Observable Features of Tachyon-Dominated Cosmology

A Friedman-Robertson-Walker cosmological model dominated by tachyonic---faster-than-light---dark matter can exhibit features similar to those of a standard dark energy/dark matter or Lambda-CDM model. For example, it can undergo expansion which decelerates to a minimum rate, passes through a `cosmic jerk,' then accelerates. But some features of a tachyon-dominated model are sufficiently distinct from those of the standard model that the two possibilities might be distinguished observationally. In this letter we examine the distance-redshift relation of such a model, which might be compared with observations of standard candles.