Cosmic Microwave Background

The Cosmic Microwave Background (CMB) is the afterglow of the Big Bang — a bath of microwave radiation permeating all of space, nearly uniform in every direction yet holding within it the fingerprints of the Universe’s birth. Discovered accidentally in 1965 by Penzias and Wilson, the CMB became the most compelling evidence for the hot Big Bang model. It represents the moment, 380,000 years after creation, when the cosmos cooled enough for protons and electrons to combine into neutral hydrogen, allowing photons to travel freely for the first time. Those photons, stretched by cosmic expansion, now whisper to us as microwaves at a temperature of about 2.725 K — the afterlight of genesis.

The CMB’s discovery was a triumph of both serendipity and theory. While Penzias and Wilson puzzled over a persistent background noise in their antenna, cosmologists Robert Dicke and Jim Peebles at Princeton were preparing to search for just such a relic radiation predicted by Big Bang models. The near-perfect blackbody spectrum measured later by the COBE satellite in 1992 confirmed that this radiation was indeed cosmic in origin. Its uniform temperature to one part in 100,000 testifies to the remarkable homogeneity of the early Universe — while those tiny anisotropies, the small deviations, became the seeds from which all galaxies and clusters would eventually form.

The CMB’s spectrum follows Planck’s law for a blackbody at \(T = 2.725\,\text{K}\), peaking at a wavelength near 1.9 millimeters. This astonishing precision — measured by COBE’s FIRAS instrument — shows that no known physical process can easily reproduce such a perfect blackbody except an early Universe once in thermal equilibrium. The energy density of this radiation today is tiny, yet it dominates the photon content of the cosmos. Each cubic centimeter of space holds roughly 411 CMB photons, relics of an epoch when radiation ruled over matter.

The minute anisotropies imprinted in the CMB carry profound information. Slight density differences — enhanced by quantum fluctuations during inflation — caused the radiation to oscillate as acoustic waves in the primordial plasma. When the Universe became transparent, these oscillations froze, leaving a detailed pattern visible today as peaks in the angular power spectrum. By analyzing these peaks, cosmologists have deduced the age, composition, and curvature of the Universe. The CMB thus acts as a cosmic Rosetta stone: a two-dimensional snapshot encoding the entire three-dimensional history of our expanding cosmos.