How ultraviolet light from the first stars and galaxies reionized hydrogen, making the universe transparent again
In the timeline of cosmic history, reionization marks the end of the so-called Dark Ages, a period after recombination when the universe was filled with neutral hydrogen atoms and no luminous sources had yet formed. As the first stars, galaxies, and quasars began to shine, their high-energy (mostly ultraviolet) photons ionized the surrounding hydrogen gas, transforming the neutral intergalactic medium (IGM) into a highly ionized plasma. This event, known as cosmic reionization, profoundly changed the transparency of the universe on large scales and set the stage for the fully illuminated cosmos we observe today.
In this article, we will explore:
- The Neutral Universe After Recombination
- First Light: Population III Stars, Early Galaxies, and Quasars
- The Ionization Process and Bubbles
- Timeline and Observational Evidence
- Open Questions and Ongoing Research
- Importance of Reionization in Modern Cosmology
2. The Neutral Universe After Recombination
2.1 The Dark Ages
From approximately 380,000 years post-Big Bang (the time of recombination) until the formation of the first luminous structures (roughly 100–200 million years later), the universe was mostly neutral, composed of hydrogen and helium left over from Big Bang nucleosynthesis. This period is referred to as the Dark Ages because, without stars or galaxies, the universe contained no significant new light sources besides the cooling cosmic microwave background (CMB).
2.2 Neutral Hydrogen Dominance
During the Dark Ages, the intergalactic medium (IGM) was almost entirely neutral hydrogen (H I)—crucial because neutral hydrogen is highly effective at absorbing ultraviolet photons. Eventually, as matter clumped into dark matter halos and primordial gas clouds collapsed, the first Population III stars began to form. Their intense radiation would soon change the state of the IGM forever.
3. First Light: Population III Stars, Early Galaxies, and Quasars
3.1 Population III Stars
Theory predicts that the first stars—Population III stars—were metal-free (composed almost exclusively of hydrogen and helium) and likely very massive, possibly ranging from tens to hundreds of solar masses. Their formation heralded the transition from the Dark Ages to the Cosmic Dawn. These stars emitted copious ultraviolet (UV) radiation capable of ionizing hydrogen.
3.2 Early Galaxies
As structure formation proceeded hierarchically, small dark matter halos merged to form larger halos, giving rise to the first galaxies. Within these galaxies, second-generation and later stars (Pop II) began to form, steadily increasing the UV photon output. Over time, galaxies—rather than Pop III stars alone—became the dominant source of ionizing radiation.
3.3 Quasars and AGN
High-redshift quasars (powered by supermassive black holes at the centers of early galaxies) also contributed to reionization, especially for helium (He II). Although their precise role in hydrogen reionization is still debated, quasars likely played a more substantial role at slightly later epochs, especially in reionizing helium at redshifts z ~ 3.
4. The Ionization Process and Bubbles
4.1 Local Ionization Bubbles
As each new star or galaxy emitted high-energy photons, these photons traveled outwards, ionizing the surrounding hydrogen. This created “bubbles” (or H II regions) of ionized hydrogen around the sources. At first, these regions were isolated and fairly small.
4.2 Overlapping Ionized Regions
Over time, more sources formed, and existing sources became more luminous. The ionized bubbles expanded, eventually overlapping one another. The once-neutral IGM became a patchwork of neutral and ionized regions. By the end of the reionization era, these H II regions coalesced, leaving the vast majority of the universe’s hydrogen in an ionized state (H II) rather than neutral (H I).
4.3 Timescale of Reionization
The duration of reionization was probably several hundred million years, roughly spanning redshifts from z ~ 10 to z ~ 6, although the exact timing remains an active area of research. By z ≈ 5–6, much of the IGM was ionized.
5. Timeline and Observational Evidence
5.1 The Gunn-Peterson Trough
A key piece of evidence for reionization comes from the Gunn-Peterson test, which examines the spectra of high-redshift quasars. Neutral hydrogen in the IGM absorbs photons at specific wavelengths (notably the Lyman-α line), leaving an absorption trough in the quasar spectrum. Observations show a significant increase in the Gunn-Peterson trough at z > 6, implying that the fraction of neutral hydrogen rises dramatically, indicating the tail end of reionization [1].
5.2 Cosmic Microwave Background (CMB) Polarization
CMB measurements also offer clues. Free electrons from reionized gas scatter CMB photons, leaving a signature in the form of large-scale polarization anisotropies. Data from WMAP and Planck have placed constraints on the average redshift and duration of reionization [2]. By measuring the optical depth τ (the probability of scattering), cosmologists can infer when most of the universe’s hydrogen became ionized.
5.3 Lyman-α Emitters
Surveys of Lyman-α emitting galaxies (galaxies whose spectra show strong emission in the Lyman-α line) are also used to probe reionization. Neutral hydrogen readily absorbs Lyman-α photons, so detecting these galaxies at high redshifts can tell us how transparent the IGM was.
6. Open Questions and Ongoing Research
6.1 The Relative Contribution of Sources
A major question is the relative contribution of different ionizing sources. While it’s clear that the earliest galaxies (with their numerous massive stars) were significant contributors, the exact fraction from Population III stars, normal star-forming galaxies, and quasars is still debated.
6.2 Low-Luminosity Galaxies
Recent evidence suggests that faint, low-luminosity galaxies— which are hard to detect—may provide a large fraction of the ionizing photons. Their role could be crucial in completing the final stages of reionization.
6.3 21-cm Cosmology
Observations of the 21-cm line from neutral hydrogen offer a unique, direct probe of the reionization epoch. Experiments like LOFAR, MWA, and HERA, and eventually the Square Kilometre Array (SKA), aim to map the spatial distribution of neutral hydrogen, revealing the topology (shape and size) of ionized bubbles as reionization progressed [3].
7. Importance of Reionization in Modern Cosmology
7.1 Galaxy Formation and Evolution
Reionization influenced how matter collapsed into structures. As the IGM became ionized, the increased heating inhibited gas collapse into small halos, affecting the formation of low-mass galaxies. Understanding reionization therefore helps clarify the hierarchical growth of galaxies.
7.2 Feedback Effects
The process of reionization was not one-way: heating and ionizing the IGM also fed back on subsequent star formation. Ionized gas is hotter and less able to collapse, leading to photoionization feedback that can suppress star formation in smaller halos.
7.3 Testing Astrophysical and Particle Physics Models
By comparing reionization data with theoretical predictions, researchers test:
- The properties of the first stars (Pop III) and early galaxies.
- The role and properties of dark matter (small-scale structure).
- The validity of cosmological models, including ΛCDM, modifications, or alternative theories.
8. Conclusion
Reionization completes the narrative arc from a neutral, dark early universe to one filled with luminous structures and transparent ionized gas. Triggered by the first stars and galaxies, ultraviolet light gradually ionized hydrogen throughout the cosmos between z ≈ 10 and z ≈ 6. Observational studies—spanning quasar spectra, Lyman-α emission, CMB polarization, and emerging 21-cm measurements —collectively provide an increasingly detailed picture of this epoch.
Still, critical questions remain: Which sources contributed most heavily to reionization? What was the exact timeline and topology of ionized regions? How did reionization feedback affect subsequent galaxy formation? Ongoing and future surveys promise to refine our understanding, potentially revealing the interplay of astrophysics and cosmology that orchestrated one of the most dramatic transformations of the early universe.
References & 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.
Through these pivotal observations and theoretical frameworks, we now view reionization as the defining event that ended the Dark Ages, paving the way for the brilliant cosmic structures filling the night sky—and offering a vital window into the universe’s earliest luminous moments.