Do early galaxies overproduce ionizing photons for reionization?
In plain words
JWST shows that early galaxies were numerous and very efficient at making ultraviolet light that strips electrons from hydrogen. Taken at face value they would have finished ionizing the universe earlier than other data allow.
Precise statement
Combining JWST UV luminosity functions, high ionizing-photon production efficiencies $\xi_{\mathrm{ion}}$ and escape fractions $f_{\mathrm{esc}}$ of order 0.1 inferred from low-redshift analogs gives a reionization history that ends too early and an optical depth above the Planck value $\tau = 0.054 \pm 0.007$, while the Lyman-alpha forest indicates completion near $z \sim 5.5 \text{ to } 6$. Determine which ingredient ($f_{\mathrm{esc}}$, $\xi_{\mathrm{ion}}$, faint-end slope, gas clumping) is overestimated, or whether $\tau$ is underestimated; an answer is an ionizing emission rate history $\mathrm{ndot}_{\mathrm{ion}}(z)$ consistent with JWST, CMB and Lyman-alpha forest data.
What would settle it
Direct escape-fraction measurements for a representative $z > 6$ sample combined with an independent determination of $\tau$.
Status in the literature
Unverified note
Munoz et al. (2024, arXiv:2404.07250) identified the overproduction; no consensus resolution exists as of 2026.