The late-time behavior of our universe is one of accelerated expansion, or that of a de Sitter space, and therefore motivates us to look for time-dependent backgrounds. Finding such backgrounds in string theory has always been a challenging problem. An even harder problem is to find time-dependent backgrounds that allow positive dark energies. As a first step to handle such scenarios, we study a time-dependent background in type IIB theory, with four-dimensional de Sitter isometries, by uplifting it to M-theory and then realizing it as a coherent, or squeezed-coherent, state over an appropriate solitonic configuration. While classically such a background does not solve the equations of motion, the corresponding Schwinger-Dyson equations reveal that there are deeper issues that may even prohibit a solution to exist at the quantum level, as long as the internal space remains time-independent. A more generic analysis is then called for, where both the effective four-dimensional space-time, the internal space, and the background fluxes are all time-dependent. We study in details such a background by including perturbative and non-perturbative as well as local and non-local quantum terms. Our analysis reveals a distinct possibility of the emergence of a four-dimensional positive curvature space-time with de Sitter isometries and time-independent Newton's constant in the landscape of type IIB string theory. We argue how the no-go and the swampland criteria are avoided in generating such a background, and compare it with other possibilities involving backgrounds with time-dependent Newton constants. These time-varying Newton constant backgrounds typically lead to unavoidable late time singularities, amongst other issues.

我们宇宙的后期行为是一种加速膨胀或德西特空间的行为，因此激励我们寻找与时间相关的背景。在弦理论中找到这样的背景一直是一个具有挑战性的问题。一个更难的问题是找到允许正暗能量的时间相关背景。作为处理此类情况的第一步，我们研究了 IIB 型理论中的时间相关背景，具有四维 de Sitter 等距，将其提升为 M 理论，然后将其实现为相干或压缩相干状态在适当的孤子配置上。虽然经典上这样的背景并不能解决运动方程，但相应的施温格-戴森方程表明存在更深层次的问题，甚至可能阻止在量子水平上存在解决方案，只要内部空间保持与时间无关。然后需要进行更通用的分析，其中有效的四维时空、内部空间和背景通量都与时间相关。我们通过包括微扰和非微扰以及局域和非局域量子项来详细研究这样的背景。我们的分析揭示了在 IIB 型弦理论的景观中出现具有德西特等距线和与时间无关的牛顿常数的四维正曲率时空的明显可能性。我们讨论了在生成这样的背景时如何避免禁止和沼泽标准，并将其与涉及具有时间相关牛顿常数的背景的其他可能性进行比较。