1. The Big Bang: The Big Bang theory states that the universe began in a very hot, dense state and has been expanding and cooling ever since. This initial hot state filled the early universe with a uniform sea of radiation.
2. Expansion of the Universe: As the universe expanded, this radiation stretched and cooled, eventually reaching a temperature of about 2.7 Kelvin, which is in the microwave part of the electromagnetic spectrum.
3. The Last Scattering Surface: About 380,000 years after the Big Bang, the universe became cool enough for protons and electrons to combine into neutral atoms. This process, known as recombination, allowed photons to travel freely through space, forming the CMB we observe today.
4. Uniformity of Early Universe: The early universe was extremely uniform, meaning the initial radiation was distributed evenly in all directions.
5. Our Position in the Universe: While the CMB is slightly hotter in some directions than others due to the Doppler effect from our motion through the universe, the overall distribution is remarkably uniform across the entire sky. This is because we are located in a relatively homogeneous region of the universe.
6. Cosmic Horizon: We can only observe radiation that has had enough time to reach us since the Big Bang. The distance light has been able to travel in the age of the universe defines our "cosmic horizon."
Therefore, the CMB is visible in all directions because it represents the afterglow of the Big Bang, which was incredibly hot and uniform, and because the expansion of the universe has cooled and stretched this radiation to microwave wavelengths, making it detectable today.
