Why Is the CMB 2.7 Kelvin? The Cosmic Microwave Background as a Window to the Early Universe
The cosmic microwave background (CMB) glows at a temperature of 2.7255 K, a relic of the hot, dense early universe. This article explains why the CMB has this temperature, how it encodes the history of cosmic expansion, and what it reveals about the key epochs from the Planck era to structure formation.