Reionization

A long time ago, the sky was dark.

The space between stars was filled with gas. This gas was not clear. It was hard for light to move through it.

Then, the first stars and galaxies formed. These new objects gave off bright light. This light hit the gas in space. It changed the gas.

The light made the gas clear again. This process is called reionization. It happened a very long time ago.

Now, the gas is clear. This lets light travel far through space. We can see the stars today because of this change.

A long time ago, the universe was dark.

Most matter was made of neutral hydrogen. This gas filled the space between stars. It was not clear. Light could not move through it easily. This time was called the Dark Ages.

Then, the first stars and galaxies began to form. These objects gave off bright light. This light was full of power. The light hit the hydrogen gas. It changed the gas back into ions. An ion is a part of an atom with an electric charge. This change is called reionization.

Reionization happened between 150 million and one billion years after the Big Bang. It did not happen all at once. Each star made a bubble of clear gas around it. These bubbles grew and met each other. Soon, the whole universe became clear.

Scientists study this using quasars. Quasars are very bright objects. They help us see how the gas changed. We also look at the cosmic microwave background. This is the oldest light in the sky. It helps us find the age of reionization.

Reionization was a huge change in the history of our universe.

Long ago, the universe was filled with neutral hydrogen gas. This gas was made of protons and electrons joined together. Because of this, the universe was not clear. Light could not move through the gas easily. This period was known as the Dark Ages. Eventually, the first stars and galaxies began to form. These new objects released very energetic light. This light changed the gas back into ions. This process is called reionization.

How does this change actually work? It happens in a step-by-step way. First, a new star forms and gives off light. This light hits the neutral hydrogen gas right next to the star. The light has enough energy to pull the electrons away. This creates a bubble of ionized gas around the star. This hot gas then expands and clears out the area. Over hundreds of millions of years, these bubbles grow larger. Eventually, the bubbles from different galaxies begin to overlap.

Soon, the entire space between galaxies becomes ionized and clear.

Scientists have worked hard to find when this happened. They use many different tools to look back in time. One way is by studying very bright objects called quasars. Quasars release an incredible amount of energy. When their light travels through neutral gas, it creates a dark gap. This gap is called a Gunn-Peterson trough. By looking at these gaps, scientists can tell if the gas was neutral. In 2001, the Sloan Digital Sky Survey found four quasars. These helped show that reionization was ending around a specific time.

There are many important numbers that help us understand this era. Reionization took place between 150 million and one billion years after the Big Bang. This time is often linked to a redshift between 6 and 20. Scientists use redshift to measure how far back in time they are looking. The Wilkinson Microwave Anisotropy Probe helped study the oldest light in the sky. This light is called the cosmic microwave background. In 2018, the Planck mission gave even more details. It suggested the process reached a middle point at a redshift of 7.68.

You can think of reionization like a fog clearing away. At first, the universe was like a very thick, dark mist. The neutral hydrogen gas acted like a heavy blanket of fog. Then, the first stars acted like bright flashlights. Their light burned through the mist bit by bit. As the light traveled, the fog turned into clear air. Today, the universe is mostly transparent just like clear air. We can see distant galaxies because the fog is gone. This allows us to study the stars and the history of space.

Reionization is a transformative era in cosmic history. It describes the process where the intergalactic medium changed from neutral hydrogen to ionized plasma.

In the very early universe, the matter was already ionized. However, as the universe expanded and cooled, protons and electrons combined. This process is called recombination. Around 379,000 years after the Big Bang, recombination left most matter as neutral hydrogen. This change made the universe opaque because photons scattered off free electrons. Once atoms formed, the universe became transparent to most light, entering the "Dark Ages." Reionization is the second major phase change that returned the universe to an ionized state.

The mechanism of reionization was driven by the first stars and galaxies. These early objects emitted energetic photons, which are particles of light. When these photons hit neutral hydrogen gas, they provided enough energy to strip electrons away from protons. This process is called ionization.

The process happens in specific stages. First, a new star forms and ionizes the gas immediately surrounding it. This creates a hot, expanding bubble of ionized gas. Over hundreds of millions of years, these ionized bubbles grow larger. Eventually, the shells of ionization from different galaxies begin to overlap. This pushes the ionization frontier out into the intergalactic medium until the entire space is ionized.

Scientists use several distinct methods to detect this era. One method involves studying the spectra of distant quasars. Quasars are among the brightest objects in the universe and release extraordinary amounts of energy. When light from a quasar travels through neutral hydrogen, it undergoes scattering. At specific wavelengths, such as the Lyman-alpha transition, the absorption is very likely. Because the universe is expanding, this light undergoes redshifting. This stretching of light creates a feature called the Gunn-Peterson trough. This dark gap in the spectrum indicates the presence of neutral hydrogen.

Another method uses the Cosmic Microwave Background, or CMB. The CMB is the oldest light in the universe. When photons from the CMB encounter free electrons, they undergo Thomson scattering. This scattering leaves a mark on the CMB anisotropy map. By comparing observed maps to theoretical models, scientists can calculate the electron density. This helps determine the age of the universe during reionization. The Wilkinson Microwave Anisotropy Probe (WMAP) provided critical data on this. While early results varied, later WMAP data suggested reionization began at a redshift of 11 and ended by a redshift of 7.

Specific numbers help define the timeline of this cosmic event. Reionization occurred between 150 million and one billion years after the Big Bang. In terms of redshift, this period is generally placed between z = 6 and z = 20. The Planck mission in 2018 provided a highly specific estimate. It yielded an instantaneous reionization redshift of 7.68, with a margin of error of 0.79. This number, known as zre, provides an estimate of the mean redshift when the process occurred. These measurements allow cosmologists to pin down exactly when the universe transitioned from dark to clear.

Lyman-alpha emission from galaxies offers another way to study this period. The Lyman-alpha line is a specific transition of neutral hydrogen. Young stars in early galaxies produce this light copiously. However, neutral hydrogen gas in the intergalactic medium can absorb this light through resonant scattering. This process obscures the light and makes it difficult to see galaxies embedded in neutral gas. If scientists detect many galaxies via Lyman-alpha light, it suggests the gas is ionized. If they see very few, it may indicate the presence of neutral regions. This method helped show that the gas was mostly ionized by a redshift of 6.5.

Finally, researchers look toward the 21-cm line for deeper understanding. This signal occurs in neutral hydrogen due to energy differences in the electron and proton states. The transition is very rare, which makes it a difficult signal to detect. However, it is highly sensitive to temperature. As objects form during the Dark Ages, they produce Lyman-alpha photons. These photons interact with neutral hydrogen through Wouthuysen-Field coupling. This creates a 21-cm signal that could reveal the very first structures in the universe. Studying this line may eventually solve remaining questions about the energy sources that drove reionization.