We study preheating in N -flation, assuming the Marčenko-Pastur mass distribution, equal energy initial conditions at the beginning of inflation and equal axion-matter couplings, where matter is taken to be a single, massless bosonic field. By numerical analysis we find that preheating via parametric resonance is suppressed, indicating that the old theory of perturbative preheating is applicable. While the tensor-to-scalar ratio, the non-Gaussianity parameters and the scalar spectral index computed for N -flation are similar to those in single field inflation (at least within an observationally viable parameter region), our results suggest that the physics of preheating can differ significantly from the single field case.

A stability analysis is made in the context of the previously discovered non-singular cosmological solution from 1-loop corrected superstring effective action. We found that this solution has an instability in graviton mode, which is shown to have a close relation to the avoidance of initial singularity via energy condition. We also estimate the condition for the breakdown of the background solution due to the overdominance of the graviton.Tracing back the history of our presently expanding universe, we are naturally led to the era of high temperature and large energy density. When the energy density approaches Planck scale, our general relativistic description of the universe is no longer considered as valid since quantum gravitational processes are thought to be significant at such a high energy scale. The leading candidate for the satisfactory theory in the very early universe is the superstring theory [1], and roughly speaking, two approaches are being made in the attempt to construct the consistent history of the early universe.Pre-big-bang universe model [2] was proposed to realize an inflation [3]"before" the initial singularity. Due to the scale factor duality [4], the equations of motion in this model have two distinct and disconnected branches of the solution, one corresponding to an ordinary Friedmann solution, and the other corresponding to the so-called super-inflation solution with increasing Hubble parameter. The super-inflationary stage, which is usually put before the Friedmann stage and characterized by the negative-power dependence of the scale factor on time, is regarded as a sort of inflation that lessens the difficulties appearing in the standard big bang model. Although this model is advantageous in that the inflation is naturally explained from one of the most promising theories at present, many difficulties, besides the serious graceful-exit problem [5], have been pointed out [6,7].Another approach in string cosmology is the search for non-singular cosmological models [8]. Initial singularity problem is inherent in the standard big bang cosmology [9], and unless we believe the universe was really generated by one sudden blow at infinite temperature, it is natural to think that the classical theory was violated in such an extreme condition and that singular situation never happened. Also, getting rid of the infinitely high temperature suggests possibilities to understand the whole history of our universe without knowing the theory of everything.Recently, non-singular cosmological model based on 1-loop corrected superstring effective action [11] was proposed [12,13]. One of the most interesting features of this model is that its solutions include a super-inflationary stage before the ordinary Friedmann-like universe, which is, just like pre-big-bang case, expected to give a natural explanation for the inflation. It should be emphasized that the transition in this model is smooth in contrast with the pre-big-bang scenario. In order to concentrate on the behavior of the metric-modulus sub-sy...

We study the evolution of fluctuations in a universe dominated by a scalar field coupled to the Gauss-Bonnet term. During the graceful exit, we found non-negligible enhancements of both curvature perturbation and gravitational wave in the long wavelength limit, and we also found a short wavelength instability for steep background superinflation just after the completion of the graceful exit. This result for one possible graceful exit mechanism would provide a significant implication on the primordial spectrum from the string cosmology.PACS number(s): 04.20. Dw, 04.50.+h, 11.25.Mj, 98.80.Hw

We study the c = −2 model of logarithmic conformal field theory in the presence of a boundary using symplectic fermions. We find boundary states with consistent modular properties. A peculiar feature of this model is that the vacuum representation corresponding to the identity operator is a sub-representation of a "reducible but indecomposable" larger representation. This leads to unusual properties, such as the failure of the Verlinde formula. Despite such complexities in the structure of modules, our results suggest that logarithmic conformal field theories admit bona fide boundary states.