Cosmography with the Einstein Telescope

The Einstein Telescope, a third-generation gravitational-wave detector under a design study, could detect millions of binary neutron star inspirals each year. A small fraction of these events might be observed as gamma-ray bursts, helping to measure both the luminosity distance D_L to and redshift z of the source. By fitting these measured values of D_L and z to a cosmological model, it would be possible to infer the dark energy equation of state to within 1.5% without the need to correct for errors in D_L caused by weak lensing. This compares favourably with 0.3-10% accuracy that can be achieved with the Laser Interferometer Space Antenna (where weak lensing will need to be dealt with) as well as with dedicated dark energy missions that have been proposed, where 3.5-11% uncertainty is expected.