This multi-wavelength image of Abell 520 shows the aftermath of a complicated collision of galaxy clusters, some of the most massive objects in the universe. In this image, the hot gas as detected by Chandra is coloured red (Image: SCIENCE SOURCE/SPL)

AFTER billions of years of runaway expansion, is the universe starting to slow down? A new analysis of nearby supernovae suggests space might not be expanding as quickly as it once was, a tantalising hint that the source of dark energy may be more exotic than we thought.

For more than a decade, astrophysicists have grappled with evidence of …