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

Michael Norman, University of California, San Diego

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Michael Norman, James Bordner, Alexei Kritsuk, Britton Smith, Daniel Reynolds, Hao Xu, Pengfei Chen, Kyungjin Ahn, Azton Wells

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 4,000:1. 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.