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Source Redshifts from Gravitational-Wave Observations of Binary Neutron Star Mergers

Monday 27th October 2014

In Einstein's theory of gravity, acceleration of mass leads to the emission of energy in the form of gravitational radiation - ripples in the very fabric of space-time that travel at the speed of light. There is world-wide effort to detect this radiation from astrophysical sources using kilometre sized laser interferometers such as the American Laser Interferometer Gravitational Wave Observatory (LIGO) and the French-Italian-Dutch Virgo.

Among the most likely sources expected to be detected by LIGO and Virgo are astrophysical binaries consisting of neutron stars and black holes which lose energy to gravitational radiation over hundreds of millions of years, causing them to spiral in and merge. The last 15 minutes of this inspiral will be observable in LIGO and Virgo.

In 1986 Cardiff physicist Professor Bernard Schutz discovered that such merging binaries are ideal standard candles that could be used to accurately measure distances to galaxies billions of light years away. To measure the expansion rate of the Universe and its dark matter and dark energy content, however, it is necessary to measure not just the distance but also the cosmological redshift of the source, namely how quickly distant galaxies are receding from us. Until recently it was believed that gravitational wave observations alone would not determine the cosmological redshifts of their sources.

Now Cardiff researcher Professor B Sathyaprakash, together with colleagues in Glasgow, Germany and the USA, has shown that in the special case when the binary consists of neutron stars it will be possible to measure both the distance and cosmological redshift using gravitational waves alone. To accurately model the dynamics of such systems and to compute the emitted gravitational radiation physicists rely on cutting-edge numerical simulations that take months to run on state-of-the-art computing facilities. These highly accurate simulations have allowed the researchers to identify characteristic frequencies in the gravitational wave signal from the merged object, called a hyper-massive neutron star.

In a paper recently published in the Journal Physical Review X, they have shown how measurement of the characteristic frequencies before and after merger, together with prior knowledge of their true values from numerical simulations, makes it possible to extract the redshift directly from gravitational wave observations. For the first time they have demonstrated that there is a cosmological application for the post-merger signal and that redshift measurements can be made from binary neutron star merger signals.

Professor Sathyaprakash said "What we have shown is that it will be possible in theory to measure redshifts of cosmological sources. To accomplish this in practice we will need more sophisticated simulations of the merger dynamics of neutron stars. For example, we don't know yet the internal structure of neutron stars and this has to be understood in detail in order for us to infer redshift from gravitational wave observations."

The results were published in Phys. Rev. X 4, 041004, DOI: http://dx.doi.org/10.1103/PhysRevX.4.041004.