  Gravitational waves (GWs) from supermassive binary black hole (BBH) inspirals
are potentially powerful standard sirens (the GW analog to standard candles)
(Schutz 1986, 2002). Because these systems are well-modeled, the space-based GW
observatory LISA will be able to measure the luminosity distance (but not the
redshift) to some distant massive BBH systems with 1-10% accuracy. This
accuracy is largely limited by pointing error: GW sources generally are poorly
localized on the sky. Localizing the binary independently (e.g., through
association with an electromagnetic counterpart) greatly reduces this
positional error. An electromagnetic counterpart may also allow determination
of the event's redshift. In this case, BBH coalescence would constitute an
extremely precise (better than 1%) standard candle visible to high redshift. In
practice, gravitational lensing degrades this precision, though the candle
remains precise enough to provide useful information about the
distance-redshift relation. Even if very rare, these GW standard sirens would
complement, and increase confidence in, other standard candles.
