At this stage Albert Einstein is a household name across the globe, his name being synonymous with the word ‘genius’. His theories and thought experiments have had an immense impact on our understanding of physics, and he seemed able to imagine ideas that no one else possibly could. This post tells the story of how, in 1929, Einstein retracted one of his theories – calling it the “biggest blunder” of his life.

Einstein had included in his equations of gravity what he called ‘the cosmoligical constant’, a constant represented by capital lambda, which allowed him to describe a static universe. This model of the universe complied with what was the generally accepted theory at the time in 1917, that the universe was indeed stationary.

Then, in 1929, Edwin Hubble (whom the Hubble telescope is named for) presented convincing evidence that the universe is in fact expanding. This caused Einstein to abandon his cosmological constant (i.e. presuming its value to be zero), believing it to be a mistake.

But that wasn’t the end of the story. Years went by and physicists repeatedly inserted, removed and reinserted lambda into the equations describing the universe, unable to decide whether or not it was necessary. Finally, in 1997/8, two teams of theorists, one led by Saul Perlmutter, published papers outlining the need for Einstein’s cosmological constant.

Through their analysis of the most distant supernovae ever observed – one of which was SN1997ap – and their redshifts, they had reached the conclusion that the distant supernovae were roughly fifteen percent farther away than where the prior models placed them. This could only mean they were accelerating away from us. The only known thing that ‘naturally’ accounts for this acceleration was Einstein’s lambda, and so it was reinserted into Einstein’s equations one last time. Einstein’s equations now perfectly matched the observed state of the universe.

So while Einstein’s initial use for the cosmological constant was incorrect, it proved vital to forming an accurate picture of our world. The great theorist had once again foreseen a factor no one else could – this time a good 70 years before anyone, including himself, was able to prove it.