The Cosmic Microwave Background and \(H_0\)

The Cosmic Microwave Background (CMB) is a fundamental component of modern cosmology, providing a wealth of information about the early universe. It is a faint radiation that permeates the entire cosmos, and it is essentially the ‘afterglow’ of the Big Bang. The CMB is composed of photons that were released approximately 380, 000 years after the Big Bang when the universe had cooled sufficiently for atoms to form (at redshift \(z \sim 1100\) ). Before that time, the universe was a hot, dense plasma where photons were constantly scattered by charged particles in a process known as Thomson scattering. The CMB was first detected in 1965 by Arno Penzias and Robert Wilson at the Bell Telephone Laboratories [45]. They were conducting experiments using a large horn antenna designed for satellite communication. Penzias and Wilson noticed a persistent background noise in their measurements, which they initially attributed to various sources of interference. However, they encountered difficulties in eliminating the noise, and after careful consideration, they realized that they were detecting a uniform radiation coming from all directions in the sky. They had inadvertently stumbled upon the CMB.