New measurement doesn’t resolve disagreement over Hubble constant

Scientific American : Two methods have been developed to determine the rate of the universe’s expansion, called the Hubble constant. But they don’t yield the same result. Researchers measuring the cosmic microwave background (CMB) radiation came up with a value of 67.3 ± 0.7 km/s/Mpc. Another group of researchers, who used the distance ladder method, now report a value of 73.02 ± 1.79 km/s/Mpc. The new study uses known cosmic distance markers, such as Cepheid variable stars and Type 1a supernovae, to calibrate measurements of galaxies’ distances and redshift, or how quickly the galaxies are moving. That technique was used by Adam Riess of the Space Telescope Science Institute in Baltimore, Maryland, and his colleagues in the 1990s to prove that the universe’s expansion was accelerating. Riess’s team is also behind the latest study, which includes several new calibration points that yield a more precise value. Both the CMB and distance ladder research teams are continuing to examine their data and calculations to try to understand why the two systems still don’t agree.