Adam Riess, co-discoverer of the accelerating expansion of the Universe due to dark energy, visited Harvard last year, where he told me a story about his time in grad school there. He recalled hearing a lecture on the uncertainty in the rate at which the Universe is expanding and thinking, “That problem will never be solved.” Twenty years on, we know the local expansion rate (called the Hubble constant, or H0) to about 4% precision, and many different, independent techniques find mutually consistent values. However, measuring the Hubble constant remains one of the most important problems in cosmology because it is intimately connected to the Universe’s contents. In particular, General Relativity means that the Universe’s contents set its expansion rate, and so precise measurement of the expansion rate can probe the amount and evolution of different components of the Universe.
Thus, it is exciting when a new, independent method of measuring the expansion rate (H0) is proposed—and even more exciting when it works. In the short paper I discuss today, the authors show that time delays in the light emitted from distant, violently variable galactic centers (“active galactic nuclei”, or AGN) can probe H0 with precision similar to that of the Hubble Space Telescope—and out to about twice the distance.