Following the Universe from Dark to Light

宇宙结构的形成和演化，包括星系、星系团和大尺度结构，是现代宇宙学的基本问题。虽然宇宙如何开始的大爆炸图景被普遍接受，但关于结构如何以及何时发展和形成的细节仍在激烈辩论中。在比星系团更大的尺度上(直径1000万光年)，人们通常认为引力是主导力量，结构是从宇宙非常早期(大爆炸后1秒)种下的小初始种子生长出来的，因为重力是有吸引力的和不稳定的。然而，在较小的尺度上，气体流体动力学、微观物理(即原子尺度上的力)和恒星形成过程在塑造和确定星系的形成和演化方面发挥着越来越重要的作用；在这些较小的尺度上，情况就不那么清楚了。在这些过程中，辐射场，特别是能够电离原子氢的紫外光，必然起着非常重要的作用。特别是，它决定了氢原子何时被完全电离，宇宙中的氢气何时被清除，变得对遥远的来源透明。这个时代被描述为“黑暗时代的结束”。由于辐射控制着气体热力学状态的演变，其中一小部分随后被并入恒星，因此在宇宙学计算机模拟中全面详细地包含辐射流体动力学是至关重要的，以便为现代宇宙学中的许多重要问题提供答案。这是一项艰巨的任务，即使在当今速度最快的超级计算机上也是如此。岑博士和他的合作者开发了一种有效的方法来计算这种复杂的情况，并开始实施一个程序，该程序应该有助于提供一些关于辐射场和物质如何随时间演变的线索。*

AST 0206299CenThe formation and evolution of cosmic structures, including galaxies, clusters of galaxies and large-scale structure, are fundamental problems of modern cosmology. While the Big Bang picture of how the Universe began is generally accepted, the details of how and when structures developed and formed are in hot debate. On scales larger than clusters of galaxies (10 million light years across) it is generally believed that gravity is the dominant force and structures grow from small initial seeds planted in the very early universe ( 1 second since the Big Bang), because gravity is attractive and unstable. However, on smaller scales gas hydrodynamics, microphysics (i.e., forces on atomic scales) and star formation processes play progressively more important roles in shaping and determining when galaxies form and how they evolve; the picture at these smaller scales is much less clear. The radiation field, especially, the ultra-violet radiation that is capable of ionizing atomic hydrogen must play a very important role in these processes. In particular, it determines when atomic hydrogen is fully ionized and the universe is cleared of hydrogen "fog" and becomes transparent to distant sources. This epoch has been characterized as the "end of the dark ages". Since radiation controls the evolution of the thermodynamic state of the gas, a fraction of which is subsequently incorporated into stars, it is essential to include radiation hydrodynamics in full detail in cosmological computer simulations in order to provide answers to many of the important questions in modern cosmology. This is a formidable task to tackle even with the fastest supercomputers today. Dr. Cen and his collaborators have developed an efficient way to compute this complicated situation and begun to carry out a program that should help provide some clues about how the radiation field and the matter evolve with time.***