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 ====== 1.4. Cosmic Microwave Background ======
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 In the earliest moments of the universe, photons were trapped in a hot, dense fog of free electrons that scattered light continuously, rendering the cosmos opaque. However, approximately 400,000 years after the Big Bang, the expanding universe cooled to roughly 3000 K, a threshold low enough for electrons to combine with protons and form neutral hydrogen atoms. This pivotal event, known as **recombination**, lifted the fog and allowed photons to decouple from matter, finally streaming freely through space primarily as visible and infrared light. Over the subsequent 14 billion years, the relentless expansion of the universe has stretched the fabric of space itself, elongating the wavelengths of these ancient photons by a factor of roughly 1,100 through cosmological redshift. Consequently, this primordial radiation has cooled and shifted from energetic light into the low-energy microwave band, permeating the cosmos today as the Cosmic Microwave Background (**CMB**) radiation with a temperature of approximately 2.73 K, often rounded to 3 K.