When and how did the universe's final phase transition happen?

The universe underwent a phase transition from predominantly neutral at recombination (z ∼ 1100), to almost fully ionized by z ∼ 6. We have not identified the sources of ionizing photons – though the most likely candidates are young stars in galaxies.

Lyman alpha emission from star-forming galaxies is absorbed by neutral gas, so observations of Lyα in high-z galaxies can constrain the timeline and morpology of reionization. I have developed a new Bayesian framework for directly inferring the neutral hydrogen fraction in the IGM from Lyα observations. I led the design, reduction and analysis of a ESO VLT Large Program with KMOS (PI: A Fontana) to measure the reionization timeline from upper limits of Lyα emission at z > 7.5.

This diagram depicts the major milestones in the evolution of the Universe since the Big Bang, about 13.8 billion years ago. It is not to scale. Credit: NAOJ