In the early universe, photons were trapped in a hot, dense fog of free electrons that continuously scattered light, rendering the cosmos opaque. However, roughly 400,000 years after the Big Bang, the expanding universe cooled to approximately 3000 K, a temperature low enough for electrons to combine with protons to form neutral hydrogen atoms. This event, known as recombination, cleared the fog and allowed photons to decouple from matter, streaming freely through space primarily as visible and infrared light. Over the last 14 billion years, the 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, pervading the cosmos today as the Cosmic Microwave Background (CMB) rdiation with a temperature of approximately 2.73 K, often quoted as just 3 K.

The definitive confirmation of the Big Bang theory arrived accidentally in 1964 when Arno Penzias and Robert Wilson (pictured above), using a 20-foot horn-shaped radio antenna (shown above) at Bell Labs in New Jersey, detected an inexplicable signal. While calibrating the instrument for satellite communications, they encountered a persistent, low-level background “hiss” that came uniformly from every direction and did not vary with time or season. After ruling out terrestrial interference and even scrubbing pigeon droppings from the antenna, they consulted Robert Dicke’s team at Princeton, who realized the “static” was the CMB. This discovery confirmed the Big Bang theory because the radiation matched the predicted “fossil remnant” of the universe’s hot, dense origin, now redshifted by cosmic expansion to a temperature of about 3 Kelvin, a phenomenon the competing Steady State theory could not explain.