Cosmology with tachyon field as dark energy

We present a detailed study of cosmological effects of homogeneous tachyon matter coexisting with nonrelativistic matter and radiation, concentrating on the inverse square potential and the exponential potential for the tachyonic scalar field. A distinguishing feature of these models (compared to other cosmological models) is that the matter density parameter and the density parameter for tachyons remain comparable even in the matter dominated phase. For the exponential potential, the solutions have an accelerating phase, followed by a phase with $a(t)\ensuremath{\propto}{t}^{2/3}$ as $\stackrel{\ensuremath{\rightarrow}}{t}\ensuremath{\infty}.$ This eliminates the future event horizon present in cold dark matter models with a cosmological constant $(\ensuremath{\Lambda}\mathrm{CDM})$ and is an attractive feature from the string theory perspective. A comparison with supernova type Ia data shows that for both the potentials there exists a range of models in which the universe undergoes an accelerated expansion at low redshifts which are also consistent with the requirements of structure formation. They do require fine-tuning of parameters but not any more than in the case of $\ensuremath{\Lambda}\mathrm{CDM}$ models or quintessence models.

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