From here to eternity: One of the first full-sky images taken from the Planck satellite
Until the early twentieth century, cosmology was more like art history than science. There were styles of universe. You could simply imagine, like the ancients, what the universe should be like. Some liked their universes infinitely old; others wanted cosmic history to have a definite beginning; while others fancied their universe as cyclic, with each new cycle springing phoenix-like from the ashes of the old — the same old thing over and over again. All these pictures had their origins in religious, artistic, or philosophical images of what things should be like, because the universe had a meaning not just a description. When observations of the stars and their motions were slowly accumulated, they were often fitted into a picture that displayed that meaning — and ours.
All this changed in 1915. For the first time Einstein’s new theory of gravity, dubbed the general theory of relativity, provided equations whose solutions were entire universes. This was remarkable. Gradually, solutions were found to Einstein’s universe equations. They revealed that the universe should be expanding, a possibility that was confirmed by the observations of Edwin Hubble and Milton Humason at Mount Wilson in 1929. All sorts of different varieties of expansion were found to be possible. At first, the expansion always looked simple and symmetrical, like an expanding sphere. Next, universes that expanded at different rates in different directions were found to be possibilities — and even universes that rotate and allow time travel into our past were allowed. In The Book of Universes (Bodley Head, £20), I tell the story of all the possible universes that Einstein’s equations revealed. Even now, few solutions of these complicated equations have ever been found and when a distinctive new one is discovered, it tends to be named after its discoverer. We have universes that expand and contract, oscillating universes, universes that are chaotic, universes that have beginnings and ends, and universes that have neither.
In 1965, two American radio astronomers, Arno Penzias and Robert Wilson, discovered the heat radiation that was predicted to exist if the universe had expanded from a hot “big bang.” Since then, cosmologists have focused increasingly on the physical processes going on in the universe during its early history, seeking out fossil evidence to confirm our reconstruction of its past. By 1980, it was clear that the universe was expanding in a mysteriously symmetrical fashion at a speed very close to the smallest rate needed to overcome the pull of its own gravity and to keep on expanding forever. There were excellent descriptions of this state of affairs from the gallery of possibilities that Einstein’s equations had already provided. What was lacking was an explanation of why the universe had these special features and also contained a special distribution of small irregularities that evolved into galaxies. For a time, cosmologists were attracted by the appealing idea that the universe began chaotically and became increasingly smooth and symmetrical as it expanded and aged because of the effect of naturally arising frictional processes in its earliest stages. Alas, this seductive theory foundered. Too much heating would result and the universe would have been too hot today if the universe was too irregular in its early moments.
In 1981, Alan Guth proposed a new expanding universe model which sidestepped these problems. He proposed that there was a brief surge of accelerated expansion, which he called “inflation”, in its very early stages. The result was remarkable: it could explain why there was such symmetrical expansion at the special rate that we see today. Soon, others showed that inflation could also generate small random irregularities in the distribution of cosmic material that were stretched by the expansion so as to provide the seeds from which the galaxies can subsequently form, ten billion years later. Remarkably, this sequence of events will leave tell-tale variations in the leftover heat radiation from the early universe in the sky today. Our satellites have found many parts of the predicted patterns. Next year we will learn if more have been found by the European Space Agency’s Planck satellite.
This phenomenon of inflation also leads us to expect that other parts of the universe, which we cannot yet see because of the finite speed of light, will be very different to our own visible part. Moreover, the whole process of inflation is self-reproducing: any region that inflates will create the conditions for further inflation of parts of itself. This process of “eternal” inflation has no end and need have no beginning. It changes our answer to the old question, “Did the universe have a beginning?” Our visible part of the entire universe will have had a beginning but the entire “multiverse” of different regions, all inflating at different rates, need not have one.
During the last decade we have steadily gathered evidence that points to a past era of inflation 13.7 billion years ago. But we have also discovered that the universe began accelerating again four billion years ago after it had expanded to about three quarters of its present extent. This change of gear from cosmic deceleration to acceleration is described with tremendous accuracy by one of the solutions to Einstein’s equations that were first found by the Belgian cosmologist and priest, Georges Lemaître, in 1927. Yet, although Lemaître’s universe is a very accurate description of what we see, again we want to know why our universe changed gear and began accelerating just a few billion years ago. Why then? One fashionable approach is to imagine that the “multiverse” of possible regions that can emerge in the eternal inflationary scenario covers all possibilities and we just happen to inhabit one of those which started accelerating late enough for galaxies, stars, planets and life to have evolved.
More interesting, potentially, is a new extension of Einstein’s theory developed by Douglas Shaw and myself in Cambridge which is just published in the journal, Physical Review Letters. By imposing the restriction of causality on a quantum cosmology, we are able to explain for the first time why the recent acceleration began when it did. We can also predict another very precise feature of the expansion that the Planck satellite’s data will be able to determine. So the magic jar of universes is not exhausted yet. Over the next two years, we will see many new pieces of data that will confirm or exclude a range of possible universes, and help us to understand why ours is just the way it is.