Einstein's 'Biggest Blunder' Came Back as the Key to Dark Energy

In 1917, shortly after developing general relativity, Albert Einstein applied his new equations to the question of how the universe evolves. At the time, the prevailing view held that the universe was static, but Einstein's equations pointed instead to a universe that must expand or contract.

To keep the universe static and match the accepted view of his era, Einstein added a term called the "cosmological constant," represented by the Greek letter Lambda, to his equations. The term acts as a built-in gravitational effect within spacetime itself, and depending on its value, it can produce either attraction or repulsion. Einstein chose a value that canceled out the pull of matter, keeping the universe stable.

The fix did not last. Within a few years, Edwin Hubble discovered that the universe is expanding, while Russian cosmologist Alexander Friedmann developed the theoretical groundwork that helped support the Big Bang theory. Einstein abandoned the cosmological constant and later called introducing it his "greatest blunder."

In 1998, two teams of astronomers studying the universe's matter content found that cosmic expansion was not slowing down as expected — it was accelerating. The amount of matter observed could not explain the acceleration, and the simplest explanation turned out to be Einstein's discarded cosmological constant, now understood as a repulsive background effect. Decades after he abandoned it, the idea returned as the leading explanation for the new data.

The cosmological constant, now also called dark energy, became the "Lambda" in LCDM (Lambda cold dark matter), the current Standard Model of Cosmology, alongside CDM, or cold dark matter, the substance believed to account for most of the mass in nearly every galaxy. Despite relying on only a handful of parameters, LCDM accounts for the universe's expansion history, the cosmic background radiation, the growth of galaxies and the development of large-scale cosmic structure.