The story of the development of the idea of the Big Bang has two separate strands, and we have to switch back and fore between them.

The one strand is the observational work of astronomers, developing techniques to measure the distance of stars and galaxies – and also their relative speed. The discovery – published by Edwin Hubble in 1929 – that the galaxies are receding from us, with the speed of recession growing with distance, is the basis of the belief of an expanding universe.

A parallel strand of investigation involves purely pencil and paper, and at the heart of this is Albert Einstein’s theory of general relativity, published in 1915. This provides a deep insight into the nature of gravity, showing that what we see as gravity’s ‘pull’ is in fact the bending of space and time by matter. Material objects such as the Sun change the very geometry of time and space around them. In that distorted landscape, other material objects, such as the planets, find their paths are not straight lines but orbits around the Sun.

These two strands, of theory and of observation, meet in the work of Sir Arthur Eddington who in 1919 provided the first confirmation of Einstein’s theory. He travelled to west Africa to measure the bending of starlight by the Sun’s gravitational field, something that could be seen directly during a solar eclipse.

Eddington’s observation confirmed the power of general relativity. The Sun had been fed into Einstein’s equations and the bending of starlight had come out. What then if we decide to take an even bigger unit of matter and see what happens? What indeed if we go so far as to take the entire universe and feed it into the equations of general relativity?

Einstein tried this; but the first result was not encouraging. It simply showed that all the matter in the universe would clump together under the effects of gravity – as we might possibly have expected. But general relativity was much too beautiful a theory to jettison just because it didn’t seem to be giving the right answer in this case. So Einstein persevered; and he suggested that in order to produce the universe we see, there had to be some kind of force of expansion to counter the gravitational attraction – a kind of a push to counter the universal gravitational pull.

Einstein could not say what this proposed new force was, just that it must somehow reside in empty space. It appeared in his equations in the form of a number called the cosmological constant – a number whose value had to be carefully selected to give the desired push-pull balance.

But this balance was so delicate that the model was unstable. It was, commented Eddington, on a knife-edge between runaway expansion and runaway contraction.

However, more theorists were coming up to tackle the problem, and the first in was the Russian physicist and mathematician Alexander Friedmann. Born in 1888 in St Petersburg, where his father was a composer and ballet dancer and his mother a pianist, he had been a pilot in World War I and was awarded the Cross of St George for bravery. Following the Russian Revolution of 1917, he was made head of an aircraft factory. After the war he became a university professor – and also a balloonist, setting a record for an ascent; and he died in 1927 of typhoid, when only 37.

Friedmann’s mathematical skills were focused in particular on meteorology, forecasting weather and looking at the dynamics of a fluid such as air. General relativity became an additional interest, and in 1922 – the same year that Stalin took power in Russia – he came up with a new solution to Einstein’s equations. He showed that if you assume that the universe is evenly filled with matter, there are three possibilities. One is Einstein’s static model, where the push and the pull balance, but there can also be two other outcomes – one for a contracting universe, and the other for an expanding one. This was something radical and extraordinary – a dynamic universe, in an immense process of change.

‘As Copernicus made the Earth go round the Sun, so Friedmann made the Universe expand,’ said the authors of a later book on his life and work. At the time, not many people read his paper. Einstein did, and first of all called it ‘suspicious.’ Friedmann wrote to him right away to explain further, and six months later Einstein announced that he had revised his opinion. ‘My criticism,’ he said, ‘was based on an error in my calculations. I consider that Mr Friedmann’s results are correct and shed new light.’

It would be seven years before the theory was confirmed, but even in 1922 there were developments in observational astronomy which were opening up questions about the scale of the universe, and the next part of the story involves the study of nebulae.