It is almost 100 years since scientists discovered the universe is expanding. In the decades that followed, the accuracy of the measurements, and interpretations and implications of this discovery, were a source of fierce debate. We now know the universe emerged dramatically from a highly compressed state in an event known as the Big Bang.

Measurements of the present-day expansion rate, known as the Hubble constant or H₀ (pronounced H-naught), have improved considerably since those early days. However, a new debate now grips the astronomy community: two independent measurements of H₀, which should agree, give different results. This situation has become known as the “H₀ tension”, or Hubble tension.

For now, our understanding of the universe continues to be dogged by disagreement in measurements of the expansion rate. One hundred years after its conception, the Hubble constant continues to confound us.