brane multiverse, the only physical force that transcends the extra dimensions is gravity, which relative to quantum forces is so feeble that the extra dimensions have escaped detection (at least, so far).
A feature of the Universe that did escape detection until about 10 years ago is the cosmological constant (or dark energy). This construct explains why the expansion of our Universe started to accelerate about 7 billion years ago. Some proponents of the multiverse have latched on to the fact that the cosmological constant is extremely small as being suggestive of an ensemble of universes, with humanity existing in one with a tiny value for the constant. But is this science?
The various multiverse approaches to cosmology take as their starting point the argument that the Universe we can observe is part of a larger whole. The universes beyond or alongside ours are not accessible, and therefore their properties can be explored only by theory and modelling. This of course invites us to do a new kind of science in which it is permissible to develop theories that are not falsifiable by observation. For that reason, the multiverse approach has its critics among philosophers of science.
In his final chapter, the author stands back and looks at the limits of enquiry. In modern physics, we know that everyday experience cannot be trusted to guide us in the quantum world or general relativity. Greene's stance is that each generation of cosmologists cannot know if, in the long run, their work will have revealed new insights or been discarded as a diversion.
This book contains a lot of information and the science is intellectually challenging. I recommend working through it carefully. Readers who already have an elementary knowledge of relativity, quantum theory and astronomy will find this an entertaining guide to nine variations on the multiverse theme.
A Pulitzer Prize finalist in 2000, author and string theorist Brian Greene was entranced by and absorbed in mathematics from a young age.
The realisation that maths was not just a game but had the capacity to describe reality offered a powerful change in perspective for the young Greene, and his interest ultimately moved towards physics.
In 1980, he gained a bachelor's degree in the discipline from Harvard University and continued his studies in the field as a Rhodes scholar at the University of Oxford, where he obtained his doctorate in 1987.
While at Oxford, Greene developed a burning desire to play Johannes Brahms' Rhapsody in G minor, despite being only a novice pianist. His friend Jack Gibbons, a concert pianist, agreed to offer support as best he could, enabling Greene to reach his goal successfully.
Greene also joined the Oxford judo team and competed against the University of Cambridge in 1986.
He now spends his free time restoring his farm in upstate New York.
The Hidden Reality: Parallel Universes and the Deep Laws of the Cosmos
By Brian Greene
Allen Lane, 384pp, £25.00
Published 14 March 2011