Cosmic Reionization

Overview

Cosmic reionization refers to the epoch in the early universe during which the predominantly neutral intergalactic medium (IGM) was ionized by the first luminous sources. This period marks one of the most significant phase transitions in the history of the universe, bridging the gap between the so-called "Dark Ages" and the universe we observe today. The study of cosmic reionization provides critical insights into the formation and evolution of the first galaxies, quasars, and other early cosmic structures.

Timeline of Cosmic Reionization

Dark Ages

Following the Big Bang, the universe underwent a period of rapid expansion known as cosmic inflation. After this, the universe cooled sufficiently for protons and electrons to combine and form neutral hydrogen during the epoch of recombination. This led to the Cosmic Microwave Background (CMB) radiation, which we can still observe today. The universe then entered the "Dark Ages," a period characterized by the absence of light-emitting sources.

Emergence of the First Luminous Sources

The end of the Dark Ages was marked by the formation of the first stars, galaxies, and quasars. These early luminous sources began to emit ultraviolet (UV) photons, which started to ionize the surrounding neutral hydrogen. This process initiated the epoch of reionization.

Epoch of Reionization

The epoch of reionization is believed to have occurred between redshifts z ~ 6 to z ~ 20, corresponding to a time period roughly between 150 million to 1 billion years after the Big Bang. During this epoch, the IGM transitioned from being mostly neutral to being almost fully ionized. This process was not uniform and occurred in patches, with ionized bubbles expanding around early galaxies and eventually overlapping.

Mechanisms of Reionization

Sources of Ionizing Photons

The primary sources of ionizing photons during reionization are thought to be:

**First Stars (Population III stars)**: These were massive, metal-poor stars that formed from primordial gas. Their high temperatures made them efficient producers of ionizing UV photons.
**Early Galaxies**: These galaxies contained young, hot stars that contributed significantly to the ionizing photon budget.
**Quasars**: Powered by supermassive black holes, quasars emitted large quantities of ionizing radiation, although their exact contribution to reionization is still debated.

Ionization Fronts

The ionizing photons from these sources created ionization fronts that propagated through the IGM. These fronts ionized hydrogen atoms, creating expanding bubbles of ionized gas. As more sources formed, these bubbles grew and eventually overlapped, leading to the complete reionization of the universe.

Observational Evidence

Cosmic Microwave Background (CMB)

The CMB provides indirect evidence for reionization. Measurements of the CMB polarization by missions such as the Wilkinson Microwave Anisotropy Probe (WMAP) and the Planck satellite have provided constraints on the timing and duration of reionization. The CMB polarization is affected by the scattering of CMB photons off free electrons, which were produced during reionization.

Quasar Absorption Spectra

The spectra of distant quasars show absorption features known as the Gunn-Peterson trough, which indicate the presence of neutral hydrogen in the IGM. The absence of the Gunn-Peterson trough in quasars at redshifts lower than z ~ 6 suggests that the universe was fully reionized by this time.

Lyman-Alpha Emitters

Lyman-alpha emitters are galaxies that emit strongly in the Lyman-alpha line of hydrogen. The visibility of these galaxies is affected by the neutral hydrogen in the IGM. Observations of Lyman-alpha emitters at different redshifts provide information about the ionization state of the IGM during reionization.

Theoretical Models

Semi-Analytic Models

Semi-analytic models use simplified physical assumptions to simulate the process of reionization. These models incorporate the formation and evolution of early galaxies, the production of ionizing photons, and the propagation of ionization fronts.

Numerical Simulations

More detailed insights into reionization are obtained from numerical simulations. These simulations solve the equations of hydrodynamics and radiative transfer to model the complex interactions between ionizing sources and the IGM. Examples include the Illustris simulation and the EAGLE project.

Radiative Transfer Codes

Radiative transfer codes are specialized tools used to model the propagation of ionizing radiation through the IGM. These codes are essential for understanding the detailed structure of ionization fronts and the spatial distribution of ionized regions.

Challenges and Uncertainties

Source Identification

One of the major challenges in studying reionization is identifying the primary sources of ionizing photons. While Population III stars, early galaxies, and quasars are all candidates, their relative contributions are still uncertain.

Escape Fraction

The escape fraction of ionizing photons from galaxies is another critical parameter. It determines how many of the photons produced by stars actually escape into the IGM to contribute to reionization. This fraction is difficult to measure and is subject to significant uncertainties.

Inhomogeneities

Reionization was a highly inhomogeneous process, with ionized bubbles forming around individual sources and eventually overlapping. Understanding these inhomogeneities requires high-resolution simulations and detailed observations.

Future Prospects

Upcoming Observations

Several upcoming observatories and missions are expected to provide new insights into reionization. These include the James Webb Space Telescope (JWST), which will observe the first galaxies and stars, and the Square Kilometre Array (SKA), which will map the distribution of neutral hydrogen during reionization.

21 cm Line Observations

The 21 cm line of neutral hydrogen is a powerful probe of the IGM during reionization. Observatories such as the Hydrogen Epoch of Reionization Array (HERA) and the Low-Frequency Array (LOFAR) are designed to detect the 21 cm signal from the early universe, providing a direct view of the reionization process.

References