Authors: Professor Brian Cox
The universe modelled in Einstein’s 1917 paper is not the one we inhabit, but the paper is of interest for the introduction of what Einstein later came to view as a mistake. Einstein tried to find a solution to his equations that would describe a finite universe, populated by a uniform distribution of matter, and stable against gravitational collapse. At the time, this was a reasonable thing to do, because astronomers knew of only a single galaxy – the Milky Way – and the stars did not appear to be collapsing inwards towards each other. Einstein also seems to have had a particular story in mind; he felt that an eternal universe was more elegant than one that had a beginning, which left open the thorny question of a creator. He discovered, however, that General Relativity does not allow for a universe with stars, planets and galaxies to be eternal. Instead, his solution told the story of an unstable universe that would collapse inwards. Einstein tried to solve this unfortunate problem by adding a new term in his equations known as the cosmological constant. This extra term can act as a repulsive force, which Einstein adjusted to resist the tendency of his model universe to collapse under its own gravity. Later, he is famously said to have remarked to his friend George Gamow that the cosmological constant was his biggest blunder.
As physicists began to search for solutions to Einstein’s equations, more and more possible universes were discovered. None, with the exception of Einstein’s universe and a universe without matter and dominated by a (positive) cosmological constant discovered in 1917 by Willem de Sitter, was static. We will return to de Sitter’s universe in a moment, but in every other case, Einstein’s equations seemed to imply continual evolution, whereas Einstein himself felt that the universe should be unchanging and eternal. As more physicists worked with the equations, things only got worse for Einstein’s static, eternal universe.
The first exact cosmological solution of Einstein’s equations for a realistic universe filled with galaxies was discovered by Russian physicist Alexander Friedmann in 1922. He reached his result by assuming something that takes us all the way back to the beginning of this chapter: a Copernican universe in the sense that nowhere in space is special. This is known as the assumption of homogeneity and isotropy, and it corresponds to solving Einstein’s equations with a completely uniform matter distribution. This may seem to be a gross oversimplification, and in the early 1920s the extent to which this assumption agreed with the observational evidence – a universe seemingly containing just a single galaxy – was tenuous. From a theoretical perspective, however, Friedmann’s assumption makes perfect sense. It’s the simplest assumption one can make, and it makes it relatively easy to do the sums! So relatively easy, in fact, that Friedmann’s work was replicated and extended quite independently by a Belgian mathematician and priest named Georges Lemaître. Lemaître planted his flag firmly in the no-man’s-land between religion and science – a strip of intellectual land occupied, whether we like it or not, by cosmology. A student of Harlow Shapley, this deeply religious man never saw a conflict between these two very different modes of human thought. He embodies the much debated and criticised modern notion, introduced by the evolutionary biologist Stephen J. Gould, that science and religion are non-overlapping magesteria, asking the same questions but operating within separate domains. My view is that this is far too simplistic a position to take; questions concerning the origin of the physical universe are of the same character as questions about the nature of the gravitational force or the behaviour of subatomic particles, and answers will surely be found by employing the methodology of science. Having said that, I am willing to recognise that romance, or wonder, or whatever the term is for that deep feeling of awe when contemplating the universe in all its immensity, is a central component of both religious and scientific experience, and perhaps there is room for both in providing the inspiration for the exploration of nature.
At least this is what Lemaître felt, and he used his twin perspectives as a guide on his intellectual journey through the cosmos throughout his distinguished career. Ordained a priest in 1923 while studying at the Catholic University in Louvain, Lemaître studied physics and mathematics alongside some of the great physicists and astronomers of the time, including Arthur Eddington and Harlow Shapley, from the University of Cambridge to Harvard and MIT, before returning to Belgium in 1925 to work with Einstein’s General Relativity.
Lemaître never met Alexander Friedmann, who died from typhoid in 1925. They never spoke or corresponded, and Lemaître was almost certainly unaware of the obscure paper Friedmann had published describing a dynamic and changing universe. He followed the same intellectual path, however, assuming an isotropic and homogeneous distribution of matter in the cosmos, and searching for solutions to Einstein’s equations that describe the story of this smooth and uniform universe. And, of course, he came to the same conclusion: such a universe cannot be static – it must either expand or contract. Lemaître met Einstein at the 1927 Solvay Conference in Brussels, and told him of his conclusions. ‘Your calculations are correct, but your physics insight is abominable’, snapped the great man. Einstein was wrong. By 1931, Lemaître was writing papers containing wonderfully vivid phrases and making clear his view that Einstein’s theory requires a moment of creation – a Big Bang. He writes of ‘a day without yesterday’, and of the universe emerging from a ‘primeval atom’.
In 1934, the Princeton physicist Howard Percy Robertson catalogued all of the possible solutions to Einstein’s equations given a uniform distribution of matter throughout the cosmos – a perfect Copernican principle according to which no place in the cosmos is special or significant. The models containing matter tend to describe either an expanding or contracting universe, and therefore suggest a quite wonderful thing: there may have been a day without a yesterday. Einstein’s equations contain within them a scientific creation story, even though their author himself resisted it.
The story of Einstein’s Theory of General Relativity, and its subsequent application to the whole universe, delivers a compelling narrative illustrating the power of physics. The theory, inspired by thinking about a man falling off a roof, predicts that there was a moment of creation. No experimental measurements are required and no observations need be made other than that things fall at the same rate in a gravitational field. There are multiple layers of irony here! The idea that such progress towards answering the most profound questions about our origins can be made by thinking alone is almost Aristotelian: a partial throwback to the lofty authority of the classical world that Bruno, Copernicus and Galileo did so much to overturn. That the equations seem to describe a universe with a necessary moment of creation, lending support, at least in Lemaître’s eyes, to the notion of a creator, would also appear to bring us full circle and back to Borman, Lovell and Anders and the creation stories of old. Indeed, Pope Pius XII, on hearing about the new cosmology, said ‘True science to an ever increasing degree discovers God, as though God was waiting behind each door opened by science’. Einstein, to his deep chagrin, having thrown a blanket of rational thought across a landscape of mythology, appeared to have replaced one creation story with another.
To finish the story of our magnificent relegation, let me briefly address these points. The theoretical prediction of an expanding universe does of course require experimental verification, and this came rapidly. On 15 March 1929, Edwin Hubble published a paper entitled ‘A relation between distance and radial velocity among extra-galactic nebulae’, in which he reported his observation that all galaxies beyond our local group are rushing away from us. Moreover, the more distant the galaxy, the higher its speed of recession. This is precisely what an expanding universe as predicted by Einstein’s theory should look like. In 1948, Alpher, Bethe and Gamow published a famous paper (with the coolest author list in the history of physics) which showed how the observed abundance of light chemical elements in the universe could be calculated assuming a very hot, dense phase in the early history of the universe. Modern calculations of these abundances are extremely precise, and agree perfectly with astronomical observations. Perhaps most compellingly of all, the afterglow of the Big Bang, known as the Cosmic Microwave Background Radiation, also predicted by Alpher and Herman in 1948, was discovered by Penzias and Wilson in 1964. We will have much to say about the Cosmic Microwave Background in the following chapters; for now, it is sufficient to say that the discovery that the universe is still glowing at a temperature of 2.7 degrees above absolute zero was the final evidence that convinced even the most sceptical scientists that the Big Bang theory was the most compelling model for the evolution of the universe.
What, though, of the thorny question of the cause of the Big Bang itself? What was the origin of Lemaître’s primeval atom? Did God really do it? The standard Big Bang cosmology of the twentieth century has no answer to this question, but twenty-first-century cosmology does. We will address the current scientific understanding of what happened before the Big Bang later on, but let me offer a tantalising hint here. It is now thought that before the Big Bang the universe underwent a period of exponential expansion known as inflation. In this time, the universe behaved in accord with de Sitter’s matter-less solution to Einstein’s equations discovered in 1917. This period of rapid expansion gave us the homogeneous and isotropic distribution of matter we see today on large distance scales, which is the reason why Friedmann and Lemaître’s simple Copernican assumptions lead to a description of the evolution of the universe after the Big Bang that fits observational data perfectly. There are no special places in the universe because the early inflationary expansion smoothed everything out. When inflation stopped, the energy contained within the field that drove it was dumped back into the universe, creating all the matter and radiation we observe today. Small fluctuations in the inflation field seeded the formation of the galaxies, uniformly distributed across the sky in their billions, each containing countless worlds, quite possibly without end beyond the visible horizon. In the words of Georges Lemaître, ‘Standing on a well-cooled cinder we see the slow fading of the suns and we try to recall the vanished brilliance of the origin of the worlds.’ Our cinder is not special; it is insignificant in size; one world amongst billions in one galaxy amongst trillions. But it has been a tremendous ascent into insignificance because, by the virtuous combination of observation and thought, we have been able to discover our place. How Giordano Bruno would have loved what we found.
Sometimes I think we are alone in the universe
and sometimes I think we’re not.
In either case the idea is quite staggering.
Arthur C. Clarke
There are questions to which knowing the answers would have a profound cultural effect. The question of our solitude is one. Are we alone in the universe – yes or no? One of these is true. The question as posed isn’t a good one, however, because it is impossible to answer in the affirmative. We have no chance, even in principle, of exploring the entire universe, which extends way beyond the visible horizon 46 billion light years away. The answer can therefore never be yes with certainty. Indeed, if the universe is infinite in extent, we have our answer! No, we are not alone. The laws of nature self-evidently allow life to exist, and no matter how improbable, life must have arisen an infinite number of times. In itself, this is quite a challenging statement, and we will explore it in more detail later on. But this isn’t really what most of us want to know.
I’ve always been interested in aliens – the ones that fly spaceships around – and I want to talk to one. On a winter afternoon in 1977 I stood in a queue that went around three sides of the Odeon cinema in Oldham with my dad, shuffling through half-frozen puddles to see
Star Wars
, and spent the next decade building Millennium Falcons out of Lego. At some point in 1979 I picked up a magazine about
Alien
, and moved on to
Nostromo
, which required more bricks. To my delight I saw
Alien
when I was 11 years old at Friday Evening Film Society at school, and it didn’t put me off. I just realised I really liked the spaceships, and didn’t care much about the organic stuff. Everyone should see
Alien
at 11. To hell with the ratings; terror, technology and Sigourney Weaver are good for the soul.
Science fiction was a natural home for my imagination. I’d been interested in astronomy for a while, I’m not sure why, but the study of the stars seemed clean and precise and romantic; something done on cold nights before Christmas with mittens and imagination.
Star Wars
,
Star Trek
,
Alien
, Arthur C. Clarke and Isaac Asimov were merged seamlessly with Patrick Moore, Carl Sagan and James Burke, and they remain so; fact and fiction are inseparable in dreams. The superficially orthogonal desires to do science and to imagine distant worlds are closely related: shadows cast by different lights.
So the question ‘Are we alone in the universe?’ might make good science fiction, but it is not well posed in a scientific sense because the universe is too big for us to explore in its entirety. If we restrict the domain of the question, however, we can address it scientifically. ‘Are we alone in the solar system?’ is a question we are actively seeking to answer with Mars rovers and future missions to the moons of Jupiter and Saturn, where the conditions necessary for life may be present on multiple worlds. But even here, the use of the word ‘alone’ in the question is problematic. Would we be alone if the universe were full of microbes? Would you feel alone stranded in a deep cave with no means of escape and a billion bacteria for company? If not being alone means having intelligent beings to communicate with – sophisticated creatures that build civilisations, have feelings, do science and respond emotionally to the universe, then we have our answer in the solar system. Yes – Earth is the only world that is home to a civilisation, and we are alone.