Dark energy, the mysterious force powering the expansion of the universe, appears to be weakening over time, according to a major cosmological survey that has thrown the laws of modern physics into doubt. Ian Sample tells Madeleine Finlay how this new finding could shed light on the ultimate fate of the cosmos, and Saul Perlmutter, who won a Nobel prize for his work proving the universe is expanding, describes how the new development could upend assumptions about how this mysterious force operates. Help support our independent journalism at theguardian.com/sciencepod