For the past 10 years, cosmologists have been left scratching their heads over why the two methods for measuring the expansion rate of the universe give completely different results. There are two possible solutions to the puzzle known as the Hubble tension: either something went wrong with the measurements, or something went wrong with our model of the universe. It was hoped that observations from the James Webb Space Telescope would shed some light on the problem, but the results released last week continued to muddy the waters. To understand why the expansion of the universe remains a mystery, and what is needed to finally map it out, Madeleine Finlay talks to Catherine Heymans, astronomer royal to Scotland and professor of astronomy at the University of Edinburgh.