Einstein's self-described ugly fudge factor came back to explain an accelerating cosmos
In 1917 Albert Einstein bolted an extra term onto his equations to hold the universe still, and he disliked it from the start. Once the cosmos was found to be expanding, he dropped it. Eight decades later, astronomers discovered the expansion is speeding up, and his discarded constant was suddenly the simplest explanation available.
General relativity, published in 1915, implied a universe that could not sit still: gravity would make a motionless cosmos collapse on itself. Everyone at the time assumed the universe was static, so Einstein added a constant, written as the Greek letter lambda, to push back against gravity. He later admitted it gave him a bad conscience and called it an ugly thing he could hardly believe nature used. The fix did not even work well, since the balance it struck was unstable: nudge such a universe outward and it keeps growing, nudge it inward and it keeps shrinking.
Others were reading the equations differently. Alexander Friedmann showed in 1922 that they worked perfectly well for a changing universe, and in 1929 Edwin Hubble's observations suggested galaxies were flying apart. According to George Gamow, Einstein called his refusal to trust his own original prediction the biggest blunder of his life. From the 1930s until the late 1990s most physicists assumed the constant was zero.
Then came a surprise. In the 1990s teams led by Saul Perlmutter, Brian Schmidt and Adam Riess tracked distant Type Ia supernovae, expecting to measure how much gravity was slowing cosmic expansion. Instead the supernovae showed it accelerating, and in 1998 both groups announced the result. The constant went straight back into the equations. Today it sits at the heart of the standard cosmological model, standing in for the dark energy thought to make up around 68 percent of everything.
A deep puzzle remains. Quantum theory says empty space seethes with fluctuating fields that should carry energy, but the predicted amount exceeds what cosmologists observe by about 120 orders of magnitude, sometimes called the worst prediction in physics. Curiously, Isaac Newton had already described, in his 1687 Principia, a force growing with distance that is mathematically equivalent to the modern constant.
Source: Cosmological constant
