1. Introduction: Cosmic Ionization History
The reionization epoch marks a fundamental transition in the universe's evolution, characterized by the transformation of the intergalactic medium (IGM) from a neutral to an ionized state. This process occurred in two distinct phases: hydrogen reionization and helium reionization, each driven by different astrophysical phenomena and having profound implications for structure formation and observational cosmology.
2. Recombination and the Dark Ages
Following the Big Bang, the universe existed in a hot, dense, and ionized state. At redshift z ≈ 1100 (approximately 380,000 years after the Big Bang), as the universe expanded and cooled to T ≈ 3000 K, protons and electrons recombined to form neutral hydrogen. This process, described by the Saha equation, marked the beginning of the "Dark Ages." During this period, the universe was dominated by neutral hydrogen, making it opaque to ultraviolet radiation.
3. Hydrogen Reionization: The Cosmic Dawn
3.1 Population III Stars and Early Galaxies
The reionization of hydrogen began with the formation of Population III stars around z ≈ 20-30. These metal-free stars, with masses potentially reaching 100-1000 M☉ and effective temperatures of T_eff ≈ 10^5 K, produced a hard UV spectrum capable of ionizing hydrogen efficiently. The ionizing photon production rate of these early sources is crucial for understanding the progress of reionization.
3.2 Stromgren Spheres and Bubble Growth
The growth of ionized regions around these first sources can be conceptualized using the Stromgren sphere formalism. Initially, these ionized bubbles are confined by the balance between ionization and recombination rates. As sources become more numerous and bubbles begin to overlap, the reionization process accelerates, leading to a percolation phase where most of the IGM becomes ionized.
3.3 Inhomogeneous Reionization and the Cosmic Web
The reionization process was highly inhomogeneous, influenced by the underlying density field of the cosmic web. Overdense regions, which hosted the first sources, ionized first, while underdense voids remained neutral for longer periods. This inhomogeneity has significant implications for the observational signatures of reionization.
3.4 Completion of Hydrogen Reionization
Observations indicate that hydrogen reionization was largely complete by redshift z ≈ 6, approximately 1 billion years after the Big Bang. This conclusion is primarily based on the Gunn-Peterson trough observed in high-redshift quasar spectra. The absence of Gunn-Peterson absorption at z < 6 suggests that the IGM was highly ionized by this time, with neutral fractions below ~10^-4.
4. Helium Reionization: The Role of Quasars
While hydrogen and HeI were ionized by the first stars and galaxies, the complete ionization of HeII required more energetic photons, primarily provided by quasars. The HeII reionization epoch, occurring at lower redshifts (z ≈ 3-4), had significant impacts on the thermal history of the IGM.
4.1 Quasar Emission Spectra and HeII Ionization
Quasars, powered by accreting supermassive black holes, emit a power-law spectrum typically modeled with a spectral index α ≈ 1.5 - 2 for the extreme UV and soft X-ray regime relevant for HeII ionization. The HeII ionization cross-section peaks at 54.4 eV and decreases as ν^(-3) at higher energies, making quasars with harder spectra more efficient at ionizing HeII.
4.2 IGM Heating During Helium Reionization
The photoionization of HeII deposited significant energy into the IGM, raising its temperature to T ≈ 20,000 - 30,000 K. This heating had profound effects on the IGM's equation of state and the Lyman-α forest observations.
5. Advanced Reionization Physics
5.1 Non-equilibrium Ionization Kinetics
The detailed ionization state of the IGM during reionization involves non-equilibrium processes. The evolution of species fractions (HI, HII, HeI, HeII, HeIII) is governed by a set of coupled differential equations involving photoionization, collisional ionization, and recombination processes.
5.2 Radiative Transfer in Reionization
Accurate modeling of reionization requires solving the radiative transfer equation in complex 3D geometries. Various numerical methods have been developed, including ray-tracing algorithms, moment methods (e.g., M1 closure), and Monte Carlo techniques. These methods account for effects such as shadowing, spectral hardening, and source clustering.
5.3 21cm Cosmology
The 21cm hyperfine transition of neutral hydrogen provides a powerful probe of the reionization epoch. The spin temperature of this transition is influenced by coupling to the CMB temperature, collisions, and the Wouthuysen-Field effect. The resulting 21cm brightness temperature relative to the CMB serves as a sensitive measure of the neutral fraction during reionization.
6. Observational Probes of Reionization
- Gunn-Peterson Trough: The complete absorption of flux blueward of Lyman-α in quasar spectra at z > 6, indicating a significantly neutral IGM.
- Thomson scattering optical depth in CMB polarization
- Kinetic Sunyaev-Zel'dovich effect
- Lyman-α emitter surveys
Each of these probes provides complementary information about the timing, duration, and morphology of the reionization process.
7. Current Frontiers and Open Questions
- Relative contributions of various source populations (Pop III stars, early galaxies, mini-quasars) to reionization
- Impact of reionization on galaxy formation, particularly for low-mass halos
- Detailed topology of reionization and its connection to the underlying cosmic web
- Nature of early quasars and their role in HeII reionization
- Detection and characterization of the 21cm signal from the Epoch of Reionization
- Influence of magnetic fields generated during reionization on subsequent structure formation
8. Conclusion
The reionization of the universe represents a critical phase transition in cosmic history, intricately linked to the formation of the first stars, galaxies, and quasars. Understanding this epoch requires a synthesis of astrophysics, plasma physics, and cosmology. Ongoing observations, particularly from next-generation telescopes like the James Webb Space Telescope (JWST) and the upcoming Square Kilometre Array (SKA), promise to refine our understanding of this pivotal era, potentially revolutionizing our view of the early universe and the processes that shaped the cosmos we observe today.