IT IS nearly 100 years since we confirmed that the universe – space-time – is expanding. But we are still struggling with a basic fact: what is the rate of the expansion? Depending on how we measure a crucial number that sets this value, we seem to get different answers. The fallout of this question could drastically change our understanding of the cosmos .

In 1929, astronomer Edwin Hubble used observations of galaxies to show that there was a correlation between their velocity and their distance from us. The further away they were, the faster they seemed to be …