Effective Tachyon Potential Research Articles

Removing the big rip singularity from anisotropic universe in super string theory

Recently, various observational data predict the possibility that dark energy could be in the form of phantom field. The positive phantom-energy density grows without limit with the expansion of the Universe and leads to a big-rip singularity at a finite future time. The main question arises: what is the origin of the big rip singularity in a four-dimensional Universe? To answer this question, in this paper, we propose a new model in super string theory that allows taking into account the Dirac and vector string tachyon in addition to the scalar one, which stretches between branes and antibranes. In this model, scalar and Dirac string tachyons cancel each other’s effects and the only effect induced by the vector tachyon can be observed in density and pressures of the universe. We observe that different scale factors, pressures, and dark energy equation of state parameters are produced in different directions because of inhomogeneous tachyon dynamics and consequently one anisotropic universe is formed. Also, these observations are given in terms of effective tachyon potential and the separation between branes and antibranes. Thus, we have shown that the expansion of the anisotropic Universe is controlled by the vector string tachyon and evolves from the non-phantom phase to the phantom one and consequently, the phantom-dominated era of the universe accelerates and ends up in a big-rip singularity.

Application of KBc Subalgebra in String Field Theory

Recently, a classical solution of open cubic string field theory (CSFT) which corresponds to the closed string vacuum is found by Erler and Schnabl. In their work, a very simple subalgebra of open string star algebra — called KBc subalgebra — plays a crucial role. In this talk, we demonstrate two applications of the KBc subalgebra. One is evaluation of classical and effective tachyon potential. It turns out that the level expansion in the KBc subalgebra terminates at a certain level, so that analytic evaluation of effective potential is available. The other application is regularization of the identity based solutions. It is demonstrated that the Okawa-Erler-Schnabl type solution naturally includes gauge invariant regularization of identity based solutions.

Tachyon Potential in KBc Subalgebra

We evaluate the classical action and the effective tachyon potential of open string field theory within KBc subalgebra, which is extensively used in analytic solution for tachyon condensation recently found by Erler and Schnabl. It is found that the level expansion of the string field terminates at level 3. We find that the closed string vacuum is a saddle point of the classical action. We also evaluate the effective potential for tachyon field. The closed string vaccum becomes stable by integrationg out an auxiliary field. It is found that the effective potential is bounded below hence has no runaway direction. We also argue validity of simple identity based solution.

The tachyon potential in the sliver frame

We evaluate the tachyon potential in the Schnabl gauge through off-shell computations in the sliver frame. As an application of the results of our computations, we provide a strong evidence that Schnabl's analytic solution for tachyon condensation in open string field theory represents a saddle point configuration of the full tachyon potential. Additionally we verify that Schnabl's analytic solution lies on the minimum of the effective tachyon potential.

Exact solution in a string cosmological model

An exact solution to the Friedmann equations with a string inspired phantom scalar matter field is constructed and the absence of the "Big Rip" singularity is shown explicitly. The notable features of the concerned model are a ghost sign of the kinetic term and a special polynomial form of the effective tachyon potential. The constructed solution is stable with respect to small fluctuations of the initial conditions and special deviations of the form of the potential.

Descent relation of tachyon condensation from boundary string field theory

We analyze how lower-dimensional bosonic D-branes further decay, using the boundary string field theory. Especially we find that the effective tachyon potential of the lower-dimensional D-brane has the same profile as that of D25-brane.

Tachyon condensation in superstring field theory

It has been conjectured that at the stationary point of the tachyon potential for the D-brane-anti-D-brane pair or for the non-BPS D-brane of superstring theories, the negative energy density cancels the brane tensions. We study this conjecture using a Wess-Zumino-Witten-like open superstring field theory free of contact term divergences and recently shown to give 60% of the vacuum energy by condensation of the tachyon field alone. While the action is non-polynomial, the multiscalar tachyon potential to any fixed level involves only a finite number of interactions. We compute this potential to level three, obtaining 85% of the expected vacuum energy, a result consistent with convergence that can also be viewed as a successful test of the string field theory. The resulting effective tachyon potential is bounded below and has two degenerate global minima. We calculate the energy density of the kink solution interpolating between these minima finding good agreement with the tension of the D-brane of one lower dimension.

Level truncation and the tachyon in open bosonic string field theory

The tachyonic instability of the open bosonic string is analyzed using the level truncation approach to string field theory. We have calculated all terms in the cubic action of the string field theory describing zero-momentum interactions of up to level 20 between scalars of level 10 or less. These results are used to study the tachyon effective potential and the nonperturbative stable vacuum. We find that the energy gap between the unstable and stable vacua converges much more quickly than the coefficients of the effective tachyon potential. By including fields up to level 10, 99.91% of the energy from the bosonic D-brane tension is cancelled in the nonperturbative stable vacuum. It appears that the perturbative expansion of the effective tachyon potential around the unstable vacuum has a small but finite radius of convergence. We find evidence for a critical point in the tachyon effective potential at a small negative value of the tachyon field corresponding to this radius of convergence. We study the branch structure of the effective potential in the vicinity of this point and speculate that the tachyon effective potential is globally non-negative.