Relic Light from the Dawn of Time
The cosmic microwave background is the oldest light in the universe, released when the cosmos cooled to ~3000 K and became transparent for the first time. Discovered accidentally by Penzias and Wilson in 1965, the CMB provides a snapshot of conditions just 380,000 years after the Big Bang. Its near-perfect blackbody spectrum confirms the hot Big Bang model, while its tiny anisotropies — temperature variations of about 30 μK — contain a wealth of cosmological information.
Acoustic Oscillations in the Primordial Plasma
Before recombination, photons and baryons were tightly coupled into a single fluid. Gravitational collapse of dark matter overdensities drove this fluid inward, while radiation pressure pushed it outward, setting up standing acoustic waves. The resulting pattern of compressions and rarefactions at the moment of recombination imprinted a characteristic series of peaks in the angular power spectrum — a cosmic barcode encoding fundamental parameters.
Decoding the Power Spectrum
Each feature of the CMB power spectrum constrains specific physics. The position of the first peak at ℓ ≈ 220 confirms the universe is spatially flat. The relative heights of odd and even peaks measure baryon density. The damping of high-ℓ power constrains photon diffusion (Silk damping) and the number of relativistic species. Together, the Planck satellite's measurements of six parameters describe the entire observable universe to percent-level precision.
Precision Cosmology
The CMB has enabled an era of precision cosmology: six numbers (Ωbh², Ωch², θs, τ, ns, As) suffice to describe the primordial fluctuations and the universe's composition. Cross-correlating CMB data with galaxy surveys, lensing maps, and baryon acoustic oscillation measurements tests the ΛCDM model at multiple redshifts and scales, searching for cracks that might reveal new fundamental physics.