In 1917, Einstein's novel equations of space-time had geometric terms on the left and energy on the right. A constant on the left kept the universe steady, suiting observations at the time. But in 1929, it became clear that the universe is expanding and Einstein dubbed the constant "the biggest mistake of my life".

Now historian Alex Harvey of New York University has re-analysed papers from the physicists, published in 1918. In one Schrödinger toyed with Einstein's equations, moving the constant from the left to the right.

The simple move transformed the constant from part of the geometry of space-time to a source of energy for the universe. "While mathematically it doesn't make any difference, physically it does," says Harvey.

Einstein responded, rather cheekily, that the properties of this new energy term were either nothing or demand a "non-observable negative density in interstellar spaces".
Pandora's Box

"That turns out to be dark energy," Harvey says – which only emerged again in 1998 to explain the universe's accelerating expansion.

Cosmologists have been seeking to pin down dark energy's true nature ever since. The discovery that the universe's expansion is accelerating garnered three physicists the 2011 Nobel prize in physics.

If Einstein had followed the mathematics, he could have predicted yet another Nobel-worthy idea from first principles. Instead, he dismissed his notion almost as soon as he conceived of it. "The course taken by Herr Schrödinger does not appear possible to me because it leads too deeply into the thicket of hypotheses," Einstein wrote.

"He simply pointed out that you're opening up a Pandora's Box here," says Harvey. "It's either trivial, or you have a headache. That headache historically turned out to be dark energy."

Harvey thinks Einstein was mildly irritated by Schrödinger's mathematical games. "His language indicates a certain degree of annoyance," he adds.