One hundred years of the expanding universe

On this day 100 years ago, the prestigious physics journal Zeitschrift für Physik received an extraordinary paper from a young Russian theorist named Alexander Friedmann.

A relatively unknown mathematician and physicist, Friedmann had become interested in applying general relativity, Einstein’s new theory of gravity, space and time, to the universe as a whole. The paper, a densely mathematical treatise, concluded with the startling prediction that our universe may not be static, but could expand or contract over time.

As so often in science, Friedmann’s paper received very little attention at first. Even Einstein, no stranger to unusual ideas, found the article beyond the pale. Indeed, Einstein published a follow-up note in the same journal claiming that Friedmann’s paper contained a basic mathematical error. It later transpired that Einstein’s criticism was incorrect, but the damage had been done and Friedmann’s work was roundly ignored.

In 1927, another young theorist, the little-known Belgian cleric Georges Lemaître, also suggested that general relativity predicted a non-stationary universe. Unaware of Friedmann’s earlier paper, Lemaître had become convinced that emerging new observations in astronomy — the recession of the galaxies — were a manifestation of a large-scale expansion of space, a phenomenon he showed was compatible with general relativity.

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Lemaître’s prediction also fell on stony ground, not least because the astronomical observations he cited to support his theory were not yet fully established. For example, Einstein rejected Lemaître’s work out of hand, declaring the notion of an expanding universe ‘abominable’. Worse, Einstein also informed the hapless cleric that the idea had already been suggested several years before!

However, the work of Friedmann and Lemaître did not languish in obscurity for long. In 1929, the American astronomer Edwin Hubble published the first definitive evidence that the distant galaxies are indeed rushing away from one another at a speed proportional to their distance. Hubble was a renowned astronomer working with the world’s most powerful telescope and his startling observations (soon known as Hubble’s law) received a great deal of attention. In particular, eminent theorists noted that Hubble’s observations could not be explained in terms of Newton’s familiar law of gravity (a law of attraction).

As so often in science, observation had caught up with theory and attention quickly turned to relativistic models of the universe. Within months, a plethora of different models of the expanding universe had been proposed, all based on the earlier models of Friedmann and Lemaître.

The story doesn’t end there. In 1931, Lemaître suggested a universe that is expanding now may have been much smaller in the past, an idea that eventually led to the modern Big Bang theory. Although the Big Bang model says nothing about the exact origin of the universe, a plethora of observations now suggests our universe was indeed once extremely small, hot and dense, expanding and cooling ever since.

Sadly, Friedmann didn’t live to see his idea taken up. In 1925, he died of typhoid fever at the young age of 37, several years before his work was adopted by other theorists and decades before his model became the basis of modern cosmology. It is sometimes said that Friedmann’s pioneering paper of 1922 might have been taken more seriously by his contemporaries if he had made some connection with astronomy; I find this view anachronistic because the relevant astronomical data were not available at the time. In any case, it is the job of the theoretician to explore all the possible paths nature may take (however strange), while it is the job of the observer or experimentalist to determine which path nature actually takes.

All in all, the story of the discovery of the expanding universe is a classic example of scientific discovery, with several scientists making a bizarre prediction from theory that was later supported by observation. To my mind, the story is one of scientific inquiry driven entirely by curiosity, from the mathematics of a brilliant young Russian theorist to the ponderings of an obscure Belgian cleric. Contrary to the assertions of some historians of science, I see no great evidence in the story of the influence of political or social considerations, simply an attempt by scientists of various backgrounds, nationalities and beliefs to expand the frontiers of knowledge.

Dr Cormac O’Raifeartaigh lectures in physics at the South East Technological University (Waterford) and is a visiting associate professor of physics at University College Dublin. He blogs at www.antimatter.ie