Perturbations and moduli space dynamics of tachyon kinks

The dynamic process of unstable D-branes decaying into stable ones with one dimension lower can be described by a tachyon field with a Dirac-Born-Infeld effective action. In this paper we investigate the fluctuation modes of the tachyon field around a two-parameter family of static solutions representing an array of brane-antibrane pairs. Besides a pair of zero modes associated with the parameters of the solution, and instabilities associated with annihilation of the brane-antibrane pairs, we find states corresponding to excitations of the tachyon field around the brane and in the bulk. In the limit that the brane thickness tends to zero, the support of the eigenmodes is limited to the brane, consistent with the idea that propagating tachyon modes drop out of the spectrum as the tachyon field approaches its ground state. The zero modes, and other low-lying excited states, show a fourfold degeneracy in this limit, which can be identified with some of the massless superstring modes in the brane-antibrane system. Finally, we also discuss the slow motion of the solution corresponding to the decay process in the moduli space, finding a trajectory which oscillates periodically between the unstable D-brane and the brane-antibrane pairs of one dimension lower

Hawking radiation via tachyon condensation and its implications to tachyon cosmology

Hawking radiation can be derived from the collapsing process of matter to form a black hole. In this work, we show in more detail that the freely infalling process of a probe (D-)particle (or point-like object) in a non-extreme black hole background is essentially a tachyon condensation process. That is, a probe D-particle will behave as an unstable D-particle in the near-horizon region of a non-extreme black hole. From this point of view, Hawking radiation can be viewed as the thermal radiation from rolling tachyon on an unstable D-particle (i.e., the infalling probe) at the Hagedorn temperature. The result has interesting implications to tachyon cosmology: the uniform tachyon rolling in cosmology can automatically create particle pairs at late times, via a mechanism just like the Hawking radiation process near a black hole. So this particle creation process can naturally give rise to a hot universe with thermal perturbations beyond tachyon inflation, providing an alternative reheating mechanism.Comment: 22 page

Cosmological evolution of a D-brane

We study the cosmological evolution of a single BPS D-brane in the absence of potential, which is in the category of the Chaplygin gas cosmological model. When such a D-brane coupled to gravity moves in the bulk with a non-vanishing velocity, it tends to slow down to zero velocity via mechanisms like gravitational waves leakage to the bulk, losing its kinetic energy to fuel the expansion of the universe on the D-brane. If the initial velocity of the D-brane is high enough, the universe on the D-brane undergoes a dust-like stage at early times and an acceleration stage at late times, as observed in the original Chaplygin gas model. When the D-brane velocity is initially zero, the D-brane will always remain fixed at some position in the bulk, with the brane tension over the Plank mass squared as a cosmological constant. Interestingly, this kind of fixed brane universe can arise as defects from tachyon inflation on a non-BPS D-brane with one dimension higherWe study the cosmological evolution of a single BPS D-brane coupled to gravity in the absence of potential. When such a D-brane moves in the bulk with non-vanishing velocity, it tends to slow down to zero velocity via mechanisms like gravitational wave leakage to the bulk, losing its kinetic energy to fuel the expansion of the universe on the D-brane. If the initial velocity of the D-brane is high enough, the universe on the D-brane undergoes a dust-like stage at early times and an acceleration stage at late times, realising the original Chaplygin gas model. When the D-brane velocity is initially zero, the D-brane will always remain fixed at some position in the bulk, with the brane tension over the Plank mass squared as a cosmological constant. It is further shown that this kind of fixed brane universe can arise as defects from tachyon inflation on a non-BPS D-brane with one dimension higher.Comment: 13 page