What is reionization?

Most of my research is about Cosmic Dawn and the Epoch of Reionization. These eras refer to the universe about 13 billion years ago, when the very first stars and galaxies formed. The light emitted by these objects ionized the low-density gas between them, heating up the gas and leaving a distinctive imprint on the large scale structure of the universe. Although we are starting to make some important measurements about this part of the universe’s history, many of the details of this process are unknown. It is also one of the final observational frontiers in astronomy: galaxies from the post-reionization universe have been cataloged and studied in great detail, and the Cosmic Microwave Background from well before reionization has been measured to exquisite precision. Reionization is one of the final frontiers!

Unfortunately, measuring reionization directly is very challenging. The stars and galaxies that drive reionization are typically much smaller than our own Milky Way galaxy, and they are much fainter because of how far away they are. Alternatively, we can use the 21-cm signal from neutral hydrogen to observe the gas between galaxies instead. This has its own set of challenges, many of which are due to emission from our own galaxy swamping the expected signal level. There are other potential ways of detecting reionization, including hints in the cosmic microwave background. So far though, a significant detection has remained elusive.

Experiments such as the Hydrogen Epoch of Reionization Array (HERA) are close to making a detection of reionization. We will definitely be able to learn a lot about these early stars from a detection, and maybe even some more fundamental cosmological information. At the same time, we might think of new questions we want to ask given our updated understanding of things. My research is focused on building ways to extract as much information about reionization as possible from measurements in the near future, and help make those measurements from HERA come to fruition. Throughout all of my projects, I make use of computational techniques that help us understand the measurements we make from different telescopes and observatories, and connect them to underlying physical models. One technique I’m very excited about is machine learning, which is being applied to cosmology research in lots of neat ways!