Cosmic Microwave Background

The cosmic microwave background (CMB) is the oldest light in the universe — thermal radiation released 380,000 years after the Big Bang, when the cosmos cooled enough for electrons and protons to combine into neutral hydrogen, making the universe transparent to photons for the first time. This recombination epoch marks the boundary between the opaque, ionized early universe and the transparent cosmos we observe today. The CMB arrives at our detectors as a nearly perfect blackbody spectrum at 2.725 K, bearing the faint imprint of primordial density fluctuations that seeded all large-scale structure.

The CMB is cosmology's Rosetta Stone. Its temperature anisotropies — mapped with extraordinary precision by the Planck satellite, WMAP, and ground-based experiments like the Atacama Cosmology Telescope — encode the geometry, contents, and initial conditions of the universe. The angular power spectrum of these anisotropies reveals the acoustic peaks of the photon-baryon fluid oscillating in the early universe's gravitational potential wells. Fitting these peaks with the Friedmann equations yields precise constraints on the Hubble constant, dark matter density, and spatial curvature.

Yet the CMB is an indirect messenger. It does not measure the present-day expansion rate directly; it measures conditions at z ≈ 1100 and relies on a theoretical model — Lambda-CDM — to extrapolate forward. The Hubble tension exposes the risk of this indirectness: if the model is incomplete, the extrapolation fails, and the CMB-derived H₀ becomes a artifact of assumptions rather than a measurement of reality.