The Topology of Cosmological Reionization

Understand

Using the largest cosmological reionization simulation to-date (~24 billion particles), we use the genus curve to quantify the topology of neutral hydrogen distribution on scales > 1 Mpc as it evolves during cosmological reionization.

• We find that the reionization process proceeds primarily in an inside-out fashion, where higher density regions become ionized earlier than lower density regions.
• There are four distinct topological phases: (1) Pre-reionization at z ~ 15, when the genus curve is consistent with a Gaussian density distribution.
• (2) Pre-overlap at 10 < z < 15, during which the number of HII bubbles increases gradually with time, until percolation of HII bubbles starts to take effect, characterized by a very flat genus curve at high volume fractions.