Behavior of tachyon in string cosmology based on gauged WZW model

We investigate a string theoretic cosmological model in the context of the gauged Wess-Zumino-Witten model. Our model is based on a product of non-compact coset space and a spectator flat space; $[SL(2,R)/U(1)]_k \times {\bf R}^2$. We extend the formerly studied semiclassical consideration with infinite Kac-Moody level $k$ to a finite one. In this case, the tachyon field appears in the effective action, and we solve the Einstein equation to determine the behavior of tachyon as a function of time. We find that tachyon field dominates over dilaton field in early times. In particular, we consider the energy conditions of the matter fields consisting of the dilaton and the tachyon which affect the initial singularity. We find that not only the strong energy but also the null energy condition is violated.Comment: 10 figure

Variation of the Number of Heat Sources in Methane Dry Reforming: A Computational Fluid Dynamics Study

To overcome the weak point of the gas type heating (failure in heating uniformly and persistently), liquid type molten salt as a concentration of solar energy was considered as a heat source for dry reforming. This high-temperature molten salt flowing through the center of the tubular reactor supplies necessary heat. The dependence on the number of heat source of the hydrogen production was investigated under the assumption of the fixed volume of the catalyst bed. By changing these numbers, we numerically investigated the methane conversion and hydrogen flow rate to find the best performance. The results showed that the methane conversion performance and hydrogen flow rate improved in proportion to the number of heating tubes. For the one heat source, the reactor surrounded by a heat source rather than that located in the center is the best in terms of hydrogen yield. In addition, this study considered the case in which the system is divided into several smaller reactors of equal sizes and a constant amount of catalyst. In these reactors, we saw that the methane conversion and hydrogen flow rate were reduced. The results indicate that the installation of as many heating tubes as possible is preferable

Closed string tachyon potential and tt∗tt^* equation

Recently Dabholkar and Vafa proposed that closed string tachyon potential for non-supersymmetric orbifold \C/\Z_3 in terms of the solution of a $tt^*$ equation. We extend this result to \C^2/\Z_n for $n=3,4,5$. Interestingly, the tachyon potentials for $n=3$ and 4 are still given in terms of the solutions of Painleve III type equation that appeared in the study of \C^1/\Z_3 with different boundary conditions. For \C^2/\Z_5 case, governing equations are of generalized Toda type. The potential is monotonically decreasing function of RG flow.Comment: 15 pages, reference added. to appear in PL

Walls in supersymmetric massive nonlinear sigma model on complex quadric surface

The Bogomol'nyi-Prasad-Sommerfield (BPS) multiwall solutions are constructed in a massive Kahler nonlinear sigma model on the complex quadric surface, Q^N=SO(N+2)/[SO(N)\times SO(2)] in 3-dimensional space-time. The theory has a non-trivial scalar potential generated by the Scherk-Schwarz dimensional reduction from the massless nonlinear sigma model on Q^N in 4-dimensional space-time and it gives rise to 2[N/2+1] discrete vacua. The BPS wall solutions connecting these vacua are obtained based on the moduli matrix approach. It is also shown that the moduli space of the BPS wall solutions is the complex quadric surface Q^N.Comment: 42 pages, 30 figures, typos corrected, version to appear in PR

Localized tachyon condensation and G-parity conservation

We study the condensation of localized tachyon in non-supersymmetric orbifold {\C^2/\Z_n}. We first show that the G-parities of chiral primaries are preserved under the condensation of localized tachyon(CLT) given by the chiral primaries. Using this, we finalize the proof of the conjecture that the lowest-tachyon-mass-squared increases under CLT at the level of type II string with full consideration of GSO projection. We also show the equivalence between the $G$-parity given by $G=[jk_1/n]+ [jk_2/n]$ coming from partition function and that given by $G=\{jk_1/n\}k_2 -\{jk_2/n\}k_1$ coming from the monomial construction for the chiral primaires in the dual mirror picture.Comment: 12 pages, 2 figures, final form to appear in JHE