一些宇宙的过去和未来
Past and future of some universes
摘要：
我们考虑一类玩具模型，其中空间平坦的宇宙充满了完美的流体。所有这些模型的动力学都是精确的。在一个家族中，完美流体是幻影类型的，我们发现宇宙首先收缩，然后膨胀，而动力学总是加速。在第二族中，宇宙首先处于加速膨胀阶段，然后处于减速膨胀阶段，直到它达到一个转折点，在此之后它以减速的方式收缩（收缩率增加），然后是另一个加速阶段（收缩率降低）。我们还考虑了将这种完美流体嵌入现实宇宙学的可能性。第一家族在传统的大爆炸宇宙中是不可能存在的，需要在非常早期的宇宙中反弹。第二个家庭是可行的范围$0<1+w_{DE,0}\lesssim 0.09$的曲率满足当前的边界的空间封闭的宇宙。虽然这个家族中的许多模型在今天还不能与一个由宇宙学常数支配的宇宙区分开来，但目前的加速膨胀是短暂的，这些宇宙将在未来进入收缩阶段之前达到一个转折点。
英文摘要：
We consider a class of toy models where a spatially flat universe is filled with a perfect fluid. The dynamics is found exactly for all these models. In one family, the perfect fluid is of the phantom type and we find that the universe is first contracting and then expanding while the dynamics is always accelerated. In a second family, the universe is first in an accelerated expansion stage, then in a decelerated expansion stage until it reaches a turning point after which it contracts in a decelerated way (increasing contraction rate) followed by another accelerated stage (decreasing contraction rate). We also consider the possibility to embed this perfect fluid in a realistic cosmology. The first family cannot be viable in a conventional big bang universe and requires a rebound in the very early universe. The second family is viable in the range $0<1+w_{DE,0}\lesssim 0.09$ for a spatially closed universe with a curvature satisfying current bounds. Though many of the models in this family cannot be distinguished today from a universe dominated by a cosmological constant, the present accelerated expansion is transient and these universes will reach a turning point in the future before entering a contraction phase.
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