The Universe is expanding faster than predicted. By observing Cepheid variable stars in the Large Magellanic Cloud, scientists using the Hubble Space Telescope announced the most accurate measurement of the Hubble Constant to date [pdf]. This measurement continues to show a discrepancy of approximately 10% between the predicted and observed values for the constant. 10% may seem small, but it’s unexplained, and the new measurement’s accuracy puts the possibility of this being a statistical fluke at 1 in 100,000. The Hubble Constant, typically written in km/s per megaparsec (1 megaparsec ≈ 3 million light-years), measures how quickly objects are currently moving away from us based on their distance—the further away the object, the faster it will be moving away from us, due to space itself expanding as the Universe grows in size. Using measurements of the Cosmic Microwave Background (CMB), the cold 2.72° K radiation permeating the Universe from when Hydrogen formed 379,000 years after the Big Bang, scientists can make predictions of the Hubble Constant we could expect to observe today. But the rate of expansion has been repeatedly observed to be higher than this prediction. Proposals for why this discrepancy exists include an unknown type of particle, dark matter interacting more with normal matter than is assumed, or unexpected “early dark energy.” Last week marked 29 years in orbit for Hubble, which continues to push science forward.