Astronomers using the Hubble Space Telescope have made their most precise measurement yet for the expansion rate of the Universe.

It comes out at 74.03km/second per megaparsec (a megaparsec is 3.26 million light-years). Or put another way - the expansion increases by 74.03km/s for every 3.26 million light-years we look further out into space.

The problem is that's 9% faster than expected. The BBC Today programme's Justin Webb asked Nobel laureate Adam Riess of the Space Telescope Science Institute and Johns Hopkins University, US, why the discrepancy mattered.