Including Absorption in Gordon's Optical Metric
  We show that Gordon's optical metric on a curved spacetime can be generalized
to include absorption by allowing the metric to become complex. We demonstrate
its use in the realm of geometrical optics by giving three simple examples. We
use one of these examples to compute corrected distance-redshift relations for
Friedman-Lema\^itre-Robertson-Walker models in which the cosmic fluid has an
associated complex index of refraction that represents grey extinction. We then
fit this corrected Hubble curve to the type Ia supernovae data and provide a
possible explanation (other than dark energy) of the deviation of these
observations from dark matter predictions.
