For the past 10 years cosmologists have been left scratching their heads over why two methods for measuring the universe’s rate of expansion provide totally different results. There are two possible solutions to the puzzle, known as the Hubble tension: either something is wrong with the measurements or something is wrong with our model of the universe. It was hoped that observations from the James Webb space telescope might shed some light on the problem, but instead results published last week have continued to muddy the waters. To understand why the expansion rate of the universe remains a mystery, and what might be needed to finally pin it down, Madeleine Finlay speaks to Catherine Heymans, the astronomer royal for Scotland and a professor of astrophysics at the University of Edinburgh. Help support our independent journalism at theguardian.com/sciencepod