Mirage Cosmology of U(1) Gauge Field on Unstable D3 Brane Universe

An unstable $D3$-brane universe governed by the DBI action of the tachyon field minimally coupled to a U(1) gauge boson is examined. The cosmological evolution of this coupled system, is further analyzed, in terms of the expansion rate of the inflating brane, which is highly affected by the presence of the tachyonic and gauge field charges. We show, that the minimal coupling makes the effective brane density less divergent. However, for some sectors of the theory the tachyon is not able to regulate it in an efficient fashion. Also, a detailed analysis of the dependance of the effective brane density on the scale factor of the universe is performed, which leads to various cosmological models.