Dark energy is usually seen as an innate property of the cosmos, at work from the very first moments. But a new model suggests it behaved like water on the verge of boiling: long unnoticed, then suddenly kicking in at a critical moment. This scenario is called 'late activation': around 5–7 billion years ago, a shift occurred, and the universe’s expansion began to accelerate.

Three landmarks helped test the idea. The cosmic microwave background, the cooled light of the Big Bang, preserved the imprint of the young universe. Explosions of supernovae, like lighthouses, allowed distances to be measured; it was these that led Adam Riess to discover the acceleration in the 1990s. The patterns of galaxies revealed the rate of universe expansion. By combining these data, scientists essentially captured the moment the cosmos 'boiled over.'

There was a surprise:

However, this doesn't resolve the main cosmological puzzle — the Hubble tension, i.e., the discrepancy in the expansion rate measured by different methods. The mystery endures, and cracking it will require even bolder ideas.