. What does it mean to say that the universe was once radiationdominated? What happened when the universe changed from being radiation-dominated to being matter-dominated? When did this happen?

When we peer into the vastness of space, we find a relic from the early universe: the Cosmic Microwave Background (CMB). This faint glow is not just widespread but also uniform, acting as a snapshot of the universe at a time when it was just 380,000 years old.

The CMB is critical evidence for the Big Bang theory. It was predicted by theorists and then discovered by accident in the 1960s. Researchers noted that the CMB holds a nearly uniform temperature of about 2.7 Kelvin, but it also has tiny fluctuations. These small variations in temperature and density were fundamental to the formation of the large-scale structures we see in the universe today, like galaxies and galaxy clusters.

Understanding the CMB provides us with critical insights into the conditions of the early universe and the forces at play.

Following the Big Bang, the universe underwent a phase where radiation was the dominant force, dictating the dynamics and evolution of the cosmos. However, as the universe expanded and cooled, it reached a pivotal moment where the balance of power shifted—this occurred at approximately 47,000 years after the Big Bang.

During the matter domination era, matter overtook radiation in density and influence on the gravitational forces shaping the universe. This shift had profound effects on the universe’s structure and future evolution, as it facilitated the formation of complex structures, such as atoms, stars, and eventually galaxies, marking the beginning of a new cosmic chapter.

The Big Bang is the most widely accepted explanation for the origin of our universe. About 13.8 billion years ago, the universe began as an infinitely dense point, or singularity, and has been expanding ever since.

Initially, the universe was a hot, dense plasma of particles and radiation. It quickly cooled as it expanded, leading to the formation of the first subatomic particles and, later, simple atoms. The evidence for the Big Bang includes the CMB, the abundance of light elements such as hydrogen and helium, and the redshift of distant galaxies, which indicates that the universe is still expanding.

In the universe's infancy, temperatures were so high that atoms could not form; nuclei and electrons existed as a plasma. As the universe cooled during the radiation-dominated era, conditions finally became right for nucleosynthesis—the formation of the first light atomic nuclei, like hydrogen and helium.

Later, as the universe descended into the matter-dominated era, it cooled sufficiently for electrons to combine with these nuclei to form neutral atoms. This process is known as recombination and marks a turning point, allowing photons to travel freely, eventually becoming the CMB we can still observe today. The formation of neutral atoms was essential for the universe to become transparent and for the later formation of stars and galaxies.

The evolution of the universe is a grand tale that begins with the Big Bang and continues through to the present day—and beyond. From the initial explosion, the universe has passed through various epochs. It moved from being hot and opaque, to the radiation-dominated era, and then to the matter-dominated era when structure formation truly began.

As gravity clumped matter together, stars ignited within vast clouds of gas, and galaxies began to form. These galaxies would go on to cluster together under gravity's inexorable pull. Today, dark energy, a mysterious force causing the accelerated expansion of the universe, plays a significant role in the universe's evolution. Researchers continue to study the cosmos, piecing together its history and unveiling the secrets of its future.