Thunder and Lightning: Using Neutron-star Mergers as Simultaneous Standard Candles and Sirens to Measure Cosmological Parameters

With the detection of gravitational wave (GW) GW170817 and its associated electromagnetic (EM) counterparts from a binary neutron star (NS) merger, the "standard siren" method for Hubble-constant measurements is expected to play a role in the Hubble-constant tension in the next few years. One intriguing proposal put forward in multiple studies is to use an NS merger's optical counterpart, known as a kilonova, as a standard candle, because its absolute magnitude can in principle be calculated from simulations. In this work, I detail the statistical framework for performing joint standard-candle and standard-siren measurements using GWs, EM follow-up data, and simulations of EM counterparts. I then perform an example analysis using GW170817 and its optical counterpart AT2017gfo to illustrate the method and the method's limitations. Crucially, the inferences using this method are only as robust as the EM counterpart models, so significant theoretical advances are needed before this method can be employed for precision cosmology.