Cosmic Microwave Background
The Cosmic Microwave Background (CMB) is electromagnetic radiation that permeates the entire universe, observable as microwave radiation coming from all directions of space. It is a relic of the early universe, emitted approximately 380,000 years after the Big Bang during a period known as recombination, when the universe had cooled enough for electrons and protons to form neutral hydrogen atoms. Before this point, the universe was opaque to radiation; the CMB represents the oldest light we can observe.
Origin and Properties
After recombination, the universe became transparent, allowing radiation to travel freely. This ancient light has been expanding and cooling with the universe ever since. Today, the CMB has cooled to approximately 2.7 Kelvin and appears as microwave radiation with a nearly uniform temperature across all directions—a key prediction of the Big Bang model. Small temperature variations in the CMB, measured in millionths of a degree, reflect tiny density fluctuations in the early universe that eventually grew into galaxies and large-scale structures.
Observational Significance
The CMB was discovered accidentally in 1964 by Arno Penzias and Robert Wilson and has become one of the most important tools in cosmology. Precise measurements from space-based observatories, particularly the WMAP and Planck satellites, have mapped the CMB’s temperature variations with extraordinary detail. These observations have enabled scientists to determine the universe’s age, composition, geometry, and expansion rate, making the CMB central to modern understanding of cosmology.