为什么有东西而不是什么都没有（出一切）？
Why there is something rather than nothing (out of everything)?
摘要：
最近提出的宇宙密度矩阵的欧几里德几何上的路径积分描述了量子宇宙学中的微正则系综。这个系综对应于真实物理变量在相空间中的均匀（权重1）分布，但就可观测的时空几何而言，它在{ \em Lorentzian}路径积分的复鞍点附近达到峰值。它们由最近获得的宇宙学瞬子表示，该瞬子被限制在宇宙学常数的有界范围内。由这些瞬子在膨胀后期产生的暴胀宇宙学经历了加速，其低能尺度可以在动态演化的额外维度的概念中实现。因此，与早期宇宙学常数的有界范围一起，这个宇宙学系综提出了限制弦真空景观的机制，同时，以宇宙微正则态的准平衡衰变的形式，提出了暗能量问题的可能解决方案。
英文摘要：
The path integral over Euclidean geometries for the recently suggested density matrix of the Universe is shown to describe a microcanonical ensemble in quantum cosmology. This ensemble corresponds to a uniform (weight one) distribution in phase space of true physical variables, but in terms of the observable spacetime geometry it is peaked about complex saddle-points of the {\em Lorentzian} path integral. They are represented by the recently obtained cosmological instantons limited to a bounded range of the cosmological constant. Inflationary cosmologies generated by these instantons at late stages of expansion undergo acceleration whose low-energy scale can be attained within the concept of dynamically evolving extra dimensions. Thus, together with the bounded range of the early cosmological constant, this cosmological ensemble suggests the mechanism of constraining the landscape of string vacua and, simultaneously, a possible solution to the dark energy problem in the form of the quasi-equilibrium decay of the microcanonical state of the Universe.
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