In the second of our Big Science Questions, Sir Martin Rees ponders the origin of the universe, while Martin Ince juggles the pieces of a cosmic puzzle.
In many cultures, the origin of the universe is traditionally explained through tales of the creation or separation of the solid Earth, the waters and the sky. The stories commonly feature a belief that there is some device or being that acts as an intermediary between heaven and Earth. The depiction of real or mythical people and animals in celestial constellations, for example.
In the modern era, most people regard these tales as colourful traditions, part of cultural history rather than science. Some, however, have survived. Christian creationists believe that the universe was made in a few days about 6,000 years ago.
The flaw with all early accounts of the origin of the universe is that they devote the bulk of their attention to the formation of the Earth and its living creatures, while in the past few hundred years it has become apparent that the Earth is a minute object in a massive universe, of interest only because we live here. It does not make even a footnote to a footnote in modern cosmology.
The study of the origin of the universe has turned, in the lifetime of contemporary adults, from a subject for speculation into a genuine experimental science, where theories are constrained by a growing range of fundamental data. The main agent of change has been the telescope. Astronomers use telescopes to look deep into space, but, for a cosmologist, telescopes can also peer into time. Light from the nearest star beyond the Sun takes four years to get here, so looking at it involves looking four years into the past. But modern telescopes look so deep into space that they can see the universe in the earliest era of its history. In addition, modern technology allows virtually the whole of the electromagnetic spectrum to be viewed, not just the small fraction that is visible to the human eye. This has led to many insights, the most startling of which was the discovery by Arno Penzias and Robert Woodrow Wilson of cosmic background radiation, which is observed at radio wavelengths. It is regarded by most cosmologists as the hole-in-one proof of the big-bang theory of the origin of the universe.
In addition, the expansion of the universe was detected early in the 20th century by Edwin Hubble and is key to our understanding of its origin. The insight that led to its discovery was the realisation that lines seen in the spectra of distant galaxies were caused by transitions in the electron energies of known atoms, but with their wavelengths altered by the Doppler effect because of their movement away from us. This was a profound insight built on decades of work in laboratory physics, which made astrophysical spectroscopy feasible. But it also depended on painstaking work to determine the scale of the universe and the distances of galaxies. In recent years, we have discovered more about the amount of matter in the universe, its age, and the crucial early period of inflation that occurred before expansion as we see it today. This has been made possible by observational astronomy working in tandem with developments in relativity and quantum theory, which can often seem arcane and well-nigh incomprehensible.
But we should recall that one of the first insights offered by relativity was the definitive solution to one of the longest-running questions in science. For millennia, a wide range of ideas has been put forward to explain how the stars, including the Sun, shine. Only in the 20th century did the discovery of nuclear fusion provide the definitive answer. It also confirmed that stars can indeed shine for the billions of years the Earth and other objects in the solar system have been independently proven to have existed. Scientists have used this knowledge base to illuminate other big questions about major aspects of the universe, including the creation and distribution of the chemical elements, most of which have been produced in the cores of stars.
In modern cosmology, the universe appears to have been roughly the same sort of place for most of its 15-billion-year existence. It is a place to which the rules of physics that we determine on Earth can both be applied and used to predict events. The appearance of the solar system - Sun, planets and satellites, asteroids and comets - about 4.5 billion years ago is a comprehensible event that can be modelled accurately and compared with observations of other stars, around which families of planets, and the dust discs from which they might form, are being discovered. By contrast, thinking about the earliest period of the universe takes us into a world where normal ideas about time and space are valueless.
But there is a way of developing ideas about matter and energy under the extreme conditions that prevailed in the very earliest moments of the universe. Much as a telescope allows us to look back billions of years into the past, so current and planned particle accelerators allow conditions in the very early universe to be replicated. The scientific community that uses them overlaps with the telescope-based cosmology community and a healthy band of theoreticians.
One of the principal challenges they face is the existence of "dark matter" unseen by telescopes or other instruments, but whose gravitational effects suggest that it may far outweigh the comparatively familiar material of which the observable universe is made. Its lack of luminosity does not mean that it is unobservable. Astronomical approaches to the problem include proposals to look for small dark-matter objects hiding stars. At the same time there have been proposals that attack the problem via a particle-physics approach. For example, it is known, that the universe is flooded by particles called neutrinos, which are emitted by stars and have long been thought to have no mass. The missing mass problem might be solved if they had even a tiny amount. It is also possible that some unknown type of material is responsible for the missing mass.
One candidate is the Wimp (weakly-interacting massive particle). Wimps are being sought by experiments in old mines and other sites far away from interference. They are inherently hard to detect (because they interact only weakly with other matter) and, if discovered, would be a major addition to our knowledge of the taxonomy of matter.
But the questions raised by the example of missing mass serve only to show the power of the "standard model" of the universe in which most scientists believe. It unites the particles and forces that make up the universe within a single set of relationships whose shape is now being finalised. Even the discovery that the majority of the universe, in mass terms, may consist of material never seen in a laboratory has not caused a fundamental rethink of its basic accuracy. Part of the reason for the apparent stability of the standard model is its wide evidence base. Astronomical and laboratory methods have produced mutually reinforcing results.
The next stage in this collaboration will involve a step change in equipment and ambition. The Large Hadron Collider under construction beneath Switzerland and France is intended to explore the possible existence of the Higgs Boson, one of the particles predicted in the standard model but which has proven elusive. The Higgs Boson is implicated in the phenomenon of mass, something that, under the standard model, ceases to be a given of the universe and becomes an explainable property. The extreme conditions within the LHC will be the closest yet created to those at the origin of the universe.
In space, a clutch of instruments will build on the successes of the 1990s, including Map, a US mission to examine the fine structure of the cosmic microwave background. The data it yields will tell us about the first moments after the big bang and about the way in which large-scale structures, such as galaxies, arose. Map is a space mission, but the new generation of ground-based telescopes is also penetrating deep into time and allowing objects from within the first billion years of the universe to be observed. New telescopes at the Earth's surface and in space, especially the successor to the Hubble Space Telescope, currently called the New Generation Space Telescope, will allow the universe to be observed at ever-earlier stages of its development.
The connections between cosmology, cosmogony and physics now being formed are so mutually confirming that it might be tempting to assume that they provide a complete picture that allows for no improvement. But anyone tempted to believe that this subject can be closed in a few years should recall the state of physics a century ago, just before the discovery of radioactivity, the photoelectric effect and relativity. At that stage, a high level of confidence that our knowledge was close to perfection was the precursor of an unprecedented era of far-reaching discovery that altered our perception of the universe completely.
Martin Ince is deputy editor, The THES