Realistic Simulations of the Intergalactic Medium: The Search for Missing Physics

在过去的十年里，对星际介质(IGM)--大爆炸中产生的星系之间的氢气和氦气--的新的、更精确的观测揭示了与我们计算模型中公认的预测的差异。特别是，使用夏威夷的凯克望远镜对IGM进行的精确观测表明，IGM的温度和电离状态并不是我们标准的宇宙学模拟所预测的：IGM要么略高于星系中恒星的紫外线辐射所能达到的温度，要么IGM在空间中的分布与模拟预测的不同，或者两者都有。我们的模型中可能缺少热源，例如衰变的暗物质粒子注入的能量。这种差异令人费解，因为标准模型非常好地预测了星系的分布。这种差异表明，标准模型缺乏一些我们简称为“缺失物理学”的基本要素。这个项目对美国的意义在于，它促进了宇宙学基础领域的科学进步，而美国在宇宙学领域处于世界领先地位。该项目正在解决这样一个问题，即我们是否忽视了宇宙质量-能量内容的一个关键组成部分。准确的答案需要强大的工具，而Blue Waters超级计算机正是完成这项工作的工具。在这项工作中，研究人员将系统地探索如何消除模型和观测之间的差异。他们将在Blue Waters超级计算机上进行前所未有的规模和物理真实感的模拟，以寻找失踪的物理学是什么。由于IGM包含了宇宙中的大部分体积和物质，一个小的差异可能会转化为一个重大的发现。由于每个维度的比例范围超过4000：1，因此需要非常大的3D模拟。他们将检查额外的热源，如来自早期星系和类星体的X射线和紫外线是否可以弥补热量不足。或者，他们还将通过对IGM进行第一次大规模模拟来检查当前模拟中有限的空间分辨率是否解释了这种差异，该模拟也详细地模拟了星系附近的气体。

In the past decade new, more precise observations of the intergalactic medium (IGM)--the hydrogen and helium gas between the galaxies produced in the Big Bang--have revealed a discrepancy with the well-established predictions of our computational models. In particular, precision observations of the IGM using the Keck telescopes in Hawaii show that the temperature and ionization state of the IGM is not what our standard cosmological simulations predict: the IGM is either somewhat hotter than ultraviolet radiation from stars in galaxies can make it, or the IGM is distributed differently in space than the simulations predict, or both. There could be missing sources of heat in our models, such as energy injection by decaying dark matter particles. The discrepancy is perplexing since the standard model predicts the galaxy distribution exceedingly well. The discrepancy suggests that the standard model lacks some essential ingredient which we refer to simply as "missing physics". The significance of this project to the nation is that it promotes the progress of science in the fundamental field of cosmology where the US is a world leader. The project is addressing the issue of whether we are overlooking a key component of the mass-energy content of the universe. Precise answers require powerful tools, and the Blue Waters supercomputer is the tool for the job. In this work, the investigators will systematically explore how the discrepancy between models and observations can be removed. They will carry out on the Blue Waters supercomputer simulations of unprecedented size and physical realism to hunt down what the missing physics is. Since the IGM comprises most of the volume and matter in the universe, a small discrepancy may translate into a big discovery. Very large 3D simulations are required because the range of scales in each dimension exceeds 4000:1. They will examine whether additional heating sources such as X-rays and ultraviolet light from early galaxies and quasars can make up the heat deficit. Alternatively, they will also examine whether the limited spatial resolution in current simulations explains the discrepancy by carrying out the first large-scale simulations of the IGM that also simulates the gas near to galaxies in detail.