A new paper in Physical Review Letters, which includes OKC co-authors Hiranya Peiris and Daniel Mortlock, proposes that observations of gravitational waves and their electromagnetic counterparts arising from neutron star mergers can resolve the tension between current measurements of the expansion rate of our Universe.

"The Hubble Constant is one of the most important numbers in cosmology because it is essential for estimating the curvature of space and the age of the universe, as well as exploring its fate,” says Professor Hiranya Peiris.

We can measure the Hubble Constant by using two methods – one observing Cepheid stars and supernovae in the local universe, and a second using measurements of cosmic background radiation from the early universe – but these methods don’t give the same values, which means our standard cosmological model might be flawed.

Posterior probability distributions for two methods of measuring the Hubble constant given simulated neutron star merger data.


When two neutron stars merge, both gravitational waves and light are emitted. The first event where both were detected occurred in August 2017. For a given system of merging neutron stars, measuring the gravitational wave signal gives a precise distance to the system. The electromagnetic signal provides the velocity of the system in the Universe's expansion. In a decade or so, once we have observations of about 50 merging neutron star systems, the authors of the paper show that with some Bayesian statistical analysis we will be able to determine whether the local measurements of the Hubble constant are systematically inaccurate or whether the standard cosmological model needs to be modified.

Researchers at the OKC will be working to discover the necessary merging neutron star systems for this research. With the support of the VR-funded GREAT (Gravitational Radiation and Electromagnetic Astrophysical Transients) research environment, they will be carrying out end-to-end simulations of the electromagnetic signals from events involving mergers of compact objects accompanied by emission of gravitational radiation. Based on these simulations, they will optimize and perform searches for electromagnetic counterparts of gravitational wave events in upcoming large astronomical surveys.