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The Cosmic Microwave Background (CMB) is essentially the afterglow of the Big Bang. It is radiation that fills the universe, seen in every direction, and is a critical piece of evidence supporting the Big Bang theory.

After the Big Bang, the universe was extremely hot and dense. As it expanded, it also cooled, leading to the formation of atoms. When this happened, photons, or particles of light, were able freely to travel through space, which is what we now detect as the CMB.

One of the most fascinating features of the CMB is its near-uniformity. The temperature across the CMB is incredibly constant at approximately 2.725 Kelvin, with only slight fluctuations. These small changes helped to form the galaxies and clusters we see today.

• The uniformity is explained by cosmic inflation, which was a rapid exponential expansion of the universe post-Big Bang. It stretched out any irregularities, creating a smooth, homogeneous background.
• Without inflation, the universe's different regions could have very different temperatures, which is not what we observe.

Big Bang Nucleosynthesis (BBN) refers to the process that created the universe's light elements during the first few minutes after the Big Bang.

During this period, temperatures were hot enough for nuclear reactions, leading to the formation of distinct elements. The most common products were hydrogen, helium, and small amounts of other light elements like lithium.

These elements form the primordial chemical composition of the universe. BBN is critical in explaining the abundance of helium relative to hydrogen throughout the cosmos.

• Helium makes up about 24% of the elemental mass in the universe. This ratio aligns with predictions made by Big Bang Nucleosynthesis models.
• BBN ceased as the universe expanded and cooled, making nuclear reactions less likely to occur, which stopped significant production of heavier elements until stars formed later.

The expansion of the universe is a process that began with the Big Bang and continues today.

Initially, this expansion was extremely rapid due to a phase known as cosmic inflation, which solved several cosmological puzzles. Even though the rate of expansion slowed after inflation, the universe has been continuously expanding ever since.

• Edwin Hubble's observations in the 1920s revealed redshift in the light from distant galaxies, indicating they are moving away from us. This was the first evidence supporting that the universe is expanding.
• Expansion affects the universe's structure, allowing for the formation of galaxies, stars, and planetary systems as mass clumps together through gravity.
• The expansion also affects cosmic time, as distances between objects in the universe increase over time, impacting phenomena like the redshift of light.

Today, we even understand that the expansion rate is accelerating, tracked by observing distant supernovae and the CMB.