The universe is expanding. That much is certain to astronomers. What continues to be up for debate is how fast the expansion is happening. Over the decades, different methods of measuring the universe’s expansion rate, also known as the Hubble constant or H0, have produced different values.
A novel analysis of a supernova provides an important new result for H0. Using a method called time-delay cosmography with a burst named Supernova Refsdal, a team of astronomers has reported in Science an H0 value of 66.6 + 4.1 – 3.3 km/s/Mpc. That compares with values of 70–74 km/s/Mpc attained through measurements of standard candles—objects such as type Ia supernovae and pulsing red giant stars called Cepheids that have known luminosities, which allow astronomers to determine their distance from Earth by measuring how bright the objects appear. Measurements of H0 made using the cosmic microwave background (CMB) radiation from the early universe, on the other hand, yield a rate of about 67 km/s/Mpc.
Although the new time-delay measurement is closer to the CMB value, there is still enough wiggle room for an agreement with standard candle measurements. “Measuring the Hubble constant using many different techniques is the way we’re going to ultimately convince ourselves that an accurate measurement has been made,” says Wendy Freedman, an observational cosmologist at the University of Chicago who was not involved in the research.
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