Cosmic reionization, the most recent phase transition in the history of the universe, is the process by which neutral cosmic gas was ionized by high-energy radiation from early galaxies. As the observational constraints on reionization are limited, theoretical modeling and numerical simulations play a critical role in reionization studies.

However, forthcoming observations from the soon-to-be-launched James Webb Space Telescope (JWST) and the currently-being-built Atacama Large Millimeter Array (ALMA) radio telescope will make all existing theoretical models of reionization obsolete. Hence, the theoretical community is now faced with the challenge of upgrading simulation technology to a qualitatively higher level to keep theory adequate for comparing with future observations.

Petascale supercomputers like Mira are enabling potential breakthroughs that were deemed impossible only a few years ago. Taking advantage of this technological progress, the Fermilab research team is carrying out simulations that model all relevant physics, from radiative transfer to gas dynamics and star formation, through its Cosmic Reionization On Computers (CROC) project, using the Adaptive Refinement Tree (ART) code as its main simulation tool.

An important objective of this research is to make predictions for future observations of the redshifted 21‑cm line of neutral hydrogen. That line, coming unimpeded even from the most remote corners of the universe, will enable researchers to map the full 3D (2D sky plus redshift as the third dimension) distribution of neutral gas in the universe, once construction of the Hydrogen Epoch of Reionization Array (HERA) is completed.

With a better understanding of cosmic reionization, researchers will be able to shed light on other aspects of modern cosmology, from probing the nature of dark matter and dark energy through Cosmic Microwave Background observations, to observing the physical state of intergalactic gas in the absorption spectra of high redshift quasars.