Gravitational waves have revealed a wealth of information about distant black holes and neutron stars — but they can also provide large-scale insights into how our universe works. A new study explores how gravitational-wave detections may soon resolve the long-lived tension in measurements of our universe’s expansion.
An Expanding Problem
Some past measurements of H0 (click to enlarge). Black data points are local-universe distance-ladder measurements, which cluster around 73 km/s/Mpc; red data points are early-universe CMB measurements, which cluster around 67 km/s/Mpc. [Renerpho]We know the universe is expanding, but we’re still not sure how quickly. The empirically derived value H0 — referred to as the Hubble constant or the Hubble-Lemaître constant — parametrizes the universe’s expansion rate. This controversial parameter, which describes how quickly galaxies are receding from us as a function of their distance from us, is traditionally measured in one of two ways:
In the local universe, by determining the distances to and recession speeds of visible astronomical objects. This method relies on the distance ladder: the distances measured to far-off objects are built upon measured distances to nearer objects.
On global scales, estimated by modeling measurements of the cosmic microwave background (CMB), relic radiation from the Big ...