标题： 宇宙膨胀中的引力坍缩与结构形成暗能量 显示英文标题

标题： Gravitational collapse and structure formation in an expanding universe with dark energy

摘要： 观测表明，宇宙的膨胀正在加速。 这要求宇宙中占主导地位的物质成分具有一些不寻常的特性，如负压。 这种奇特的成分被命名为暗能量。 我们使用暗能量的最简单模型，即爱因斯坦引入的宇宙学常数。 我们在这样的模型中研究球形高密度区域的演化，并表明存在一个最小的高密度值以使坍缩发生：振幅较小的扰动不会坍缩。 这个阈值很有趣，因为即使具有正高密度和负能的扰动也不会在有限时间内坍缩。 此外，我们还表明，振幅大于但与阈值相当的扰动需要非常长的时间才能坍缩。 我们将这些解与暗能量不存在的情况进行比较。 显示英文摘要

摘要： Observations show that the expansion of the Universe is accelerating. This requires that the dominant constituent of matter in the Universe has some unusual properties like negative pressure. This exotic component has been given the name dark energy. We work with the simplest model of dark energy, the cosmological constant introduced by Einstein. We study the evolution of spherical over-densities in such a model and show that there is a minimum over-density required for collapse: perturbations with a smaller amplitude do not collapse. This threshold is interesting as even perturbations with a positive over-density and negative energy do not collapse in finite time. Further, we show that perturbations with an amplitude larger than, but comparable to the threshold value, take a very long time to collapse. We compare the solutions with the case when dark energy is absent.