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.
