How ultraviolet light from the first stars and galaxies re-ionized hydrogen, making the Universe transparent
In cosmic history regionalization marks Dark Ages The end of the 20th century – the period after recombination, when the Universe was filled with neutral hydrogen atoms and there were no bright sources (stars, galaxies) yet. When the first stars, galaxies and quasars began to shine, their high-energy (mostly ultraviolet) photons ionized the surrounding cloud of hydrogen gas, 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 our familiar, light-filled Universe.
In this article we will discuss:
- Neutral Universe after recombination
- First Light: Population III Stars, Early Galaxies, and Quasars
- Ionization process and bubble formation
- Time course and observational evidence
- Unanswered questions and current research
- The importance of reionization in modern cosmology
2. Neutral Universe after recombination
2.1 The Dark Ages
From about 380,000 years after the Big Bang (when it happened recombination) until the first light sources were formed (after about 100–200 million years) The universe was largely neutral, composed of hydrogen and helium, leftover from the nucleosynthesis of the Big Bang. This period is called In the Dark Ages, because there were no significant new sources of light other than stars or galaxies, except for 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 (HI), which is a good absorber of ultraviolet photons. As matter began to accumulate in dark matter halos and ancient gas clouds collapsed, the first galaxies formed. Population III starsTheir abundant radiation fluxes subsequently fundamentally changed the state of the IGM.
3. First Light: Population III Stars, Early Galaxies, and Quasars
3.1 Population III stars
Theoretically, the first stars are predicted to be Population III stars – were metal-free (made almost entirely of hydrogen and helium) and were probably very massive, perhaps tens or hundreds of solar masses. They marked the end of the Dark Ages, often called the Cosmic dawnThese stars emitted abundant ultraviolet (UV) radiation that can ionize hydrogen.
3.2 Early galaxies
As the structures formed hierarchically, small halos of dark matter merged to form larger ones, from which the first galaxies. Population II stars formed in them, further increasing the UV photon flux. Over time, these galaxies—not just 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 of helium (He II). Although their effect on hydrogen reionization is still debated, it is believed that quasars became especially important at somewhat later times, for example, in reionizing helium at z ~ 3.
4. Ionization process and bubbles
4.1 Local ionization bubbles
As each new star or galaxy began to emit high-energy photons, these photons spread outward, ionizing surrounding hydrogen.This created isolated "bubbles" (or H II areas) of ionized hydrogen around the sources. Initially, these bubbles were solitary and quite small.
4.2 Interaction between bubbles
As the number of new sources and their luminosity grew, these ionized bubbles expanded and mergedThe once neutral IGM became a patchwork of primarily neutral and ionized media. As the reionization epoch neared its end, the H II regions coalesced, leaving most of the Universe's hydrogen ionized (H II) rather than neutral (HI).
4.3 Reionization time scale
The reionization is thought to have lasted for several hundred million years, spanning redshifts from about z ~ 10 until z ~ 6Although the exact dates remain a subject of research, at z ≈ 5–6 most of the IGM was already ionized.
5. Time course and observational evidence
5.1 Gunn–Peterson effect
An important indicator of regionalization is the so-called Gunn–Peterson test, studying the spectra of distant quasars. Neutral hydrogen in the IGM absorbs photons well at certain wavelengths (especially Lyman-α line), which causes an absorption band to appear in the quasar's spectrum. Observations show that at z > 6 this Gunn–Peterson effect becomes strong, indicating a much larger fraction of neutral hydrogen and emphasizing the end of reionization [1].
5.2 Cosmic Microwave Background (CMB) and Polarization
KMF measurements also provide clues. Free electrons from the reionized medium scatter the KMF photons, leaving behind large angular scale polarizations trace. Data from WMAP and Planck constrains the average time and duration of reionization [2]. By measuring the optical thickness τ (the scattering probability), cosmologists can determine when most of the hydrogen in the Universe became ionized.
5.3 Lyman-α emitters
Galaxies that emit strong Lyman-α Observations of the Lyman-α line (called Lyman-α emitters) also provide information about reionization. Neutral hydrogen readily absorbs Lyman-α photons, so the detection of these galaxies at high redshifts indicates how transparent the IGM was.
6. Unanswered questions and current research
6.1 Ratio of contributions from different sources
One of the key questions is – different ionizing sources contribution ratio. While it is clear that the earliest galaxies (due to the massive stars they formed) were important, how much did they contribute to reionization Population III stars, normal star-bearing galaxies and quasars – remains a subject of debate.
6.2 Faint galaxies
Recent data suggest that a significant proportion of ionizing photons may have been emitted by faint, poorly observed galaxies, which are difficult to detect. Their role may have been crucial in ending the reionization.
6.3 21 cm cosmology
Observations 21 cm hydrogen lines opens up the possibility of directly studying the reionization epoch. Experiments such as LOFAR, MWA, HERA and the future Square Kilometre Array (SKA) aims to map the distribution of neutral hydrogen, showing how ionized bubbles are during the next reionization [3].
7. The importance of reionization in modern cosmology
7.1 Formation and evolution of galaxies
Reionization affected how matter could contract into structures. As the IGM became ionized, higher temperatures made it more difficult for gas to collapse into tiny haloes. Therefore, to understand hierarchical development of galaxies, it is necessary to assess the impact of reionization.
7.2 Feedback
Reionization is not unidirectional: ionization and heating of the gas inhibits subsequent star formation. Hotter, ionized medium collapses more slowly, so photoionization feedback can suppress star formation in the smallest haloes.
7.3 Testing Astrophysics and Particle Physics Models
By comparing reionization data with theoretical models, scientists can verify:
- First stars (population III) and properties of early galaxies.
- Dark matter role and its fine-scale structure.
- Cosmological models (e.g. ΛCDM) accuracy, possible corrections or alternative theories.
8. Conclusion
Reionization completes 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, and their ultraviolet light gradually ionized hydrogen throughout the cosmos (among z ≈ 10 and z ≈ 6). Observation data – from quasar spectra, Lyman-α lines, KMF polarizations up to date 21 cm line observations – recreates this era more and more accurately.
However, many fundamental questions remain: What were the main sources of regionalization? What was the exact evolution and structure of the ionized regions? How did reionization affect the subsequent formation of galaxies? New and upcoming research promises to provide deeper understanding, highlighting how astrophysics and cosmology intertwined to create one of the greatest transformations of the early Universe.
References and further reading
- Gunn, JE, & Peterson, BA (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, SR, Oh, SP, & Briggs, FH (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, CL, & 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, we see reionization as a unique event, ended the Dark Ages and paved the way for the spectacular cosmic structures visible in the night sky, while providing an invaluable opportunity to study the early moments of light in the Universe.