Some classes of inflationary models naturally introduce two distinct metrics/frames, and their equivalence in terms of observables has often been put in question. D-brane inflation proposes candidates for an inflaton embedded in the string theory and possesses descriptions on the brane and bulk metrics/frames, which are connected by a conformal/disformal transformation that depends on the inflaton and its derivatives. It has been shown that curvature perturbations generated by the inflaton are identical in both frames, meaning that observables such as the spectrum of cosmic microwave background (CMB) anisotropies are independent of whether matter fields---including those in the standard model of particle physics---minimally couple to the brane or the bulk metric/frame. This is true despite the fact that the observables are eventually measured by the matter fields and that the total action including the matter fields is different in the two cases. In contrast, in curvaton scenarios, the observables depend on the frame to which the curvaton minimally couples. Among all inflationary scenarios, we focus on two models motivated by the KKLMMT fine-tuning problem: a slow-roll inflation with an inflection-point potential and a model of a rapidly rolling inflaton that conformally couples to gravity. In the first model, the difference between the frames in which the curvaton resides is encoded in the spectral index of the curvature perturbations, depicting the nature of the frame transformation. In the second model, the curvaton on the brane induces a spectral index significantly different from that in the bulk and is even falsified by the observations. This work thus demonstrates that two frames connected by a conformal/disformal transformation lead to different physical observables such as CMB anisotropies in curvaton models.
Statistical Anisotropy from Vector Curvaton in D-brane Inflation
我们研究了在IIB型弦论的暴胀D膜模型中嵌入向量曲率范式的可能性。向量曲率等同于U（1）规范场，它存在于D3-膜的世界体积上，D3-膜可以是静止的，也可以在内部空间中进行一般运动。该矢量场被认为是一个旁观者场，它调节着矢量场的演化。在这种设置中，假设微扰子演化为e^（-ε）\propto a ^2，其中a（t）是比例因子，则矢量曲率能够在曲率扰动的谱和双谱中产生可测量的统计各向异性。我们的工作构成了探索这种独特的特征是如何从自然出现在宇宙学的弦论模型中的几个光场的存在中产生的第一步。
Gravitational Shock Waves and Their Scattering in Brane-Induced Gravity
本文继hep-th/0501028之后，详细推导和讨论了在膜诱导引力作用下局域于张膜上的相对论性粒子的精确引力场解。我们的推导产生正常分支和自膨胀分支DGP braneworlds的度量。他们推广了德西特空间中的四维引力激波，因此我们将它们与相应的四维广义相对论解以及引力只存在于五维体中且没有膜定域引力子动力学项的情况进行了比较。我们写下来的解决方案，在两个变量的超几何函数，并发现在短距离的冲击波轮廓看起来完全相同的4D闵可夫斯基空间，从而恢复的限制，如果重力是介导的亚稳态，但寿命长，批量共振。远离源的校正不同于4D de Sitter空间中的长距离校正，具有距离的奇次幂。我们详细讨论了自膨胀分支上的极限情况下，当重力被修改正好在德西特半径，能量可以通过共振过程损失到大部分。最后，我们考虑了膜上的普朗克散射，并发现对于足够小的碰撞参量，它非常接近通常的4D描述。
