A merger to measure expansion of the universe

This image, from a computerised simulation, shows the formation of an inner disc of matter and a wide hot disc of matter 5.5 milliseconds after the merger of a neutron star and a black hole.

Mergers between a neutron star and a black hole may provide an ideal way to measure how fast the universe is expanding, according to a paper in the latest issue of “Physical Review Letters” by Salvatore Vitale of the Massachusetts Institute of Technology (MIT) and Hsin-Yu Chen of Harvard University.

The universe’s expansion rate, or Hubble constant, is a crucial parameter in cosmological models. Since the two main methods for determining it, on the basis of observations of the cosmic microwave background or of supernovae, give conflicting results, researchers would like to have a third independent method to resolve the tension. Calculations by the above two researchers show that a specific kind of gravitational-wave source, the merger of a black hole with a neutron star, could be the best target for precise determination of the expansion rate.