What ended the cosmic dark ages?

01 · TRANSPARENT, THEN DARK

The universe could transmit light before it had stars

About 380,000 years after the big bang, electrons and nuclei combined into neutral atoms and the universe became transparent to the light now seen as the cosmic microwave background.

That event did not illuminate the sky with stars. Matter was mostly neutral hydrogen and helium, density fluctuations were growing, and no long-lived points of light had yet formed. “Dark ages” names this interval between last scattering and cosmic dawn.

02 · COSMIC DAWN

The first sources switched on locally, not everywhere

Gravity gathered gas into the first dark-matter haloes. Stars formed, then galaxies and accreting black holes. Their spectra contained energetic ultraviolet photons capable of removing electrons from neutral hydrogen.

One source could transform only its neighbourhood. The early universe was not a curtain lifted all at once; bright islands appeared inside a much larger neutral medium.

03 · A PATCHWORK TRANSITION

Ionized bubbles grew, touched and overlapped

An ionizing photon that reached intergalactic hydrogen could split an atom into a proton and electron. Recombination worked in the opposite direction, so each region reflected a competition among photon production, gas density, clumping and time.

Around groups of galaxies, ionized regions expanded. As sources accumulated, separate bubbles connected and the remaining neutral gas retreated into islands. This is why reionization has a duration and geography rather than one universal date.

The first stars ended the absence of light sources. Overlapping bubbles ended the mostly neutral intergalactic medium. Those are related, but not identical milestones.

04 · THE PHOTON BUDGET HAS A GATE

Producing ultraviolet light is not enough

Young massive stars can make many ionizing photons, but gas and dust inside their own galaxy may absorb them. The relevant quantity is the escape fraction: the share that reaches intergalactic space.

A galaxy population can dominate reionization by combining abundance, photon-production efficiency and escape. A few bright galaxies are easy to see; vast numbers of faint galaxies can still win the total budget.

05 · WHAT WEBB CHANGED

Faint galaxies look more efficient—and far more important

In the lensed sample analysed by Atek and colleagues, faint galaxies produced about four times more ionizing photons per unit ultraviolet luminosity than values commonly assumed in earlier budgets.

The study used JWST observations behind the lensing cluster Abell 2744 to reach intrinsically faint systems. Extrapolating a lensed field to the whole universe still requires luminosity functions, completeness corrections and an assumed escape fraction; “dwarfs did it” is a strong case, not a photograph of every ionizing photon.

06 · THREE WINDOWS ON ONE ERA

Galaxies, quasars and the microwave background constrain different pieces

JWST spectra tell us about early galaxies and their ionizing efficiency. Absorption in distant quasar spectra traces neutral hydrogen near the end of reionization. CMB polarization records the integrated scattering by free electrons across the transition.

Under Planck’s simple instantaneous-transition parameterization, the midpoint of reionization lies at about redshift 7.7. That number is model-dependent: a midpoint inferred under a rapid-transition shape is not proof that reionization happened instantaneously.

07 · THE MAP WE STILL LACK

The next goal is to see neutral and ionized regions in three dimensions

Redshifted 21-centimetre emission from neutral hydrogen should map the gas that galaxies were transforming. Current experiments are pushing toward statistical detections while foreground radio emission remains a severe challenge.

Combining 21-centimetre maps with JWST galaxy positions can test bubble sizes, source clustering and timing. The open question is no longer simply whether galaxies mattered, but which galaxies, with what escape fractions, in which environments.