Astronomers have measured the Universe's current rate of expansion to within 5% — twice as a precisely as before. Adam Riess of the Space Telescope Science Institute in Baltimore, Maryland, and his team used the Hubble Space Telescope to refine previous estimates of the Hubble constant, which relates the speed at which galaxies race apart to their distances from each other. They found the constant to be 74.2 kilometres per second per million parsecs.
Because true cosmic yardsticks are hard to find, astronomers chain together different observations to get the great distances needed to determine the constant. The researchers eliminated some sources of error using observations of 240 stars called cepheids, which provided precise distance measurements for seven galaxies. The new measurements are consistent with observations that the Universe is full of dark energy, a repulsive force that is accelerating the Universe's expansion — and changing the Hubble constant slightly with time.