Knowing how fast the universe is expanding is fundamental for astronomers. The Hubble constant sets the cosmic scale for our measurements, enabling us to determine the true distance and size of astrophysical objects. Even more profoundly, it tells us about how the universe began and how it could end.  Given its importance, astronomers have been trying to measure the Hubble constant with a variety of methods.  However, the highest-precision measurements disagree – significantly.  One set of measurements puts the Hubble constant at 67–68km/s per megaparsec whereas another set yielded 72–74km/s per megaparsec. This is referred to as the Hubble tension.

Kelly Gourdji describes a new study adopting a different approach in an attempt to pin down the Hubble constant, using gravitational waves caused by the merger of two neutron stars.  Critical information is provided tracking the aftermath of the GW170817 gravitation wave event using a worldwide network of radio telescopes.  By reanalysing the extraordinarily precise telescope observations of the merger’s aftermath in greater detail, they found that models commonly used in earlier studies struggled to match the data.

The new result agrees more closely with the lower value of the Hubble constant and suggests that the tension may arise from subtle calibration issues affecting other methods.  But this result, from a single gravitsational wave event, is still four times less precise than the leading measurements. It will be necessary to detect more neutron star collisions to definitively settle the Hubble tension using gravitational waves. Such events are rare, so it may be a while – but for now, the new study provides an important new clue in one of astronomy’s biggest problems.