Timing, duration, and topology of cosmic reionization

Determine the timing, duration, and topology of cosmic reionization, which remain unresolved aspects of the transition from a predominantly neutral intergalactic medium to a highly ionized one.

Background

The paper identifies the timing, duration, and topology of cosmic reionization as unresolved properties of the Universe’s reionization history. These properties describe when reionization occurred, how long the transition lasted, and how the resulting ionized regions were spatially organized.

Although the paper focuses on modeling optical-depth fluctuations and separating patchy-ionization and baryon-density contributions, it does not determine these broader reionization-history properties. Their unresolved status motivates continued observational and theoretical study of reionization morphology and evolution.

References

Some of the more pressing open questions in this field are: How do galaxies grow? How do metals distribute around galaxies? How do molecules distribute around galaxies? Are H outflows common at the disk-halo interface and in the CGM? How far from the disk can they survive in the CGM, and what factors promote or inhibit their survival? Do H outflows have any connection to the observed neutral and low-ionization metal outflows? What is the location of Galactic and Magellanic high-velocity clouds? What is the thermodynamic state of the CGM? Can we chemically trace the origins of the gas and feedback? Can we characterise the chemical and kinematical properties of infalling gas and outflows? How did the \ reionization proceed and when was it completed?

— The future of high-resolution UV spectroscopy: Science with a UV Échelle spectrograph on the Habitable Worlds Observatory, or a dedicated mission  (2609.09329 - Cia et al., 8 Sep 2026) in Section 5, subsection “The Circumgalactic and Intergalactic Medium,” especially “Open questions in CGM/IGM science”

The process was largely complete by $z\approx 6$ \citep{Fan2006,McGreer2015}, but the timing, duration, and topology of reionization remain open questions.

— The Challenge of Observing Patchy Reionization with CMB Optical-Depth Fluctuations  (2608.19307 - Takoudes et al., 19 Aug 2026) in Introduction, Section 1

The redshift evolution and spatial variation of the volume-averaged neutral fraction $\bar{x}_{\rm H\,I}$ remain uncertain.

— 3.5-meter Segmented-Mirror Robotic Space Telescope Mission White Paper II. Key Scientific Mission: Wide-Field Cosmology and Galaxy Evolution  (2609.02574 - Kim et al., 2 Sep 2026) in Section 4.5, “Redshift Reach for Reionization”