An open or hyperbolic Friedmann-Robertson-Walker spacetime dominated by tachyonic dark matter can exhibit an ``inflected'' expansion -- initially decelerating, later accelerating -- similar but not identical to that of now-standard $\Lambda$CDM models dominated by dark energy. The features of the tachyonic model can be extracted by fitting the redshift-distance relation of the model to data obtained by treating Type Ia supernovae as standard candles. Here such a model is fitted to samples of 186 and 1048 Type Ia supernovae from the literature. The fits yield values of $H_0=(66.6\pm1.5)~\hbox{ km/s/Mpc}$ and $H_0=(69.6\pm0.4)~\hbox{ km/s/Mpc}$, respectively, for the current-time Hubble parameter, and $t_0=(8.35\pm0.68)~\hbox{ Gyr}$ and $t_0=(8.15\pm0.36)~\hbox{ Gyr}$, respectively, for the comoving-time age of the Universe. Tests of the model against other observations will be undertaken in subsequent works.
Categories: astro-ph.CO gr-qc