What is the speed at which the universe is (allegedly) expanding?
Gravitational waves created by the collision of two stars have allowed the speed at which the Universe is expanding to be determined.
A group of papers recently reported the merger of two neutron stars spotted by gravitational-wave detectors, as well as observations of this event in γ-ray, X-ray, optical and infrared radiation. A group of collaborations, led by the LIGO Scientific Collaboration and the Virgo Collaboration, used this wealth of data to directly determine the Hubble constant — the current expansion rate of the Universe. They used the gravitational-wave signal to calculate the distance to the merging stars, and a property of the light called redshift to calculate the velocity of the signal’s host galaxy.
Combining these gives a result of 70 kilometres per second per megaparsec, which is consistent with previous estimates and confirms a prediction from the 1980s that gravitational waves could be used to determine the Hubble constant. Although at present the error margin is large, the result could be improved as more events are observed, the authors say.