Einstein's discarded constant resurfaces as the driver of cosmic acceleration

In 1917 Einstein introduced a cosmological constant, denoted Lambda, to counteract gravity and allow a static universe, a move he later dismissed as a blunder. The discovery of cosmic expansion by Edwin Hubble rendered that solution unnecessary, and the constant fell out of favor. Decades later, two research teams found that the universe’s expansion is not slowing but accelerating, a result that could not be explained by matter alone.

The re-introduction of Lambda, interpreted as dark energy, provided a natural fit, giving rise to the ΛCDM model that also incorporates cold dark matter. This model successfully reproduces the expansion history, cosmic microwave background, galaxy distribution and other key observations. Despite its successes, the framework remains provisional, with physicists aware that it may still miss crucial aspects of cosmic physics.