Observable Features of Tachyon-Dominated Cosmology

A Friedmann-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. 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. As a demonstration of concept, the distance-redshift relation of such a model is compared here with some observations of Type Ia supernovae. Other measures of the third time derivative of the cosmic scale factor -- the true cosmic jerk -- might be found to test a tachyonic-dark-matter hypothesis.