Branes, Strings and Defects in Cosmology

多年来，暴胀和拓扑缺陷一直被认为是现代宇宙学的两个不相容或相互竞争的方面。从历史上看，提出暴胀的原因之一是为了将标准的热大爆炸模型从热问题中拯救出来。然而，这样的机制也会稀释宇宙弦，除非它们是在最后或暴胀之后产生的。后来，暴胀和拓扑缺陷作为两种替代机制相互竞争，产生密度扰动，导致观测到的大尺度结构和宇宙微波背景（CMB）的各向异性。在过去的十年里，宇宙学取得了巨大的进步。大量关于CMB的数据揭示了一个与暴胀基本预测惊人一致的早期宇宙，而拓扑缺陷模型的预测与CMB数据之间存在明显的不一致。这表明了对通货膨胀的明显偏好。然而，即使暴胀提供了一个强大的场论机制，解决了热大爆炸宇宙学模型的缺点，并与CMB实验惊人的一致，它仍然缺乏一个精确的理论模型。随着宇宙学数据的不断完善，寻找一个具有坚实理论基础的暴胀模型变得至关重要。目前，人们普遍认为弦理论是所有物质和力的基础理论，包括一个一致的量子引力部门。如果是这样的话，那么在弦理论中必然存在一个自然暴胀的情形。这种方法将允许识别暴胀子并确定其性质，同时宇宙学测量将为我们宇宙的精确弦描述提供重要的见解。自从狄利克雷（D）膜的“发现”以来，我们的宇宙在弦理论中的一个自然实现就是膜世界方案。在这种背景下，一个简单，现实和动机良好的暴胀模型是膜暴胀，其中暴胀发生在两个膜相互移动时，它们的湮灭释放了膜张力能量，加热了宇宙，开始了热大爆炸时代。通常，在碰撞期间可以产生所有大小和类型的串。大的基本弦（F）和/或D1-膜（D-弦）在宇宙演化中幸存下来，成为宇宙超弦。通过观察天空中的弦，我们将能够第一次（也许是唯一一次）检验弦理论。现在人们普遍认为宇宙弦一般都是在超对称大统一理论和膜暴胀中形成的，其中暴胀子势是混合型的，暴胀阶段以相变结束，留下一个宇宙弦网络。因此，弦和通货膨胀必须调和。我们的建议的目的是确定一个成功的宇宙学模型的弦论，它可以解释时空的维度和起源的暴胀模型，以及寻求弦理论的观测测试，主要是通过宇宙超弦的印记。在IIB型弦理论的框架内，我们将发展一种机制，它可以通过膜相互作用来解释时空维度的起源。在这样的背景下，膜暴胀可能会出现，它的唯象特征将限制模型。膜湮灭将释放膜的能量，重新加热宇宙，从而进入辐射主导的时代。通过研究这些网络的性质，它们与场论宇宙弦有许多相似之处，但也有许多不同之处，以及它们的现象学后果，我们希望能确定成功的和“自然的”暴胀模型。

For a number of years, inflation and topological defects have been considered either as two incompatible or as two competing aspects of modern cosmology. Historically, one of the reasons for which inflation was proposed is to rescue the standard hot Big Bang model from the monopole problem. However, such a mechanism could also dilute cosmic strings unless they were produced at the end or after inflation. Later on, inflation and topological defects competed as the two alternative mechanisms generating the density perturbations which led to the observed large-scale structure and the anisotropies in the Cosmic Microwave Background (CMB). The past decade has seen tremendous advances in cosmology. A plethora of data on the CMB has revealed an early universe in striking agreement with the basic predictions of inflation, while there is a clear inconsistency between predictions from topological defect models and CMB data. This indicates a clear preference for inflation. However, even though inflation provides a robust field theoretic mechanism which answers the shortcomings of the hot Big Bang cosmological model and is in a surprising agreement with CMB experiments, it still lacks a precise theoretical model. As the cosmological data keep improving fast, it becomes essential to find a specific inflationary model having a solid theoretical foundation. At present, it is widely believed that string theory is the fundamental theory of all matter and forces, including a consistent quantum gravity sector. If this is the case, then there must exist a natural inflationary scenario within string theory. Such an approach will allow the identification of the inflaton and the determination of its properties, while at the same time cosmological measurements will provide important insight on the precise stringy description of our universe. Since the 'discovery' of Dirichlet (D) branes, a natural realisation of our universe in string theory is the brane-world scenario. In this context, a simple, realistic and well-motivated inflationary model is brane inflation, where inflation takes place while two branes move towards each other, and their annihilation releases the brane tension energy that heats up the universe to start the hot Big Bang era. Typically, strings of all sizes and types may be produced during the collision. Large fundamental (F) strings and/or D1-branes (D-strings) that survive the cosmological evolution become cosmic superstrings. By observing strings in the sky, we will be able to test, for the first (and maybe only) time, string theory. It is now widely accepted that cosmic strings are generically formed in both supersymmetric grand unified theories and brane inflation, where the inflaton potential is of the hybrid type and the inflationary phase ends with a phase transition, leaving behind a network of cosmic strings. Thus, strings and inflation must reconciliate. The aim of our proposal is to identify a successful cosmological model motivated by string theory, which can explain the dimensionality of space-time and give origin to an inflationary model, as well as to seek observational tests of string theory, through mainly the imprint of cosmic superstrings. In the framework of type IIB string theory we will develop a mechanism which can account for the origin of space-time dimensionality through brane interactions. In such a context brane inflation may appear and its phenomenological characteristics will constrain the model. Brane annihilation will release the energy of the branes, reheating the universe which will thus enter the radiation-dominated era. By studying properties of such networks, which have a number of similarities but also a number of differences from field theory cosmic strings, and their phenomenological consequences, we expect to pin down the successful and 'natural' inflationary model.