WHERE ARE WE? - Human Universe - Brian Cox, Andrew Cohen

Human Universe - Brian Cox, Andrew Cohen (2014)


We shall not cease from exploration,

And the end of all our exploring

Will be to arrive where we started

And know the place for the first time.

T. S. Eliot


For me, it was an early 1960s brick-built bungalow on Oakbank Avenue. If the wind was blowing from the east you could smell vinegar coming from Sarson’s Brewery - although these were rare days in Oldham, a town usually subjected to Westerlies dumping Atlantic moisture onto the textile mills, dampening their red brick in a permanent sheen against the sodden sky. On a good day, though, you’d take the vinegar in return for sunlight on the moors. Oldham looks like Joy Division sounds - and I like Joy Division. There was a newsagent on the corner of Kenilworth Avenue and Middleton Road and on Fridays my granddad would take me there and we’d buy a toy - usually a little car or truck. I’ve still got most of them. When I was older, I’d play tennis on the red cinder courts in Chadderton Hall Park and drink Woodpecker cider on the bench in the grounds of St Matthew’s Church. One autumn evening just after the start of the school year, and after a few sips, I had my first kiss there - all cold nose and sniffles. I suppose that sort of behaviour is frowned upon these days; the bloke in the off-licence would have been prosecuted by Oldham Council’s underage cider tsar and I’d be on a list. But I survived, and, eventually, I left Oldham for the University of Manchester.

Everyone has an Oakbank Avenue; a place in space at the beginning of our time, central to an expanding personal universe. For our distant ancestors in the East African Rift, their expansion was one of physical experience alone, but for a human fortunate to be born in the latter half of the twentieth century in a country like mine, education powers the mind beyond direct experience - onwards and outwards and, in the case of this little boy, towards the stars.

As England stomped its way through the 1970s, I learned my place amongst the continents and oceans of our blue planet. I could tell you about polar bears on Arctic ice flows or gazelle grazing on central plains long before I physically left our shores. I discovered that our Earth is one planet amongst nine (now redefined as eight) tracing out an elliptical orbit around an average star, with Mercury and Venus on the inside and Mars, Jupiter, Saturn, Uranus and Neptune beyond. The Sun is one star amongst 400 billion in the Milky Way Galaxy, itself just one galaxy amongst 350 billion in the observable universe. Later, at university, I discovered that physical reality extends way beyond the 90-billion-light-year visible sphere into - if I had to guess based on my 46-year immersion in the combined knowledge of human civilisation - infinity.

This is my ascent into insignificance; a road travelled by many and yet one that remains intensely personal to each individual who takes it. The routes we follow through the ever-growing landscape of human knowledge are chaotic; the delayed turn of a page in a stumbled-upon book can lead to a lifetime of exploration. But there are common themes amongst our disparate intellectual journeys, and the relentless relegation from centre stage that inevitably followed the development of modern astronomy has had a powerful effect on our shared experience. I am certain that the voyage from the centre of creation to an infinitesimally tiny speck should be termed an ascent, the most glorious intellectual climb. Of course, I also recognise that there are many who have struggled - and continue to struggle - with such a dizzying physical relegation.

John Updike once wrote that ‘Astronomy is what we have now instead of theology. The terrors are less, but the comforts are nil’. For me, the choice between fear and elation is a matter of perspective, and it is a central aim of this book to make the case for elation. This may appear at first sight to be a difficult challenge - the very title Human Universe appears to demonstrate an unjustifiable solipsism. How can a possibly infinite reality be viewed through the prism of a bunch of biological machines temporarily inhabiting a mote of dust? My answer to that is that Human Universe is a love letter to humanity, because our mote of dust is the only place where love certainly exists.

This sounds like a return to the anthropocentric vision we held for so long, and which science has done so much to destroy in a million humble cuts. Perhaps. But let me offer an alternative view. There is only one corner of the universe where we know for sure that the laws of nature have conspired to produce a species capable of transcending the physical bounds of a single life and developing a library of knowledge beyond the capacity of a million individual brains which contains a precise description of our location in space and time. We know our place, and that makes us valuable and, at least in our local cosmic neighbourhood, unique. We don’t know how far we would have to travel to find another such island of understanding, but it is surely a long long way. This makes the human race worth celebrating, our library worth nurturing, and our existence worth protecting.

Building on these ideas, my view is that we humans represent an isolated island of meaning in a meaningless universe, and I should immediately clarify what I mean by meaningless. I see no reason for the existence of the universe in a teleological sense; there is surely no final cause or purpose. Rather, I think that meaning is an emergent property; it appeared on Earth when the brains of our ancestors became large enough to allow for primitive culture - probably between 3 and 4 million years ago with the emergence of Australopithecus in the Rift Valley. There are surely other intelligent beings in the billions of galaxies beyond the Milky Way, and if the modern theory of eternal inflation is correct, then there is an infinite number of inhabited worlds in the multiverse beyond the horizon. I am much less certain that there are large numbers of civilisations sharing our galaxy, however, which is why I use the term ‘isolated’. If we are currently alone in the Milky Way, then the vast distances between the galaxies probably mean that we will never get to discuss our situation with anyone else.

We will encounter all these ideas and arguments later in this book, and I will carefully separate my opinion from that of science - or rather what we know with a level of certainty. But it is worth noting that the modern picture of a vast and possibly infinite cosmos, populated with uncountable worlds, has a long and violent history, and the often visceral reaction to the physical demotion of humanity lays bare deeply held prejudices and comfortable assumptions that sit, perhaps, at the core of our being. It seems appropriate, therefore, to begin this tour of the human universe with a controversial figure whose life and death resonates with many of these intellectual and emotional challenges.

Giordano Bruno is as famous for his death as for his life and work. On 17 February 1600, his tongue pinioned to prevent him from repeating his heresy (which recalls the stoning scene in Monty Python’s Life of Brian when the admonishment ‘you’re only making it worse for yourself’ is correctly observed to be an empty threat), Bruno was burned at the stake in the Campo de’ Fiori in Rome and his ashes thrown into the Tiber. His crimes were numerous and included heretical ideas such as denying the divinity of Jesus. It is also the opinion of many historians that Bruno was irritating, argumentative and, not to put too fine a point on it, an all-round pain in the arse, so many powerful people were simply glad to see the back of him. But it is also true that Bruno embraced and promoted a wonderful idea that raises important and challenging questions. Bruno believed that the universe is infinite and filled with an infinite number of habitable worlds. He also believed that although each world exists for a brief moment when compared to the life of the universe, space itself is neither created nor destroyed; the universe is eternal.

Although the precise reasons for Bruno’s death sentence are still debated amongst historians, the idea of an infinite and eternal universe seems to have been central to his fate, because it clearly raises questions about the role of a creator. Bruno knew this, of course, which is why his return to Italy in 1591 after a safe, successful existence in the more tolerant atmosphere of northern Europe remains a mystery. During the 1580s Bruno enjoyed the patronage of both King Henry III of France and Queen Elizabeth I of England, loudly promoting the Copernican ideal of a Sun-centred solar system. Whilst it’s often assumed that the very idea of removing the Earth from the centre of the solar system was enough to elicit a violent response from the Church, Copernicanism itself was not considered heretical in 1600, and the infamous tussles with Galileo lay 30 years in the future. Rather, it was Bruno’s philosophical idea of an eternal universe, requiring no point of creation, which unsettled the Church authorities, and perhaps paved the way for their later battles with astronomy and science. As we shall see, the idea of a universe that existed before the Big Bang is now central to modern cosmology and falls very much within the realm of observational and theoretical science. In my view this presents as great a challenge to modern-day theologians as it did in Bruno’s time, so it’s perhaps no wonder that he was dispensed with.

Bruno, then, was a complex figure, and his contributions to science are questionable. He was more belligerent free-thinker than proto-scientist, and whilst there is no shame in that, the intellectual origins of our ascent into insignificance lie elsewhere. Bruno was a brash, if portentous, messenger who would likely not have reached his heretical conclusions about an infinite and eternal universe without the work of Nicolaus Copernicus, grounded in what can now clearly be recognised as one of the earliest examples of modern science, and published over half a century before Bruno’s cinematic demise.


Nicolaus Copernicus was born in the Polish city of Torun in 1473 and benefited from a superb education after being enrolled at the University of Cracow at 18 by his influential uncle, the Bishop of Warmia. In 1496, intending to follow in the footsteps of his uncle, Copernicus moved to Bologna to study canon law, where he lodged with an astronomy professor, Domenica Maria de Novara, who had a reputation for questioning the classical works of the ancient Greeks and in particular their widely accepted cosmology.

The classical view of the universe was based on Aristotle’s not unreasonable assertion that the Earth is at the centre of all things, and that everything moves around it. This feels right because we don’t perceive ourselves to be in motion and the Sun, Moon, planets and stars appear to sweep across the sky around us. However, a little careful observation reveals that the situation is in fact more complicated than this. In particular, the planets perform little loops in the sky at certain times of year, reversing their track across the background stars before continuing along their paths through the constellations of the zodiac. This observational fact, which is known as retrograde motion, occurs because we are viewing the planets from a moving vantage point - the Earth - in orbit around the Sun.

This is by far the simplest explanation for the evidence, although it is possible to construct a system capable of predicting the position of the planets months or years ahead and maintain Earth’s unique stationary position at the centre of all things. Such an Earth-centred model was developed by Ptolemy in the second century and published in his most famous work, Almagest. The details are extremely complicated, and aren’t worth describing in detail here because the central idea is totally wrong and we won’t learn anything. The sheer contrived complexity of an Earth-centred description of planetary motions can be seen in Ptolemy’s Model, which shows the apparent motions of the planets against the stars as viewed from Earth. This tangled Ptolemaic system of Earth-centred circular motions, replete with the arcane terminology of epicycles, deferents and equant, was used successfully by astrologers for thousands of years to predict where the planets would be against the constellations of the zodiac - presumably allowing them to write their horoscopes and mislead the gullible citizens of the ancient world. And if all you care about are the predictions themselves, and your philosophical prejudice and common-sense feeling of stillness require the Earth to be at the centre, then everything is fine. And so it remained until Copernicus became sufficiently offended by the sheer ugliness of the Ptolemaic model to do something about it.

Copernicus’s precise objections to Ptolemy are not known, but sometime around 1510 he wrote an unpublished manuscript called the Commentariolus in which he expressed his dissatisfaction with the model. ‘I often considered whether there could perhaps be found a more reasonable arrangement of circles, from which every apparent irregularity would be derived while everything in itself would move uniformly, as is required by the rule of perfect motion.’

The Commentariolus contained a list of radical and mostly correct assertions. Copernicus wrote that the Moon revolves around the Earth, the planets rotate around the Sun, and the distance from the Earth to the Sun is an insignificant fraction of the distance to the stars. He was the first to suggest that the Earth rotates on its axis, and that this rotation is responsible for the daily motion of the Sun and stars across the sky. He also understood that the retrograde motion of the planets is due to the motion of the Earth and not the planets themselves. Copernicus always intended Commentariolus to be the introduction to a much larger work, and included little if any detail about how he had come upon such a radical departure from classical ideas. The full justification for and description of his new cosmology took him a further 20 years, but by 1539 he had finished most of his six-volume De revolutionibus, although the completed books were not published until 1543. They contained the mathematical elaborations of his heliocentric model, an analysis of the precession of the equinoxes, the orbit of the Moon, and a catalogue of the stars, and are rightly regarded as foundational works in the development of modern science. They were widely read in universities across Europe and admired for the accuracy of the astronomical predictions contained within. It is interesting to note, however, that the intellectual turmoil caused by our relegation from the centre of all things still coloured the view of many of the great scientific names of the age. Tycho Brahe, the greatest astronomical observer before the invention of the telescope, referred to Copernicus as a second Ptolemy (which was meant as a compliment), but didn’t accept the Sun-centred solar system model in its entirety, partly because he perceived it to be in contradiction with the Bible, but partly because it does seem obvious that the Earth is at rest. This is not a trivial objection to a Copernican solar system, and a truly modern understanding of precisely what ‘at rest’ and ‘moving’ mean requires Einstein’s theories of relativity - which we will get to later! Even Copernicus himself was clear that the Sun still rested at the centre of the universe. But as the seventeenth century wore on, precision observations greatly improved due to the invention of the telescope and an increasingly mature application of mathematics to describe the data, and led a host of astronomers and mathematicians - including Johannes Kepler, Galileo and ultimately Isaac Newton - towards an understanding of the workings of the solar system. This theory is good enough even today to send space probes to the outer planets with absolute precision.

At first sight it is difficult to understand why Ptolemy’s contrived mess lasted so long, but there is a modern bias to this statement that is revealing. Today, a scientifically literate person assumes that there is a real, predictable universe beyond Earth that operates according to laws of nature - the same laws that objects obey here on Earth. This idea, which is correct, only emerged fully formed with the work of Isaac Newton in the 1680s, over a century after Copernicus. Ancient astronomers were interested primarily in predictions, and although the nature of physical reality was debated, the central scientific idea of universal laws of physics had simply not been discovered. Ptolemy created a model that makes predictions that agree with observation to a reasonable level of accuracy, and that was good enough for most people. There had been notable dissenting voices, of course - the history of ideas is never linear. Epicurus, writing around 300 BCE, proposed an eternal cosmos populated by an infinity of worlds, and around the same time Aristarchus proposed a Sun-centred universe about which the Earth and planets orbit. There was also a strong tradition of classic orthodoxy in the Islamic world in the tenth and eleventh centuries. The astronomer and mathematician Ibn al-Haytham pointed out that, whilst Ptolemy’s model had predictive power, the motions of the planets as shown in the figure here represent ‘an arrangement that is impossible to exist’.

The end of the revolution started by Copernicus around 1510, and the beginning of modern mathematical physics, can be dated to 5 July 1687, when Isaac Newton published the Principia. He demonstrated that the Earth-centred Ptolemaic jumble can be replaced by a Sun-centred solar system and a law of universal gravitation, which applies to all objects in the universe and can be expressed in a single mathematical equation:

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The equation says that the gravitational force between two objects - a planet and a star, say - of masses m1 and m2 can be calculated by multiplying the masses together, dividing by the square of the distance r between them, and multiplying by G, which encodes the strength of the gravitational force itself. G, which is known as Newton’s Constant, is, as far as we know, a fundamental property of our universe - it is a single number which is the same everywhere and has remained so for all time. Henry Cavendish first measured G in a famous experiment in 1798, in which he managed (indirectly) to measure the gravitational force between lead balls of known mass using a torsion balance. This is yet another example of the central idea of modern physics - lead balls obey the same laws of nature as stars and planets. For the record, the current best measurement of G = 6.67 × 10-11 N m2/kg2, which tells you that the gravitational force between two balls of mass 1kg each, 1 metre apart, is just less than ten thousand millionths of a Newton. Gravity is a very weak force indeed, and this is why its strength was not measured until 71 years after Newton’s death.



Force between the masses


Gravitational constant


First mass


Second mass


Distance between the centres of the masses

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This is a quite brilliant simplification, and perhaps more importantly, the pivotal discovery of the deep relationship between mathematics and nature which underpins the success of science, described so eloquently by the philosopher and mathematician Bertrand Russell: ‘Mathematics, rightly viewed, possesses not only truth, but supreme beauty - a beauty cold and austere, like that of sculpture, without appeal to any part of our weaker nature, without the gorgeous trappings of painting or music, yet sublimely pure, and capable of a stern perfection such as only the greatest art can show. The true spirit of delight, the exaltation, the sense of being more than Man, which is the touchstone of the highest excellence, is to be found in mathematics as surely as in poetry.’

Nowhere is this sentiment made more clearly manifest than in Newton’s Law of Gravitation. Given the position and velocity of the planets at a single moment, the geometry of the solar system at any time millions of years into the future can be calculated. Compare that economy - you could write all the necessary information on the back of an envelope - with Ptolemy’s whirling offset epicycles. Physicists greatly prize such economy; if a large array of complex phenomena can be described by a simple law or equation, this usually implies that we are on the right track.

The quest for elegance and economy in the description of nature guides theoretical physicists to this day, and will form a central part of our story as we trace the development of modern cosmology. Seen in this light, Copernicus assumes even greater historical importance. Not only did he catalyse the destruction of the Earth-centred cosmos, but he inspired Brahe, Kepler, Galileo, Newton and many others towards the development of modern mathematical physics - which is not only remarkably successful in its description of the universe, but was also necessary for the emergence of our modern technological civilisation. Take note, politicians, economists and science policy advisors of the twenty-first century: a prerequisite for the creation of the intellectual edifice upon which your spreadsheets, air-conditioned offices and mobile phones rest was the curiosity-driven quest to understand the motions of the planets and the Earth’s place amongst the stars.


Matching the observations of the wandering stars - the planets - of the night sky with the idea that the Earth was at the centre of the solar system required extremely complex models. In the case of Venus, combining the Earth at the centre with the observations meant that Venus had a circular orbit around a point midway between the Earth and the Sun, so-called epicycles, with all the other planets having similar complicated orbits around various points scattered around the solar system. Placing the Sun at the centre of the solar system, with the planets arranged in their familiar order, with the Moon orbiting the Earth, gave a much simpler system.

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1968 was a difficult year on planet Earth. The Vietnam War, the bloodiest of Cold War proxy tussles, was at its height, ultimately claiming over three million lives. Martin Luther King Jr. was assassinated in Memphis, prompting presidential hopeful Bobby Kennedy to ask the people of the United States ‘to tame the savageness of man and make gentle the life of this world’. Kennedy himself was assassinated before the year was out. Elsewhere, Russian tanks rolled into Prague, and France teetered on the edge of revolution. As I approached my first Christmas, my parents could have been forgiven for wondering what kind of world their son would inhabit in 1969. And then, as Christmas Eve drifted into Christmas morning, an unexpected snowfall decorated Oakbank Avenue and Borman, Lovell and Anders, 400,000 kilometres away, saved 1968.

Apollo 8 was, in the eyes of many, the Moon mission that had the most profound historical impact. It was a terrific, noble risk; a magnificent roll of the dice; a distillation of all that is great about exploration; a tribute to the sheer balls of the astronauts and engineers who decided that, come what may, they would honour President Kennedy’s pledge to send ‘a giant rocket more than 300 feet tall, the length of this football field, made of new metal alloys, some of which have not yet been invented, capable of standing heat and stresses several times more than have ever been experienced, fitted together with a precision better than the finest watch, carrying all the equipment needed for propulsion, guidance, control, communications, food and survival, on an untried mission, to an unknown celestial body, and then return it safely to Earth, re-entering the atmosphere at speeds of over 40,000 kilometres per hour, causing heat about half that of the temperature of the Sun - almost as hot as it is here today - and do all this, and do it right, and do it first before this decade is out’. If I heard that from a leader today I’d be first on the rocket. Instead I have to listen to vacuous diatribes about ‘fairness’, ‘hard-working families’, and how ‘we’re all in it together’. Sod that, I want to go to Mars.

To set Apollo 8 in context, Apollo 7, the first manned test flight of the Apollo programme, was flown by Schirra, Eisele and Cunningham in October 1968. Apollo 8 was supposed to be a December test flight for the Lunar Lander, conducted in the familiar surroundings of Earth orbit, but delivery delays meant that it was not ready for flight and the aim of meeting Kennedy’s deadline looked to be dead. But this wasn’t the twenty-first century, it was the 1960s and NASA was run by engineers. The programme manager was George Low, an army veteran and aeronautical engineer who knew the spacecraft inside out and had the strength of character to make decisions. Why not send Apollo 8 directly to the Moon without the Lunar Lander, proposed Low, allowing Apollo 9 to test-fly the LEM (Lunar Excursion Module) in Earth orbit in early 1969 when it became available and pave the way for a landing before the decade was out? Virtually every engineer at NASA is said to have agreed, and so it was that only the second manned flight of the Apollo spacecraft lifted off from Kennedy on 21 December, ten short weeks after Apollo 7, bound for the Moon. The crew later said that they estimated their chance of succeeding to be fifty-fifty.

Borman: Oh my God!

Look at that picture over there.

Here’s the Earth coming up.

Wow, is that pretty.

Anders: Hey, don’t take that,

it’s not scheduled.

Borman: (laughing) You got a
color film, Jim?

Anders: Hand me that roll of
color quick, will you …?

Lovell: Oh, man, that’s great!

Precisely 69 hours, 8 minutes and 16 seconds after launch, the Command Module’s engine fired to slow the spacecraft down and allow it to be captured by the Moon’s gravity, putting the three astronauts into lunar orbit. Newton’s almost 300-year-old equations were used to calculate the trajectory. This was a spectacular, practically unbelievable engineering triumph. Less than a decade after Yuri Gagarin became the first human to orbit the Earth, three astronauts travelled all the way to the Moon. But the mission’s powerful and enduring cultural legacy rests largely on two very human actions by the crew. One was the famous and moving Christmas broadcast, the most-watched television event in history at that time, when distant explorers read the first lines from the Book of Genesis: ‘We are now approaching lunar sunrise, and for all the people back on Earth, the crew of Apollo 8 has a message that we would like to send to you,’ began Anders. ‘In the beginning God created the heaven and the Earth. And the Earth was without form, and void; and darkness was upon the face of the deep.’ Borman concluded with a sentence clearly spoken by a lonely man 400,000 kilometres from home. ‘And from the crew of Apollo 8, we close with goodnight, good luck, a Merry Christmas - and God bless all of you, all of you on the good Earth.’

The mission’s most potent legacy, however, is NASA image AS8-14-2383, snapped by Bill Anders on a Hasselblad 500 EL at f/11 and a shutter speed of 1/250th of a second on Kodak Ektachrome film. It was, in other words, a very bright photograph. The image is better known as Earthrise. When viewed with the lunar surface at the bottom, Earth is tilted on its side with the South Pole to the left, and the equator running top to bottom. Little landmass can be seen through the swirling clouds, but the bright sands of the Namib and Saharan deserts stand out salmon pink against the blackness beyond. Just 368 years and 10 months after a man was burned at the stake for dreaming of worlds without end, here is Earth, a fragile crescent suspended over an alien landscape, the negative of a waxing Moon in the friendly skies of Earth. This is an unfamiliar, planetary Earth, no longer central; just another world. When Kennedy spoke of Apollo as a journey to an unknown celestial body, he meant the Moon. But we discovered Earth and, in the words of T. S. Eliot, came to know the place for the first time.


Newton’s laws are the keys to understanding our place in our local neighbourhood. Coupled with precision observations of the motion of the planets and moons, they allow the scale and geometry of the solar system to be deduced, and their positions to be calculated at any point in the future. The nature and location of the stars, however, requires an entirely different approach because at first sight they appear to be point-like and fixed. The observation that the stars don’t appear to move is important if you know something about parallax, as the ancients did. Parallax is the name given to a familiar effect. Hold your finger up in front of your face and alternately close each of your eyes, keeping your finger still. Your finger appears to move relative to the more distant background, and the closer your finger is to your face, the more it appears to move. This is not an optical illusion; it’s a consequence of viewing a nearby object from two different spatial positions; in this case the two slightly different positions of your eyes. We don’t normally perceive this parallax effect because the brain combines the inputs from the eyes to create a single image, although the information is exploited to create our sense of depth. Aristotle used the lack of stellar parallax to argue that the Earth must be stationary at the centre of the universe, because if the Earth moved then the nearby stars would be observed to move against the background of the more distant ones. Thousands of years later, Tycho Brahe used a similar argument to refute the conclusions reached by Copernicus. Their logic was completely sound, but the conclusion is wrong because the nearby stars do move relative to the more distant background stars as the Earth orbits the Sun, and indeed as the Sun orbits the galaxy itself. You just have to look extremely carefully to see the effect.

Amongst the thousands of stars visible to the naked eye, 61 Cygni is one of the faintest. It’s not without interest, being a binary star system of two orange K-type dwarf stars, slightly smaller and cooler than the Sun, orbiting each other at the lethargic rate of around 700 years. Despite the pair’s relative visual anonymity, however, 61 Cygni has great historical significance. The reason for this quiet fame is that this faint star system was the first to have its distance from Earth measured by parallax.

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Friedrich Bessel is best known to a physicist or mathematician for his work on the mathematical functions that bear his name. Pretty much any engineering or physical problem that involves a cylindrical or spherical geometry ends up with the use of Bessel functions, and, in blissful ignorance, you will probably encounter some piece of technology that has relied on them in the design process at some point today. But Bessel was first and foremost an astronomer, being appointed director of the Königsberg Observatory at the age of only 25. In 1838, Bessel observed that 61 Cygni shifted its position in the sky by approximately two-thirds of an arcsecond over a period of a year as viewed from Earth. That’s not very much - an arcsecond is one 3600th of a degree. It is enough, however, to do a bit of trigonometry and calculate that 61 Cygni is 10.3 light years away from our solar system. This compares very favourably with the modern measurement of the distance, 11.41 ± 0.02 light years. Parallax is so important in astronomy that there is a measurement system completely based on it, which allows you to do these sums in your head. Astronomers use a distance measurement known as a parsec - which stands for ‘per arcsecond’. This is the distance of a star from the Sun that has a parallax of 1 arcsecond. One parsec is 3.26 light years. Bessel’s measurement of the parallax of 61 Cygni was 0.314 arcseconds, and this immediately implies that it’s around 10 light years away.

Even today, stellar parallax remains the most accurate way of determining the distance to nearby stars, because it is a direct measurement which uses only trigonometry and requires no assumptions or physical models. On 19 December 2013 the Gaia space telescope was launched on a Soyuz rocket from French Guiana. The mission will measure, by parallax, the positions and motions of a billion stars in our galaxy over five years. This data will provide an accurate and dynamic 3D map of the galaxy, which in turn will allow for an exploration of the history of the Milky Way, because Newton’s laws, which govern the motions of all these stars under the gravitational pull of each other, can be run backwards as well as forwards in time. Given precise measurements of the positions and velocities of 1 per cent of the stars in the Milky Way, it is possible to ask what the configuration of the stars looked like millions or even billions of years ago. This enables astronomers to build simulations of the evolution of our galaxy, revealing its history of collisions and mergers with other galaxies over 13 billion years, stretching back to the beginning of the universe. Newton and Bessel would have loved it.

Stellar parallax, when deployed using a twenty-first-century orbiting observatory, is a powerful technique for mapping our galaxy out to distances of many thousands of light years. Beyond our galaxy, however, the distances are far too great to employ this direct method of distance measurement. In the mid-nineteenth century, this might have appeared an insurmountable problem, but science doesn’t proceed by measurement alone. As Newton so powerfully demonstrated, scientific progress often proceeds through the interaction between theory and observation. Newton’s Law of Gravitation is a theory; in physics this usually means a mathematical model that can be applied to explain or predict the behaviour of some part of the natural world. How might we measure the mass of a planet? We can’t ‘weigh’ it directly, but given Newton’s laws we can determine the planet’s mass very accurately if it has a moon. The logic is quite simple - the moon’s orbit clearly has something to do with the planet’s gravity, which in turn has something to do with its mass. These relationships are encoded in Newton’s law, and careful observation of the moon’s orbit around the planet therefore allows for the planet’s mass to be determined. For the more mathematical reader, the equation is:

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where a is the (time-averaged) distance between the planet and the moon, G is Newton’s gravitational constant and P is the period of the orbit. (This equation is in fact Kepler’s third law, discovered empirically by Kepler in 1619. Kepler’s laws can be derived from Newton’s law of gravitation.) Under the assumption that the mass of the planet is far larger than the mass of the Moon, this equation allows for the mass of the planet to be measured. This is how theoretical physics can be used to extract measurements from observation, given a mathematical model of the system. To measure the distance to objects that are too far away to use parallax, therefore, we need to find a theory or mathematical relationship that allows for a measurement of something - anything - to be related to distance. The first relationship of this type, which opened the door to all other methods of distance measurement out to the edge of the observable universe, was discovered at the end of the nineteenth century by an American astronomer named Henrietta Leavitt.


The Earth is replete with features named after rogues, because history is the province of the rich and powerful and the deserving rarely become either. To find more worthy geographical nomenclature it is necessary to look further afield, to a place that escaped the attention of the vain. The dark side of the Moon is such a place, because nobody had seen it until the Soviet spacecraft Luna 3 photographed it in 1959. It isn’t dark, by the way; it permanently faces away from Earth due to an effect called tidal locking, and receives the same amount of sunlight as the familiar Earth-facing side. The first humans to see it were the crew of Apollo 8, when Bill Anders memorably described it as looking like ‘a sand pile my kids have played in for some time. It’s all beat up, no definition, just a lot of bumps and holes.’ Lacking the smooth lunar maria, the dark side is an expanse of craters, and many of these have been named entirely appropriately after deserving scientists. Giordano Bruno is there, of course, alongside Pasteur, Hertz, Millikan, D’Alembert, Planck, Pauli, Van der Waals, Poincaré, Leibnitz, Van der Graaf and Landau. Arthur Schuster, the father of the physics department at the University of Manchester, is honoured. And tucked away in the southern hemisphere, next to a plain named Apollo, is a 65-kilometre-wide partly eroded crater called Leavitt.

Henrietta Swan Leavitt was one of the ‘Harvard Computers’, a group of women employed to work at the Harvard College Observatory by Professor Edward Charles Pickering. By the late nineteenth century Harvard had collected a large amount of data in the form of photographic plates, but the professional astronomers had neither the time nor resources to process the reams of material. Pickering’s answer was to hire women as skilled, and cheap, analysts. Scottish astronomer Williamina Fleming was his first recruit, whom he employed after proclaiming that ‘even his maid’ could do a better job than the overworked males at the observatory. Fleming became a respected astronomer; she was made an honorary member of the Royal Astronomical Society in London and, amongst many important published works, discovered the Horsehead Nebula in Orion. Buoyed by this successful policy, Pickering continued to expand his ‘computers’ throughout the later years of the nineteenth century, bringing Henrietta Leavitt into the team in 1893. Pickering assigned her to the study of stars known as variables, whose brightness changes over a period of days, weeks or months. In 1908, Leavitt published a paper based on a series of observations of variable stars in the Small Magellanic Cloud, which we now know to be a satellite galaxy of the Milky Way. It consists of a detailed list of the positions and periods of 1777 variable stars, and towards the end, a brief but extremely important observation: ‘It is worthy of note that in Table VI the brighter variables have the longer periods. It is also noticeable that those having the longest periods appear to be as regular in their variations as those which pass through their changes in a day or two.’

The history of astronomy is
a history of receding horizons.

Edwin Hubble

This discovery immediately caught the interest of Pickering, and for good reason. If a star’s intrinsic brightness is known, then it is a simple matter to calculate its distance. Put very simply, the further away an object is, the dimmer it appears! Leavitt and Pickering published a more detailed study in 1912, in which they proposed a simple mathematical relationship between the period and intrinsic brightness of 25 variable stars. This relationship is known as the period-luminosity relation. All that was required to calibrate the relation was a parallax measurement of the distance to a single variable of the type observed by Leavitt. If this could be achieved, then the distance to the Small Magellanic Cloud could be obtained. In 1913, the Danish astronomer Ejnar Hertzsprung, in a spectacularly accurate piece of astronomical observing, managed to measure the distance by parallax to the well-known variable star Delta Cephei. Delta Cephei has a period of 5.366341 days, and lies 890 light years from Earth, according to modern measurements by the Hubble Space Telescope. Because of its historic place as the first of Leavitt’s variable stars to have its distance measured, these stars are now known as Cepheid variables. Inexplicably, even though Hertzsprung managed to get the parallax measurement and the distance to Delta Cephei correct, his published paper quotes the distance to the Small Magellanic Cloud as 3000 light years, which is badly wrong; the modern-day measurement is 170,000 light years. There is speculation that Hertzsprung made a simple typographical error in the paper, and for some reason couldn’t be bothered to correct it. In any case, the technique had been established, and two years later Harlow Shapley published the first of a series of papers that refined the method and led him to the first measurements of the size and shape of the Milky Way. He concluded that the galaxy is a disc of stars, around 300,000 light years in extent, with the Sun positioned around 50,000 light years from the centre. This is roughly correct - the Milky Way is around 100,000 light years across and the Sun is about 25,000 light years from the centre. This was an important moment in the history of astronomy, because it was the first measurement that relegated the solar system from being the centre of everything. It’s true that few if any astronomers would have claimed otherwise by the turn of the twentieth century, but science is a subject that relies on measurement rather than opinion. The journey into insignificance had begun.


With the size and shape of the galaxy measured, the question of our place in creation now shifted from the position of the Sun within the galaxy to the nature of the universe itself. If the progress from Copernicus through Newton to Leavitt and Shapley appears relatively fast, certainly when viewed in the context of the glacial progress throughout the 2000-year dominance of Aristotelian thinking, then the decade that followed Shapley’s determination of the size of the Milky Way might be described as an intellectual avalanche. The revolution was fuelled from two sides. A new generation of telescopes and the increasingly sophisticated observational techniques developed by astronomers like Leavitt, Hertzsprung and Shapley provided the data, and in parallel theoretical physics experienced a revolution. Claims of revolutions or paradigm shifts have to be made with great care in science - indeed the terminology is quite unfashionable in certain academic circles. But from a physicist’s perspective there is no doubt that physics experienced a revolution in 1915, because in November of that year Albert Einstein presented a new theory of gravity to the Prussian Academy of Science.

The theory is known as General Relativity, and it replaces Newton’s law of universal gravitation. Many physicists regard General Relativity as the most beautiful piece of physics yet devised by the human mind, and we will explore why this is so a little later. For now, let us note that the Big Bang, the expanding universe, black holes, gravitational waves and the whole evocative landscape of twenty-first-century cosmological language began, absolutely, with the publication of General Relativity. The parallels with the Newtonian revolution are clear. Without Newton’s laws, there is no deep understanding of the solar system and the motions of the planets. Without General Relativity, there is no deep understanding of the large-scale structure and behaviour of the universe. But we are getting ahead of ourselves. As the second decade of the twentieth century dawned, the size and shape of the Milky Way galaxy was established, albeit with rather large errors, but the true extent of the universe beyond our galaxy was still hotly debated. Could we, at least, cling to a sort of token pre-Copernican fig leaf and place our galaxy at the centre of the universe? The desire to be special runs deep. The last intellectual rearguard action against our demotion can, rather theatrically, be said to have played out on a single evening on 26 April 1920 in the Baird auditorium at the Smithsonian Museum of Natural History, Washington DC. This is, of course, an oversimplification, but allow me a minute to enjoy the sound of the outraged shaking jowls of a thousand historians of science before I qualify and partially justify this hyperbolic claim.


The history of science is littered with crunching moments of conflict, debates and disagreements that divided opinion in the most passionate of battles. The wonderful thing about science, however, is that the debates can be settled when facts become available. Science and ‘conservative common sense’ famously clashed in 1860 when Thomas Huxley and Samuel Wilberforce fulminated over the new theory of evolution published by Darwin seven months earlier. I imagine Wilberforce’s indignant reddening cheeks shaking with righteous outrage as he denied the repugnant possibility that his grandfather was a monkey. None of his relatives was a chimpanzee, by the way; we simply share a common ancestor with them around 6 or 7 million years ago. But the ‘unctuous, oleaginous and saponaceous’ bishop, as Disraeli once called him, was having none of it. This might be a little unfair to the great Victorian orator and bishop of the Church of England, but in the case of evolution he was firmly on the wrong side of reality. Few great leaps in knowledge occur without dividing opinion, and this is entirely appropriate. Extraordinary claims require extraordinary evidence, and the great scientific discoveries we are celebrating here are utterly extraordinary. The trick as an educated citizen of the twenty-first century is to realise that nature is far stranger and more wonderful than human imagination, and the only appropriate response to new discoveries is to enjoy one’s inevitable discomfort, take delight in being shown to be wrong and learn something as a result.

The world of astronomy had its moment of intellectual sumo in what has become known as the Great Debate. The year was 1920, and two eminent astronomers found themselves stuck on a train together travelling the 4000 kilometres from California to Washington to discuss the greatest cosmological question of the day. The younger of the two men, Harlow Shapley, we have already met. He had just published his data suggesting that the Milky Way galaxy was much larger than previously suspected. This, however, was where he believed the universe stopped; Shapley was convinced our galaxy was the beginning and end of the cosmos. His travelling companion thought otherwise. Heber Curtis had been studying a misty patch of light known as the Andromeda Nebula. He was convinced that this was not part of our galaxy, but a separate island universe of billions of other stars.

It is not known what they discussed on the train, but the debate itself took place at the Smithsonian Museum of Natural History throughout the day and night of 26 April. At stake was the scale of the universe itself, and both men knew that the question would ultimately be settled by evidence rather than debating skills. The human race had already been shunted from the centre of the universe by Copernicus, and now faced the possibility that the Milky Way galaxy itself was part of a multitude, stretching across millions of light years of space. The question wasn’t settled that evening, but the experienced Curtis, perceived as the underdog because of the magnitude of what he was suggesting, landed significant blows. Curtis observed that the Andromeda Nebula contains a number of novae - exploding stars that shine temporarily, but brightly, in the night sky - but he also noted that the novae in Andromeda appeared on average to be ten times fainter than any others. Curtis asserted that Andromeda’s novae appear dimmer simply because they are perhaps half a million light years further away than those in the Milky Way. Andromeda is therefore another galaxy, claimed Curtis, which strongly implied that the other so-called nebulae were other galaxies too. This was the very definition of an extraordinary claim, and the extraordinary evidence came only four years later.

In 1923 a photo of Andromeda, taken by a 33-year-old astronomer called Edwin Hubble, further fuelled the Great Debate. It’s only a photograph but, just like Anders’ Earthrise, it belongs to a rarefied group of images that have transformed our perspective. Aside from their scientific merit, such images assume great cultural significance because of the ideas they generate and the philosophical and ideological challenges they pose. They also carry with them, in the shadows, personal stories. Someone would have taken a photograph of Andromeda, someday, and discovered what Hubble did. But Hubble took this one, and his story therefore becomes inextricably intertwined with it. Some don’t like their history presented in this way, but science is richer when its stories include people as well as ideas; curiosity is, after all, a human virtue. Hubble may never have taken the photograph had he followed through on a promise to his father to practise law. Reading jurisprudence at Queen’s College, Oxford, as one of the first Rhodes Scholars, Hubble aimed to fulfil his father’s wishes, but John Hubble died before Edwin finished his degree. The death of his father encouraged Edwin to ditch law and revisit his childhood passion for astronomy. He left Oxford for the University of Chicago, joined the Yerkes Observatory and received his PhD in 1917 with a thesis entitled ‘Photographic Investigations of Faint Nebulae’. After brief service in the US Army at the end of World War One, Hubble obtained a position at the Mount Wilson Observatory. He found himself at the controls of the largest, most powerful telescope on the planet, and with the knowledge and good sense to point it at the most intriguing and controversial object in the night sky: Andromeda. Just like Curtis before him, Hubble could make out distinct features within the misty patch, but the newly commissioned 100-inch Hooker telescope allowed him to see much more detail. On 5 October 1923 he took a 45-minute exposure, found three unidentified specks that he assumed were new novae and marked them all with an ‘N’.

To confirm his findings Hubble needed to compare this plate with previous images of Andromeda taken at Mount Wilson. The following day he made the journey down to the basement archive where the observatory’s collection of images was catalogued and stored. To Hubble’s delight, two of the specks were indeed newly discovered novae - what we now know to be the bright nuclear flares of white dwarf stars as they accrete gas and dust from a nearby companion. But it was the third speck that he found most interesting when he compared it to previous images. As Hubble scanned back through the Mount Wilson catalogue he discovered that the star had been captured before; in some plates it appeared brighter, whereas in others it appeared dim or not present at all. Hubble immediately grasped the importance of his discovery. The third speck was a Cepheid variable, the type of star Henrietta Leavitt had studied two decades earlier. In one of the most famous corrections in scientific history, Hubble crossed out the letter ‘N’ and replaced it in red ink with the letters ‘VAR’ followed by a very understated exclamation mark.

Hubble had discovered a cosmic yardstick in Andromeda, and it was a trivial matter to calculate the distance. The new star varied with a period of 31.415 days, which, following Leavitt, implied its intrinsic brightness was 7000 times that of our Sun, and yet it appeared so dim in the night sky that it was invisible to all but the most powerful of telescopes. Hubble’s initial calculations revealed that the star was over 900,000 light years away from Earth, a staggering distance when the size of our own galaxy was estimated to be no more than 100,000 light years across. Hubble, with the help of Leavitt’s ruler, laid the Great Debate to rest. Andromeda, the distant patch of light in the night sky, is a galaxy; an island, according to current estimates, of a trillion suns. Current measurements put the giant spiral at a distance of 2.5 million light years from the Milky Way, one of around 54 galaxies gravitationally bound together to form our galactic neighbourhood known as The Local Group.


What is science? There are many answers, and whole academic careers are devoted to a complex analysis of the historical and sociological development of the subject. To a working scientist, however, I think the answer is quite simple and illuminating because it reveals a lot about how scientists see themselves and what they do. The great (an overused adjective, but not in this case) physicist Richard Feynman gave a characteristically clear and simple description in his Messenger Lectures delivered at Cornell University in 1964: ‘In general, we look for a new law by the following process: First we guess it. Then we - now don’t laugh, that’s really true - then we compute the consequences of the guess to see what, if this is right, if this law that we guessed is right, to see what it would imply. And then we compare the computation results to nature, or we say compare to experiment or experience, compare it directly with observations to see if it works. If it disagrees with experiment, it’s wrong. In that simple statement is the key to science. It doesn’t make any difference how beautiful your guess is, it doesn’t make any difference how smart you are, who made the guess, or what his name is. If it disagrees with experiment, it’s wrong. That’s all there is to it.’

Why do I like this so much? The reason is that it is modest - almost humble in its simplicity - and this, in my opinion, is the key to the success of science. Science isn’t a grandiose practice; there are no great ambitions to understand why we are here or how the whole universe works or our place within it, or even how the universe began. Just have a look at something - the smallest, most trivial little thing - and enjoy trying to figure out how it works. That is science. In a famous BBC Horizon film broadcast in 1982 called ‘The Pleasure of Finding Things Out’, Feynman went further: ‘People say to me, “Are you looking for the ultimate laws of physics?” No, I’m not. I’m just looking to find out more about the world and if it turns out there is a simple ultimate law which explains everything, so be it; that would be very nice to discover. If it turns out it’s like an onion with millions of layers and we’re just sick and tired of looking at the layers, then that’s the way it is … My interest in science is to simply find out more about the world.’

The remarkable thing about science, however, is that it has ended up addressing some of the great philosophical questions about the origin and fate of the universe and the meaning of existence without actually setting out to do so, and this is no accident. You won’t discover anything meaningful about the world by sitting on a pillar for decades and contemplating the cosmos, although you may become a saint. No, a truly deep and profound understanding of the natural world has emerged more often than not from the consideration of much less lofty and profound questions, and there are two reasons for this. Firstly, simple questions can be answered systematically by applying the scientific method as outlined by Richard Feynman, whereas complex and badly posed questions such as ‘Why are we here?’ cannot. But more importantly, and rather more profoundly, it turns out that the answers to simple questions can overturn centuries of philosophical and theological pontificating quite by accident. Reputations count for naught in the face of observation. The famous story of Galileo’s clashes with the Inquisition at the height of the Copernican debate, which he certainly did not expect (nobody does), is the archetypal example.

Galileo began his university career with the study of medicine, but his imagination was captured by art and mathematics. Between studying Medicine in Pisa and returning to his hometown in 1589 to become Professor of Mathematics, Galileo spent a year in Florence teaching perspective and in particular a technique called chiaroscuro. Chiaroscuro is the study of light and shadow, and how it can be used to create a sense of depth by accurately representing the way that light sources illuminate objects. Chiaroscuro was one of the most important new artistic techniques to emerge during Galileo’s time, allowing a new sense of realism to be portrayed on canvas.

Although Galileo spent only a brief time in Florence, the skills he acquired had a great impact on his scientific work. In particular, his carefully developed ability to understand the delicate play of light on three-dimensional shapes, when applied to his later astronomical studies, played an important role in undermining the Aristotelian cosmological edifice which formed a cornerstone of the teachings of the Roman Catholic Church.

The small and seemingly innocuous theological thread on which Galileo unwittingly tugged was made available to him on a visit to Venice in 1609, when he purchased the lenses required to build his first telescope. One of the first objects he turned his ‘perspective tube’ towards was the Moon. With the mind of a mathematician and the eye of an artist, Galileo drew a series of six watercolours representing what he saw.

These images are both beautiful and revolutionary. Catholic dogma asserted that the Moon and the other heavenly bodies were perfect, unblemished spheres. Previous astronomers who had viewed the Moon, either with the naked eye or through telescopes, had drawn a two-dimensional blotchy surface, but Galileo saw the patterns of light and dark differently. His training in chiaroscuro revealed to him an alien lunar landscape of mountain ranges and craters.

‘I have been led to the conclusion that … the surface of the Moon is not smooth, even and perfectly spherical - as the great crowd of philosophers have believed about this and other heavenly bodies - but, on the contrary, to be uneven and rough and crowded with depression and bulges. And it is like the face of the Earth itself, which is marked here and there with chains of mountains and depths of valleys.’

Galileo shared the watercolours with his long-standing friend from Florence, the artist Cigoli, who was inspired to represent this new and radical view of the Moon in the grandest of settings. Built in the year 430 by Pope Sixtus III, the Pauline Chapel in Rome documents the changing artistic styles and techniques used to represent the natural world across many centuries; a place filled with shifting examples of how the three-dimensional world can be represented on a two-dimensional surface. Covering the dome of the Pauline Chapel is Cigoli’s final masterpiece - a striking fresco depicting a familiar scene of the Virgin Mary bathed in a shaft of golden light surrounded by cherubs and angels. The fresco depicts Mary over what was, for the first time, a detailed, textured and cratered moon. The Vatican named it the Assumption of the Virgin, unaware perhaps of the philosophical challenge it represented. Here was art representing scientific knowledge - a type of knowledge radically different to historical or scriptural authority, based on observation rather than dogma and presented unashamedly in a grand setting for all in Rome to see. It is undoubtedly true that Galileo didn’t intend to challenge the very theological foundations of the Church of Rome by observing the Moon through a telescope. But scientific discoveries, however innocuous they may seem at first sight, have a way of undermining those who don’t much care for facts. Reality catches up with everyone eventually.

With his depictions of the Moon completed, Galileo turned his ever more powerful lenses to other celestial bodies. Between 7 and 13 January 1610, he became the first human to observe Jupiter’s four largest moons - Io, Europa, Ganymede and Callisto - now known as the Galilean Satellites. For Galileo, this was further evidence to support the work of Copernicus and the physical reality of the heliocentric model. If moons were orbiting Jupiter, Galileo reasoned, it was impossible to argue that the Earth was at the centre of the universe, because heavenly bodies existed that did not circle the Earth.

Galileo published these observations in the spring of 1610 in ‘The Starry Messenger’, and from his correspondence with Kepler his irritation with the discontent it caused amongst philosophers was clear. ‘My dear Kepler, I wish that we might laugh at the remarkable stupidity of the common herd. What do you have to say about the principal philosophers of this academy who are filled with the stubbornness of an asp and do not want to look at either the planets, the Moon or the telescope, even though I have freely and deliberately offered them the opportunity a thousand times? Truly, just as the asp stops its ears, so do these philosophers shut their eyes to the light of truth.’

To Galileo’s mind, absolute confirmation of Copernicus’s heliocentric model was provided by his studies of Venus. Beginning in September 1610, Galileo observed Venus over the course of months and, like the Moon, he observed that Venus had phases. Sometimes the planet was lit completely by the Sun, but at other times only a crescent appeared to be illuminated. The only plausible explanation for this observation was that Venus was orbiting the Sun. This was surely final compelling evidence of a solar system with the Sun at its heart and the planets orbiting around it.

It wasn’t that simple, of course. Galileo, in what was certainly an ill-judged move, decided to move beyond reporting his scientific observations and instead champion a particular theological and philosophical interpretation of the data - namely that the Church was wrong and that the Earth was most definitely not the centre of the universe. This he seems to have done because he wanted to be famous, and famous he became. Copernicus’s De revolutionibuswas banned until ‘corrected’ (the full version was not removed from the banned list until 1758!) and Galileo ordered not to repeat his ‘foolish and absurd’ conclusions. Galileo didn’t keep quiet, and he achieved his historical notoriety by being put under house arrest in 1633, where he stayed for the remainder of his life.

Many historians characterise Galileo as a bit of an egotistical social climber who brought it all on himself, which is partly true and yet also desperately unfair. He was undoubtedly a great scientist and a supremely talented astronomical observer. In particular, he was the first to clearly state the principle of relativity which lies at the heart of Newton’s laws of motion; namely that there is no such thing as absolute rest or absolute motion. This is why we don’t feel the movement of the Earth around the Sun, and why Aristotle et al. were misled into reading far too much into their stationary feelings. In the hands of Albert Einstein, the principle of relativity can be generalised to freely falling objects in a gravitational field, and this ultimately leads to modern cosmology and the Big Bang theory. But we are jumping ahead again. The purpose of recounting the story of Galileo is not to attack the easy target of the Inquisition (which nobody expects). Rather, it is to highlight the fact that the smallest and most modest of scientific observations can lead to great philosophical and theological shifts that in turn can have a tremendous impact on society. Galileo, by looking through a telescope, doing some drawings and thinking about what he saw, helped to undermine centuries of autocratic idiocy and woolly thinking. In doing so, he got himself locked up, but also bridged the gap between Copernicus and Kepler, and paved the way for Isaac Newton and ultimately Albert Einstein to construct a complete description of the universe and our place within it.


Scientific progress, then, is often triggered by rather innocuous discoveries or simple realisations. There is a terrible cliché about scientists exhibiting a ‘childlike’ fascination with nature, but I can’t think of a better way of putting it. The sense in which the cliché rings true is that children are occasionally in the habit of focusing on a very small thing and continuing to ask the question ‘Why?’ until they get an answer that satisfies their curiosity. Adults don’t seem to do this as much. Good scientists do, however, and if I have a thesis in this chapter then it is as follows: by focusing on tiny but interesting things with honesty and clarity, great and profound discoveries are made, often by flawed human beings who don’t initially realise the consequences of their investigations. The absolutely archetypal example of such an approach can be found at the beginning of Einstein’s quest to replace Newton’s Theory of Gravity.

Einstein is most famous for his equation E=mc2, which is contained within the special theory of relativity he published in 1905. At the heart of the theory is a very simple concept that dates all the way back to Galileo. Put simply, there is no way that you can tell whether you are moving or not. This sounds a bit abstract, but we all know it’s true. If you are sitting in a room at home reading this book, then it feels the same as if you are sitting in an aircraft reading this book, as long as there is no turbulence and the aircraft is in level flight. If you aren’t allowed to look out of the window, then nothing you can do in the room or on the plane will tell you whether or not you are ‘sitting still’ or moving. You might claim that your room is self-evidently not moving, whereas a plane obviously is because otherwise it wouldn’t take you from London to New York. But that’s not right, because your room is moving in orbit around the Sun, and indeed it is spinning around the Earth’s axis, and the Sun itself is in orbit around the galaxy, which is moving relative to other galaxies in the universe. Einstein discovered his famous equation E=mc2 by taking this seemingly pedantic reasoning seriously and asserting that NO experiment you can ever do, even in principle, using clocks, radioactive atoms, electrical circuits, pendulums, or any physical object at all, will tell you whether or not you are moving. Anyone has the absolute right to claim that they are at rest, as long as there is no net force acting on them causing them to accelerate. You are claiming it now, no doubt, if you are reading this book sitting comfortably on your sofa. Pedantry is very useful sometimes, because without Einstein’s theory of special relativity we wouldn’t have E=mc2, we wouldn’t really understand nuclear or particle physics, how the Sun shines or how radioactivity works. We wouldn’t understand the universe.

Something important bothered Einstein after he published his theory in 1905, however. Newton’s great achievement - the all-conquering Universal Law of Gravitation - did not fit within the framework of special relativity, and therefore one or the other required modification. Einstein’s response to this problem was typically Einsteinian: he thought about it very carefully, and, in November 1907, whilst sitting in his chair in the patent office in Bern, he found the right thread to pull. Looking back at the moment in an article written in 1920, Einstein described his idea with beautiful, and indeed child-like, simplicity.

‘Then there occurred to me the “glücklichste Gedanke meines Lebens”, the happiest thought of my life, in the following form. The gravitational field has only a relative existence in a way similar to the electric field generated by magnetoelectric induction. Because for an observer falling freely from the roof of a house there exists - at least in his immediate surroundings - no gravitational field [his italics]. Indeed, if the observer drops some bodies then these remain relative to him in a state of rest or of uniform motion, independent of their particular chemical or physical nature (in this consideration the air resistance is, of course, ignored). The observer therefore has a right to interpret his state as “at rest”.’

I am well aware that you might object quite strongly to this statement, because it appears to violate common sense. Surely an object falling under the action of the gravitational force is accelerating towards the ground, and therefore cannot be said to be ‘at rest’? Good, because if you think that then you are about to learn a valuable lesson. Common sense is completely worthless and irrelevant when trying to understand reality. This is probably why people who like to boast about their common sense tend to rail against the fact that they share a common ancestor with a monkey. How, then, to convince you that Einstein was, and indeed still is, correct?

Most of the time, books are better at conveying complex ideas than television. There are many reasons for this, some of which I’ll discuss in a future autobiography when my time on TV is long over. But when done well, television pictures can convey ideas with an elegance and economy unavailable in print. Human Universe contains, I hope, some of these moments, but there is one sequence in particular that I think fits into this category.

NASA’s Plum Brook Station in Ohio is home to the world’s largest vacuum chamber. It is 30 metres in diameter and 37 metres high, and was designed in the 1960s to test nuclear rockets in simulated space-like conditions. No nuclear rocket has ever been fired inside - the programme was cancelled before the facility was completed - but many spacecraft, from the Skylab nosecone to the airbags on Mars landers, have been tested inside this cathedral of aluminium. To my absolute delight, NASA agreed to conduct an experiment using their vacuum chamber to demonstrate precisely what motivated Einstein to his remarkable conclusion. The experiment involves pumping all the air out of the chamber and dropping a bunch of feathers and a bowling ball from a crane. Both Galileo and Newton knew the result, which is not in question. The feathers and the bowling ball both hit the ground at the same time. Newton’s explanation for this striking result is as follows. The gravitational force acting on a feather is proportional to its mass. We’ve already seen this written down in Newton’s Law of Gravitation. That gravitational force causes the feather to accelerate, according to Newton’s other equation, F=ma. This equation says that the more massive something is, the more force has to be applied to make it accelerate. Magically, the mass that appears in F=ma is precisely the same as the mass that appears in the Law of Gravitation, and so they precisely cancel each other out. In other words, the more massive something is, the stronger the gravitational force between it and the Earth, but the more massive it is, the larger this force has to be to get it moving. Everything cancels out, and so everything ends up falling at the same rate. The problem with this explanation is that nobody has ever thought of a good reason why these two masses should be the same. In physics, this is known as the equivalence principle, because ‘gravitational mass’ and ‘inertial mass’ are precisely equivalent to each other.

Einstein’s explanation for the fact that both the feathers and the bowling ball fall at the same rate in the Plum Brook vacuum chamber is radically different. Recall Einstein’s happiest thought. ‘Because for an observer falling freely from the roof of a house there exists … no gravitational field’. There is no force acting on the feathers or the ball in freefall, and therefore they don’t accelerate! They stay precisely where they are: at rest, relative to each other. Or, if you prefer, they stand still because we are always able to define ourselves as being at rest if there are no forces acting on us. But, you are surely asking, how come they eventually hit the ground if they are not moving because no forces are acting on them? The answer, according to Einstein, is that the ground is accelerating up to meet them, and hits them like a cricket bat! But, but, but, you must be thinking, I’m sitting on the ground now and I’m not accelerating. Oh yes you are, and you know it because you can feel a force acting on you. It’s the force exerted by the chair on which you may be sitting, or the ground on which you are standing. This is obvious - if you stand up long enough then your feet will hurt because there is a force acting on them. And if there is a force acting on them, then they are accelerating. There is no sleight of hand here. The very beautiful thing about Einstein’s happiest thought is that, once you know it, it’s utterly obvious. Standing on the ground is hard work because it exerts a force on you. The effect is precisely the same as sitting in an accelerating car and being pushed back into your seat. You can feel the acceleration viscerally, and if you switch off your common sense for a moment, then you can feel the acceleration now. The only way you can get rid of the acceleration, momentarily, is to jump off a roof.

This is wonderful reasoning, but of course it does raise the thorny question of why, if there is no such thing as gravity, the Earth orbits the Sun. Maybe Aristotle was right after all. The answer is not easy, and it took Einstein almost a decade to work out the details. The result, published in 1916, is the General Theory of Relativity, which is often cited as the most beautiful scientific theory of them all. General Relativity is notoriously mathematically and conceptually difficult when you get into the details of making predictions that can be compared with observations. Indeed, most physics students in the UK will not meet General Relativity until their final year, or until they become postgraduates. But having said that, the basic idea is very simple. Einstein replaced the force of gravity with geometry - in particular, the curvature of space and time.

Imagine that you are standing on the surface of the Earth at the equator with a friend. You both start walking due north, parallel to each other. As you get closer to the North Pole, you will find that you move closer together, and if you carry on all the way to the Pole you will bump into each other. If you don’t know any better, then you may conclude that there is some kind of force pulling you both together. But in reality there is no such force. Instead, the surface of the Earth is curved into a sphere, and on a sphere, lines that are parallel at the equator meet at the Pole - they are called lines of longitude. This is how geometry can lead to the appearance of a force.

Einstein’s theory of gravity contains equations that allow us to calculate how space and time are curved by the presence of matter and energy and how objects move across the curved spacetime - just like you and your friend moving across the surface of the Earth. Spacetime is often described as the fabric of the universe, which isn’t a bad term. Massive objects such as stars and planets tell the fabric how to curve, and the fabric tells objects how to move. In particular, all objects follow ‘straight line’ paths across the curved spacetime that are known in the jargon as geodesics. This is the General Relativistic equivalent of Newton’s first law of motion - every body continues in a state of rest or uniform motion in a straight line unless acted upon by a force. Einstein’s description of the Earth’s orbit around the Sun is therefore quite simple. The orbit is a straight line in spacetime curved by the presence of the Sun, and the Earth follows this straight line because there are no forces acting on it to make it do otherwise. This is the opposite of the Newtonian description, which says that the Earth would fly through space in what we would intuitively call a ‘straight line’ if it were not for the force of gravity acting between it and the Sun. Straight lines in curved spacetime look curved to us for precisely the same reason that lines of longitude on the surface of the Earth look curved to us; the space upon which the straight lines are defined is curved.

This is all well and good, but there may be a question that has been nagging away in your mind since I told you that the ground accelerated up and hit the feathers and the bowling ball at Plum Brook like a cricket bat. How could it possibly be that every piece of the Earth’s surface is accelerating away from its centre, and yet the Earth stays intact as a sphere with a fixed radius? The answer is that if a little piece of the Earth’s surface at Plum Brook were left to its own devices, it would do precisely the same thing as the feather and the bowling ball; it would follow a straight line through spacetime. These straight lines point radially inwards towards the centre of the Earth. This is the ‘state of rest’, if you like - the natural trajectory that would be followed by anything. The geodesics point radially inwards because of the way that the mass of the Earth curves spacetime. So a collapsing Earth would be the natural state of things without any forces acting - one in which, ultimately, all the matter would collapse into a little black hole. The thing that prevents this from happening is the rigidity of the matter that makes up the Earth, which ultimately has its origin in the force of electromagnetism and a quantum mechanical effect called the Pauli Exclusion Principle. In order to stay as a big, spherical, Earth-sized ball, a force must act on each little piece of ground and this must cause each piece of ground to accelerate. Every piece of big spherical things like planets must continually accelerate radially outwards to stay as they are, according to General Relativity.

From what I’ve said so far, it might seem that General Relativity is simply a pleasing way of explaining why the Earth orbits the Sun and why objects all fall at the same rate in a gravitational field. General Relativity is far more than that, however. Very importantly, it makes precise predictions about the behaviour of certain astronomical objects that are radically different from Newton’s. One of the most spectacular examples is a binary star system known rather less than poetically as PSR J0348+0432. The two stars in this system are exotic astrophysical objects. One is a white dwarf, the core of a dead star held up against the force of gravity by a sea of electrons. Electrons behave according to the Pauli Exclusion Principle, which, roughly speaking, states that electrons resist being squashed together. This purely quantum mechanical effect can halt the collapse of a star at the end of its life, leaving a super-dense blob of matter. White dwarfs are typically between 0.6 and 1.4 times the mass of our Sun, but with a volume comparable to that of the Earth. The upper limit of the mass of a white dwarf is known as the Chandrasekhar limit, and was first calculated by the Indian astrophysicist Subrahmanyan Chandrasekhar in 1930. The calculation is a tour de force of modern physics, and relates the maximum mass of these exotic objects to four fundamental constants of nature - Newton’s gravitational constant, Planck’s constant, the speed of light and the mass of the proton. After almost a century of astronomical observations, no white dwarf has ever been discovered that exceeds the Chandrasekhar limit. Almost all the stars in the Milky Way, including our Sun, will end their lives as white dwarfs. Only the most massive stars will produce a remnant that exceeds the Chandrasekhar limit, and the vast majority of these will produce an even more exotic object known as a neutron star. In the PSR J0348+0432 system, quite wonderfully, the white dwarf has a neutron star companion, and this is what makes the system so special.

If the remains of a star exceed the Chandrasekhar limit, the electrons are squashed so tightly onto the protons in the star that they can react together via the weak nuclear force to produce neutrons (with the emission of a particle called a neutrino). Through this mechanism, the whole star is converted into a giant atomic nucleus. Neutrons, just like electrons, obey the Pauli Exclusion Principle and resist being squashed together, leading to a stable dead star. Neutron stars can have masses several times that of our Sun, but quite astonishingly are only around 10 kilometres in diameter. They are the densest stars known; a teaspoonful of neutron star matter weighs as much as a mountain.

Imagine, for a moment, this exotic star system. The white dwarf and neutron star are very close together; they orbit around each other at a distance of 830,000 kilometres - that’s around twice the distance to the Moon - once every 2 hours and 27 minutes. That’s an orbital velocity of around 2 million kilometres per hour. The neutron star is twice the mass of our Sun, around 10 kilometres in diameter, and spins on its axis 25 times a second. This is a star system of unbelievable violence. Einstein’s Theory of General Relativity predicts that the two stars should spiral in towards each other because they lose energy by disturbing spacetime itself, emitting what are known as gravitational waves. The loss of energy is minuscule, resulting in a change in orbital period of eight millionths of a second per year. In a triumph of observational astronomy, using the giant Arecibo radio telescope in Puerto Rico, the Effelsberg telescope in Germany and the European Southern Observatory’s VLT in Chile, astronomers measured the rate of orbital decay of PSR J0348+0432 in 2013 and found it to be precisely as Einstein predicted. This is quite remarkable. Einstein could never have dreamt of the existence of white dwarfs and neutron stars when he had his happiest thought in 1907, and yet by thinking carefully about falling off a roof he was able to construct a theory of gravity that describes, with absolute precision, the behaviour of the most exotic star system accessible to twenty-first-century telescopes. And that, if I really need to say it, is why I love physics.

Einstein’s Theory of General Relativity has, at the time of writing, passed every precision test that scientists have been able to carry out in the century since it was first published. From the motion of feathers and bowling balls in the Earth’s gravitational field to the extreme astrophysical violence of PSR J0348+0432, the theory comes through with flying colours.

There is rather more to Einstein’s magisterial theory than the mere description of orbits, however. General Relativity is fundamentally different to Newton’s theory because it doesn’t simply provide a model for the action of gravity. Rather, it provides an explanation for the existence of the gravitational force itself in terms of the curvature of spacetime. It’s worth writing down Einstein’s field equations, because they are (to be honest) deceptively simple.

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Here, the right-hand side describes the distribution of matter and energy in some region of spacetime, and the left-hand side describes the shape of spacetime as a result of the matter and energy distribution. To calculate the orbit of the Earth around the Sun one would put a spherical distribution of mass with the radius of the Sun into the right-hand side of the equation, and (roughly speaking) out would pop the shape of spacetime around the Sun. Given the shape of spacetime, the orbit of the Earth can be calculated. It’s not completely trivial to do this by any means, and the notation above hides great complexity. But the point is simply that, given some distribution of matter and energy, Einstein’s equations let you calculate what spacetime looks like. But here is the remarkable point that draws us towards the end of our story. Einstein’s equations deal with the shape of spacetime - the fabric of the universe. The first thing to note is that we are dealing with spacetime, not just space. Space is not a fixed arena within which things happen with a big universal clock marking some sort of cosmic time upon which everyone agrees. The fabric of the universe in Einstein’s theory is a dynamical thing. Very importantly, therefore, Einstein’s equations don’t necessarily describe something that is static and unchanging. The second thing to note is that nowhere have we restricted the domain of Einstein’s theory to the region of spacetime around a single star, or even a double star system such as PSR J0348+0432. Indeed, there is no suggestion in Einstein’s theory that such a restriction is necessary. Einstein’s equations can be applied to an unlimited region of spacetime. This implies that they can, at least in principle, be used to describe the shape and evolution of the entire universe.


Storytelling is an ancient and deeply embedded human impulse; we learn, we communicate, we connect across generations through stories. We use them to explore the minutiae of human life, taking delight in the smallest things. And we tell grander tales of origins and endings. History is littered with stories about the creation of the universe; they seem as old as humanity itself. Multifarious gods, cosmic eggs, worlds emerging from chaos or order, from the waters or the sky or nothing at all - there exist as many creation myths as there are cultures. The impulse to understand the origin of the universe is clearly a powerful unifying idea, although the very existence of many different mythologies continues to be a source of division. It is an unfortunate testament to the emotional power of creation narratives that so much energy is spent arguing about old ones rather than using the increasingly detailed observational evidence available to twenty-first-century citizens to construct new ones. We live in a very privileged and exciting time in this sense, because observational evidence for creation stories was scant even a single lifetime ago. When my grandparents were born in Oldham at the turn of the twentieth century, there was no scientific creation story. Astronomers were not even aware of a universe beyond the Milky Way, which makes it all the more remarkable that the modern scientific approach to the description of the universe emerged almost fully formed from Einstein’s Theory of General Relativity before Edwin Hubble published the discovery of his Cepheid variable star in Andromeda and settled Shapley and Curtis’s Great Debate.

One of the beautiful things about mathematical physics is that equations contain stories. If you think of equations in terms of the nasty little things you used to solve at school on a damp autumn afternoon, then that may sound like a strange and abstract idea. But equations like Einstein’s field equations are much more complex animals. Recall that Einstein’s equations will tell you the shape of spacetime, given some distribution of matter and energy. That shape is known as a solution of the equations, and it is these solutions that contain the stories. The first exact solution to Einstein’s field equations was discovered in 1915 by the German physicist Karl Schwarzschild. Schwarzschild used the equations to calculate the shape of spacetime around a perfectly spherical, non-rotating mass. Schwarzschild’s solution can be used to describe planetary orbits around a star, but it also contains some of the most exotic ideas in modern physics; it describes what we now know as the event horizon of a black hole. The well-known tales of astronauts being spaghettified as they fall towards oblivion inside a supermassive collapsed star are to be found in Schwarzschild’s solution. The calculation was a remarkable achievement, not least because Schwarzschild completed it whilst serving in the German Army at the Russian Front. Shortly afterwards, the 42-year-old physicist died of a disease contracted in the trenches.

There were two ways of
arriving at the truth;
I decided to follow them both.

Georges Lemaître

The most remarkable stories waiting to be found inside Einstein’s equations reveal themselves when we take an audacious and seemingly reckless leap. Instead of confining ourselves to describing the spacetime around spherical blobs of matter, why not think a little bigger? Why not try to use Einstein’s equations to tell us about all of spacetime? Why can’t we apply General Relativity to the entire universe? Einstein noticed this as a possibility very early in the development of his theory, and in 1917 he published a paper entitled ‘Cosmological Considerations of the General Theory of Relativity’. It’s a big step, of course, from thinking about someone falling off a roof to telling the story of the universe, and Einstein appears to have been uncharacteristically wobbly. In a letter to his friend Paul Ehrenfest a few days before he presented his paper to the Prussian Academy, he wrote ‘I have … again perpetrated something about gravitation theory which somewhat exposes me to the danger of being confined in a madhouse.’

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.