How ultraviolet light from the first stars and galaxies reionized hydrogen again, making the Universe transparent
In cosmic history, reionization marks the end of the Dark Ages — the period after recombination when the Universe was filled with neutral hydrogen atoms and bright sources (stars, galaxies) had not yet appeared. When the first stars, galaxies, and quasars began to shine, their high-energy (mostly ultraviolet) photons ionized the surrounding hydrogen gas clouds, turning the neutral intergalactic medium (IGM) into a highly ionized plasma. This phenomenon, called cosmic reionization, significantly changed the large-scale transparency of the Universe and set the stage for the familiar, light-filled Universe we know.
In this article, we will discuss:
- The Neutral Universe after Recombination
- The First Light: Population III Stars, Early Galaxies, and Quasars
- The Ionization Process and Bubble Formation
- The Passage of Time and Observational Evidence
- Unanswered Questions and Current Research
- The Importance of Reionization in Modern Cosmology
2. The Neutral Universe after Recombination
2.1 The Dark Ages
From about 380,000 years after the Big Bang (when recombination occurred) until the formation of the first light sources (about 100–200 million years later), the Universe was largely neutral, composed of hydrogen and helium left over from Big Bang nucleosynthesis. This period is called the Dark Ages because, without stars or galaxies, there were no significant new sources of light except the cooling cosmic microwave background (CMB).
2.2 Dominance of neutral hydrogen
During the Dark Ages, the intergalactic medium (IGM) was almost entirely neutral hydrogen (H I), which strongly absorbs ultraviolet photons. As matter began to accumulate in dark matter halos and ancient gas clouds collapsed, the first Population III stars formed. Their abundant radiation flux later significantly changed the state of the IGM.
3. The first light: Population III stars, early galaxies, and quasars
3.1 Population III stars
Theoretically, the first stars – Population III stars – had no metals (consisting almost entirely of hydrogen and helium) and were probably very massive, possibly tens or hundreds of solar masses. They marked the end of the Dark Ages, often called the Cosmic Dawn. These stars emitted abundant ultraviolet (UV) radiation capable of ionizing hydrogen.
3.2 Early galaxies
With hierarchical structure formation, small dark matter halos merged to form larger ones, from which the first galaxies formed. These hosted Population II stars, which further increased the UV photon flux. Over time, these galaxies – not only Population III stars – became the main source of ionizing radiation.
3.3 Quasars and AGN
High redshift quasars (active galactic nuclei powered by supermassive black holes) also contributed to reionization, especially regarding helium (He II). Although their impact on hydrogen reionization is still debated, quasars are thought to have become particularly important at later times, for example, reionizing helium around z ~ 3.
4. The ionization process and bubbles
4.1 Local ionization bubbles
When each new star or galaxy began emitting high-energy photons, these photons traveled outward, ionizing the surrounding hydrogen. This created isolated “bubbles” (or H II regions) of ionized hydrogen around the sources. Initially, these bubbles were solitary and quite small.
4.2 Interaction between bubbles
As the number and brightness of new sources increased, these ionized bubbles expanded and merged. The once neutral IGM first became a patchwork of neutral and ionized media. As the reionization epoch neared its end, H II regions merged and most of the Universe's hydrogen remained ionized (H II), not neutral (H I).
4.3 Reionization timescale
It is believed that reionization lasted several hundred million years, covering redshifts from about z ~ 10 to z ~ 6. Although exact dates remain a subject of research, by z ≈ 5–6 most of the IGM was already ionized.
5. Timeline and observational evidence
5.1 Gunn–Peterson effect
An important indicator of reionization is the so-called Gunn–Peterson test, which examines the spectra of distant quasars. Neutral hydrogen in the IGM strongly absorbs photons at certain wavelengths (especially the Lyman-α line), causing an absorption trough in the quasar spectrum. Observations show that at z > 6 this Gunn–Peterson effect becomes strong, indicating a much higher fraction of neutral hydrogen and highlighting the end of reionization [1].
5.2 Cosmic microwave background (CMB) and polarization
CMB measurements also provide clues. Free electrons from the ionized medium scatter CMB photons, leaving a large angular scale polarization imprint. Data from WMAP and Planck constrain the average timing and duration of reionization [2]. By measuring the optical depth τ (scattering probability), cosmologists can determine when most of the Universe's hydrogen became ionized.
5.3 Lyman-α emitters
Observations of galaxies that emit strong Lyman-α lines (called Lyman-α emitters) also provide information about reionization. Neutral hydrogen easily absorbs Lyman-α photons, so detecting these galaxies at high redshifts indicates how transparent the IGM was.
6. Unanswered questions and current research
6.1 Contribution ratio of different sources
One of the key questions is the ratio of contributions from different ionizing sources. While it is clear that the earliest galaxies (due to massive stars formed within them) were important, how much Population III stars, normal star-forming galaxies, and quasars contributed to reionization remains a subject of debate.
6.2 Faint galaxies
Recent data suggest that a significant portion of ionizing photons could have been provided by faint, barely observable galaxies that are difficult to detect. Their role may have been crucial in completing reionization.
6.3 21 cm cosmology
Observations of the 21 cm hydrogen line open the possibility to directly study the epoch of reionization. Experiments such as LOFAR, MWA, HERA, and the upcoming Square Kilometre Array (SKA) aim to map the distribution of neutral hydrogen, showing how ionized bubbles changed during reionization [3].
7. The importance of reionization in modern cosmology
7.1 Galaxy formation and evolution
Reionization acted as matter could collapse into structures. When the IGM became ionized, the higher temperature hindered gas collapse into small halos. Therefore, to understand the hierarchical development of galaxies, it is necessary to assess the impact of reionization.
7.2 Feedback
Reionization is not one-way: gas ionization and heating inhibit later star formation. A hotter, ionized medium collapses less effectively, so photoionization feedback can suppress star formation in the smallest halos.
7.3 Testing astrophysical and particle physics models
By comparing reionization data with theoretical models, scientists can test:
- Properties of the first stars (Population III) and early galaxies.
- Role of dark matter and its small-scale structure.
- Accuracy of cosmological models (e.g., ΛCDM), possible corrections or alternative theories.
8. Conclusion
Reionization complements the history of the Universe – from a neutral, dark initial state to a light-filled, ionized intergalactic medium. This process was driven by the first stars and galaxies, whose ultraviolet light gradually ionized hydrogen throughout the cosmos (between z ≈ 10 and z ≈ 6). Observational data – from quasar spectra, Lyman-α lines, CMB polarization to the latest 21 cm line observations – increasingly accurately reconstruct this epoch.
However, many fundamental questions remain: Who were the main sources of reionization? What was the exact evolution and structure of the ionized regions? How did reionization affect subsequent galaxy formation? New and upcoming studies promise to provide deeper understanding, highlighting how astrophysics and cosmology intertwined to create one of the greatest early Universe transformations.
Links and further reading
- Gunn, J. E., & Peterson, B. A. (1965). “On the Density of Neutral Hydrogen in Intergalactic Space.” The Astrophysical Journal, 142, 1633–1641.
- Planck Collaboration. (2016). “Planck 2016 Intermediate Results. XLVII. Planck Constraints on Reionization History.” Astronomy & Astrophysics, 596, A108.
- Furlanetto, S. R., Oh, S. P., & Briggs, F. H. (2006). “Cosmology at Low Frequencies: The 21 cm Transition and the High-Redshift Universe.” Physics Reports, 433, 181–301.
- Barkana, R., & Loeb, A. (2001). “In the Beginning: The First Sources of Light and the Reionization of the Universe.” Physics Reports, 349, 125–238.
- Fan, X., Carilli, C. L., & Keating, B. (2006). “Observational Constraints on Cosmic Reionization.” Annual Review of Astronomy and Astrophysics, 44, 415–462.
Based on these important observations and theoretical models, reionization is seen as a unique event that ended the Dark Ages and opened the way to impressive cosmic structures visible in the night sky, while providing an invaluable opportunity to explore the early light moments of the Universe.