The Biggest Controversy in Cosmology Just Got Bigger

Two rival teams have led the effort to measure the cosmic expansion rate, which is known as the Hubble constant, or H0. One of these teams, led by Adam Riess of Johns Hopkins University, has consistently measured H0 to be about 8 percent higher than the theoretical prediction for how fast space should be expanding, based on the cosmos’s known ingredients and governing equations. This discrepancy, known as the Hubble tension, suggests that the theoretical model of the cosmos might be missing something—some extra ingredient or effect that speeds up cosmic expansion. Such an ingredient could be a clue to a more complete understanding of the universe.