Cosmic Microwave Background
The Cosmic Microwave Background (CMB) is electromagnetic radiation that fills the entire observable universe. It is detected as microwave radiation arriving from all directions of space with nearly uniform intensity. The CMB is a relic of the early universe, emitted approximately 380,000 years after the Big Bang during the recombination epoch, when the universe had cooled sufficiently for electrons and protons to combine into neutral hydrogen.
Origin and Detection
Before recombination, the early universe was opaque to light, filled with a dense plasma of charged particles. As the universe expanded and cooled, electrons bound to protons to form neutral atoms, allowing photons to travel freely for the first time. This released radiation is observed today as the CMB, redshifted from its original ultraviolet wavelengths into the microwave region of the electromagnetic spectrum. The radiation was first detected in 1964 by Arno Penzias and Robert Wilson and has since been precisely measured by space-based observatories including COBE, WMAP, and Planck.
Scientific Significance
The CMB provides direct observational evidence for the Big Bang theory and serves as a snapshot of the universe’s earliest observable state. Slight variations in temperature across the sky, measured at levels of one part in 100,000, correspond to density fluctuations in the primordial universe that seeded the formation of galaxies and large-scale structures. By analyzing the CMB’s properties, including its temperature, polarization, and power spectrum, cosmologists have determined fundamental parameters of the universe such as its age, composition, and geometry.