Title: Construction of nonsingular cosmological solutions in string theories Show Chinese title

Title: 弦理论中非奇异宇宙学解的构造

Abstract: We study nonsingular cosmological scenarios in a general $D$-dimensional string effective action of the dilaton-modulus-axion system in the presence of the matter source. In the standard dilatonic Brans-Dicke parameter ($\omega=-1$) with radiation, we analytically obtain singularity-free bouncing solutions where the universe starts out in a state with a finite curvature and evolves toward the weakly coupled regime. We apply this analytic method to the string-gas cosmology including the massive state in addition to the leading massless state (radiation), with and without the axion. We numerically find bouncing solutions which asymptotically approach an almost radiation-dominant universe with a decreasing curvature irrespective of the presence of the axion, implying that inclusion of the matter source is crucial for the existence of such solutions for $\omega=-1$. In the theories with $\omega \ne -1$, it is possible to obtain complete regular bouncing solutions with a finite dilaton and curvature in both past and future asymptotics for the general dimension, $D$. We also discuss the case where dilatonic higher-order corrections are involved to the tree-level effective action and demonstrate that the presence of axion/modulus fields and the matter source does not significantly affect the dynamics of the dilaton-driven inflation and the subsequent graceful exit. Show Chinese abstract

Abstract: 我们研究在存在物质源的情况下，标量-模量-轴子系统的广义$D$维弦有效作用下的非奇异宇宙学情景。 在标准的标量-布兰斯-迪克参数（$\omega=-1$）伴随辐射的情况下，我们解析地获得无奇点的反弹解，其中宇宙从一个具有有限曲率的状态开始，并演化到弱耦合区域。 我们将这种解析方法应用于包括除主要无质量态（辐射）之外的高质量态的弦气体宇宙学，包括有和没有轴子的情况。 我们数值地找到反弹解，这些解无论是否存在轴子，都会渐近地趋近于一个几乎由辐射主导的宇宙，且曲率不断减小，这表明包含物质源对于$\omega=-1$的此类解的存在至关重要。 在具有$\omega \ne -1$的理论中，可以得到完整的正则反弹解，在一般维度$D$的过去和未来渐进行为中，标量和曲率都是有限的。 我们还讨论了标量高阶修正涉及树级有效作用的情况，并证明轴子/模量场和物质源的存在不会显著影响标量驱动的暴胀以及随后的平滑退出的动力学。