Black holes are normally and naturally associated to the end-point of gravitational collapse. Yet, alternatives have been proposed and a particularly interesting one is that of gravitational condensate stars, or gravastars. We here revisit the gravastar model and increase the degree of speculation by considering new solutions that are inspired by the original model of gravastars with anisotropic pressure, but also offer surprising new features. In particular, we show that it is possible to nest two gravastars into each other and obtain a new solution of the Einstein equations. Since each gravastar essentially behaves as a distinct self-gravitating equilibrium, a large and rich space of parameters exists for the construction of nested gravastars. In addition, we show that these nested-gravastar solutions can be extended to an arbitrarily large number of shells, with a prescription specified in terms of simple recursive relations. Although these ultra-compact objects are admittedly very exotic, some of the solutions found, provide an interesting alternative to a black hole by having a singularity-free origin, a full matter interior, a time-like matter surface, and a compactness C→(1/2)− .

中文翻译：

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引力凝聚星的嵌套解

黑洞通常和自然地与引力坍缩的终点相关。然而，已经提出了替代方案，其中一个特别有趣的方案是引力凝聚星，或引力星。我们在这里重新审视 Gravastar 模型，并通过考虑新的解决方案来增加推测程度，这些解决方案受到具有各向异性压力的 Gravastars 原始模型的启发，但也提供了令人惊讶的新功能。特别是，我们证明可以将两个格拉瓦星相互嵌套并获得爱因斯坦方程的新解。由于每个 gravastar 本质上都表现为独特的自引力平衡，因此存在大量且丰富的参数空间用于构建嵌套的 gravastar。此外，我们还表明，这些嵌套的 gravastar 解决方案可以扩展到任意数量的 shell，并根据简单的递归关系指定规定。尽管这些超紧凑物体确实非常奇特，但发现的一些解决方案通过具有无奇点起源、完整的物质内部、类时间物质表面和致密性，提供了黑洞的有趣替代品 C→（1/2）- 。